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Hydrologic and Geomorphic Effects of Beaver Dams and Their Influence on Influence on Fishes


Abstract and Figures

Beaver dams alter the hydrology and geomorphology of stream systems and affect habitat for fishes. Beaver dams measurably affect the rates of groundwater recharge and stream discharge, retain enough sediment to cause measurable changes in valley floor mor-phology, and generally enhance stream habitat quality for many fishes. Historically, beaver dams were frequent in small streams throughout most of the Northern Hemisphere. The cumulative loss of millions of beaver dams has dramatically affected the hydrology and sediment dynamics of stream systems. Assessing the cumulative hydrologic and geomorphic effects of depleting these millions of wood structures from small and medium-sized streams is urgently needed. This is particularly important in semiarid climates, where the widespread removal of beaver dams may have exacerbated effects of other land use changes, such as livestock grazing, to accelerate incision and the subsequent lowering of groundwater levels and ephemeralization of streams.
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American Fisheries Society Symposium 37:XXX–XXX, 2003
Hydrologic and Geomorphic Effects of BeaHydrologic and Geomorphic Effects of Bea
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National Oceanic and Atmospheric Administration, Northwest Fisheries Science Center
2725 Montlake Boulevard E., Seattle, Washington 98112, USA
Abstract.—Beaver dams alter the hydrology and geomorphology of stream systems and affect
habitat for fishes. Beaver dams measurably affect the rates of groundwater recharge and
stream discharge, retain enough sediment to cause measurable changes in valley floor mor-
phology, and generally enhance stream habitat quality for many fishes. Historically, beaver
dams were frequent in small streams throughout most of the Northern Hemisphere. The
cumulative loss of millions of beaver dams has dramatically affected the hydrology and
sediment dynamics of stream systems. Assessing the cumulative hydrologic and geomorphic
effects of depleting these millions of wood structures from small and medium-sized streams
is urgently needed. This is particularly important in semiarid climates, where the widespread
removal of beaver dams may have exacerbated effects of other land use changes, such as
livestock grazing, to accelerate incision and the subsequent lowering of groundwater levels and
ephemeralization of streams.
In most of the temperate Northern Hemisphere,
beaver historically altered low-gradient, small-
stream ecosystems by constructing millions of
dams made primarily of wood. Almost every
northern temperate ecosystem that had trees or
shrubs growing along streams also once had bea-
ver dams. In Eurasia, evidence of beaver has been
found in streams as far south as Iraq and Turkey,
in the Arctic, and stretching from Scotland in the
west to Kamchatka in the east (Figure 1; Halley
and Rosell 2002). In North America, beaver were
once found far south into the arid environments
of Arizona and northern Mexico along rivers such
as the San Pedro, Colorado, and the Rio Grande
(Pattie 1833; Leopold 1972; Fredlake 1997) and
occupied all biomes north of the border from coast
to coast, except for the Arctic, peninsular Florida,
and the dry Great Basin and desert country of
Nevada and southern California (Figure 1; Jenkins
Historically, beaver dams created streams
systems with slow, deep water and floodplain
wetlands dominated by emergent vegetation and
shrubs. Geomorphology and plant communities
of small low-gradient streams were much changed
throughout much of the Northern Hemisphere
after reduction of beaver populations (Rea 1983;
Naiman et al. 1988). In both Eurasia and North
America, beaver populations have generally de-
clined as human populations have increased. In
both continents, only small populations survived
by the end of the 19th century (Naiman et al. 1988;
MacDonald et al. 1995; Nolet and Rosell 1998;
Halley and Rosell 2002). The primary reasons for
the declines were that people trapped beavers ei-
ther because they were resources for fur or oil or
competitors for productive valley bottom lands
(MacDonald et al. 1995; Mackie 1997; Halley and
Rosell 2002). More recently, however, there has
been widespread recognition that beaver dams
play a vital role in maintaining and diversifying
stream and riparian habitat (Naiman et al. 1988;
Pollock et al. 1994; Gurnell 1998; Collen and Gibson
2001). In the past century, land managers through-
out the Northern Hemisphere have attempted to
reintroduce beaver in areas where they have been
extirpated. Today, beaver populations are re-
bounding throughout North America, with the
population estimated to be about 10 million and
reoccupying most of its former range (Naiman et
al. 1988). Throughout Eurasia, recovery has been
slower, with the Eurasian beaver population be-
FIGURE 1. Estimated current and historic distribution of beaver in North America (A) and Eurasia (B). Isolated
populations in peninsular Florida and Southern California are not shown. In Eurasia, cross hatching delineates
current distribution. In North America, the current and historic distributions are approximately coincidental.
(Based on Jenkins 1979; Halley and Rosell 2002; MacDonald et al. 1995 ).
tween a half to one million, with large geographic
areas where no beaver are present or where the
current status is unknown (Halley and Rosell 2002).
North American beaver have also become
established in regions outside their geographic
range, such as the Tierra del Fuego region of Ar-
gentina and Chile where they are spreading rap-
idly (Jaksic et al. 2002). The current beaver popu-
lation in Tierra del Fuego is estimated to be around
70,000 (Jaksic et al. 2002). Several North Ameri-
can beaver populations have also been established
in Europe prior to the recognition of the Eurasian
and North American beavers as separate species
(Figure 1; Nolet and Rosell 1998; Halley and Rosell
2002). In such areas, beaver are exotic species and
may have undesirable impacts on native species
and ecosystem processes.
The primary instream habitat value of bea-
ver dams is that they impound water to form
large pools and ponds. These impoundments trap
sediment, help to create productive and diverse
wetland environments on adjacent floodplains,
improve water quality, and facilitate groundwa-
ter recharge. All these functions are ultimately
the result of the dams reducing stream velocities
and spreading water over a large surface area.
Here, we review how beaver dams have al-
tered stream hydrology and morphology, both
in the past when dams were much more abun-
dant and under current conditions. We estimate
the historic abundance and location of beaver
dams in watersheds throughout the Northern
Hemisphere, assess the cumulative hydrologic
effects of multiple beaver dams, evaluate the geo-
morphic consequences of those effects, and ex-
amine what influence these physical effects have
on fishes. Most of the scientific literature on the
ecological effects of beaver dams come from stud-
ies of the North American beaver while literature
on the ecological effects of the Eurasian beaver
dams are rarer, if for no other reason than that
the Eurasian beaver itself is still quite rare
throughout much of its former range. However,
available studies indicate that the two species are
quite similar in most respects, though some evi-
dence suggests the Eurasian beaver builds fewer
dams and lodges and has a lower reproductive
rate (Danilov and Kan’shiev 1983; Collen and
Gibson 2001; Halley and Rosell 2002). Studies of
the hydrologic and geomorphic effects of North
American beaver dams should be applicable to
Eurasian beaver, but studies of the effects on fishes
may not be directly applicable, depending on what
fish species were studied. Reviews of the many
additional ecosystem impacts of beaver that are
not directly related to their dams, such as their
effects on riparian communities, can be found else-
where (Naiman et al. 1988, 1994; Pollock et al. 1994;
Gurnell 1998; Collen and Gibson 2001).
The Historical he Historical
he Historical he Historical
he Historical Abundance ofAbundance of
Abundance ofAbundance of
Abundance of
ver Damser Dams
er Damser Dams
er Dams
Contemporary studies of protected or remote
beaver populations support the contention that,
historically, dams were very common in most
small, low-gradient streams, throughout North
America and Eurasia, but that the frequency of
these dams varied considerably. Where beaver
populations are undisturbed, localized dam fre-
quencies range from 7.5 per km to as high as 74
per km, with frequencies of around 10 dams per
km being more typical in low-gradient streams
(Table 1; Warren 1926; Scheffer 1938). Frequen-
cies may decrease when larger areas are consid-
ered. Two studies examining dam occurrence
across entire, multiple watersheds found frequen-
cies of 2.5 per km for the 750 km2 Kabetogama
Peninsula in Minnesota and 9.6 dams per km for
an 85 km2 area encompassing two watersheds in
Wyoming (Skinner et al. 1984; Johnston and
Naiman 1990b).
Estimates of historic dam densities over large
geographical areas can be calculated if colony den-
sities are known and the average dams built per
colony can be estimated. Reported colony densi-
ties of remote or protected populations show a
trend of lower densities in subarctic regions and
higher densities in more temperate regions, with
an overall average of a little less than 0.5 colonies
per km2 (Table 2). Johnston and Naiman (1990b)
used remote sensing on the Kabetogama Penin-
sula and estimated a colony density of 0.92 km2
and a beaver pond density of 2.96 km2, which gives
an average of 3.2 dams per colony. Recognizing
both the high spatial variation and uncertainty of
estimates of both colony density and dams per
colony, these studies suggest that, before the ar-
rival of Europeans, at least 25 million beaver dams
spanned small to medium-sized streams through-
out the 15.5 × 106 km2 of the North American con-
tinent where beaver once existed (Figure 1; Jenkins
1979). Historic dam densities were probably simi-
lar throughout Eurasia, though there is some evi-
dence to suggest that the remnant Eurasian bea-
ver build fewer dams than their North American
counterparts (Danilov and Kan’shiev 1983).
Using Collen and Gibson’s (2001) estimated
overall mean of 5.2 individuals per colony of North
American beaver, and using a continental mean
of 0.5 colonies per km2 (based on Table 2), pro-
vides an historic population estimate of North
American lodge-building beaver of around 40
million individuals. Only about 75% of beaver
live in lodges, and the rest burrow into banks
(Danilov and Kan’shiev 1983); we estimate a to-
tal historic population of around 55 million. This
estimate is on the low end of, but not inconsis-
tent with, Seton’s (1929) often cited population
estimate of 60–400 million, which was derived
primarily from the qualitative data. However,
we will never be certain whether current esti-
mates of individuals per colony or colony densi-
ties are applicable to historic populations. Because
colony density is affected by habitat quality, it is
not unreasonable to assume that, historically,
overall colony densities were much higher, and
thus populations much greater, when the entire
vast, productive lowland river bottom habitat
throughout North America had not yet been al-
tered by humans and was available for beavers
TABLE 1. Beaver dam density reported in the literature for pristine, remote, or protected areas.
Dams Range Surveyed
Source /km dams/km length/km Location Gradient Comments
(Naiman et al. 1986) 10.6 8.6–16.0 4.3 Quebec low Pristine
(Smith 1950) 12.0 n/a 4.4 Colorado 1–3% Remote
(Smith 1950) 19.1 n/a 4.4 Colorado 1–3% Includes inac-
tive dams
(Warren 1926) 73.7 n/a 0.3 Colorado 12.5% Pristine
(Warren 1926) 41.6 n/a 1.2 Colorado 3.5% Pristine
(Scheffer 1938) 7.5 n/a 8.0 Washington low Transplanted
(Scheffer 1938) 35.4 n/a 0.6 Washington low Transplanted
(Skinner et al. 1984) 9.6 2.9–41.1 43.3 Wyoming 85 km2 area Remote
(Woo and Wadding- 14.3 5-19 n/a Ontario low Remote, pris-
ton 1990) tine
(Naiman et al. 1988) 2.5 2.0–2.9 n/a Minnesota 750 km2 area Protected
(Hering et al. 2001) 27.8 n/a 0.9 W. Germany n/a Recovering
(Hering et al. 2001) 12.3 n/a 1.3 S. Germany n/a Recovering
(Medwecka-Kornas 20.0 n/a 1.0 Poland n/a Recovering
and Hawro 1993) population
TABLE 2. Colony density estimates for North American and Eurasian beaver for protected, unprotected, and
recovering populations.
Source Location Density km2Status
(Howard and Larson 1985) Massachusetts 0.92 Protected
(Johnston and Naiman 1990a) Minnesota 0.92 Protected
(McCall et al. 1996) Maine 0.15–0.32 Managed
(Voigt et al. 1976) Ontario 0.38–0.76 (3.0) Remote
(Bergerud and Miller 1977) Newfoundland 0.2–0.3 (4.2) Remote
(Aleksiuk 1970) Northwest Territories 0.38 Remote
(MacDonald et al. 1995) Latvia 0.37 Recovering
(Hartman 1994) Sweden 0.25 Recovering
(Zurowski and Kasperczyk 1986) Poland 0.15 Recovering
(Jaksic et al. 2002) Argentina 1.9 Introduced
to use. The Eurasian beaver has a slower repro-
ductive rate and a lower average colony size (3.8);
so, historic estimates of populations and densi-
ties cannot be directly extrapolated from studies
of the North American beaver (Danilov and
Kan’shiev 1983). However, recovering popula-
tions of European beaver suggest that high den-
sities may be achievable in the long term if man-
agement authorities desire such a goal (Halley
and Rosell 2002).
Location of Dams inLocation of Dams in
Location of Dams inLocation of Dams in
Location of Dams in
Beaver prefer to dam small, low-gradient streams
with unconfined valleys, but they can also dam
both large and high-gradient streams. Retzer et
al. (1956) studied 365 reaches in 61 streams
throughout Colorado to determine the physical
factors determining beaver pond location. Bea-
ver built dams on 82% of all the low-gradient (1–
3%) streams surveyed, 73% of reaches with 4–6%
gradients, and 61% of reaches with 7–9% gradi-
ents (Figure 2). Use of streams with a slope greater
than 9% dropped off dramatically. On streams with
gradients greater than 15%, just one active dam
was found on a 16% stream slope and one aban-
doned dam was found on a 21% stream slope.
Beaver overwhelmingly preferred to dam streams
in valleys wider than 46 m (150 ft); they used 85%
of such streams, but only 35% of streams with
narrower valleys, regardless of their slope. These
results are confounded because wide valleys tend
to have low-gradient streams.
Consistent with these results, Pollock and Pess
(1998) studied 341 beaver ponds in the 1,741 km2
Stillaguamish watershed, Washington and found
that 91% were in low-gradient (4%) streams with
unconfined (>4 channel widths) valleys, and al-
most all of them were in watersheds less than 15
km2. In this study, 16% was the steepest gradient
where a beaver pond was found. Similarly, Suzuki
and McComb (1998) studied 170 beaver dams in
the Drift Creek basin, Oregon and consistent with
these results found that more than 90% were on
stream gradients of less than 6%. Beier and Barrett
(1987) also found that geomorphic and hydrologic
conditions were the best predictors of dam-site
suitability, with gradient, stream depth, and stream
width the most important factors. They found that
biological factors, such as food availability, did not
provide additional explanatory power as to the
location of dam sites.
The maximum size of streams that beaver
can dam is not well documented, and it will cer-
tainly vary from region to region, depending on
hydrologic conditions. In Washington, historic
FIGURE 2. Beaver–dam frequency based on stream gradient from a survey of 356 stream reaches in the
mountainous regions of Colorado, showing that beaver strongly prefer to dam low-gradient streams (adapted from
Retzer et al. 1956).
1-3 4-6 7-9 10-12 13-15 16-18 19-21 21
Amount of surveyed habitat currently or formerly
used (%)
records note a beaver dam on a stream with a
drainage area of 107 km2. Naiman et al. (1984)
report seasonal dams being built on the lower
reaches of the 207 km2 Muskrat River watershed,
Quebec. In 1998, one of us (Pollock) noted a stable
beaver dam crossing the entire South Fork of the
Little Colorado River with an upstream drainage
area of 280 km2. We suspect that unreported dams
exist on larger and more powerful streams, but
stream size or drainage area are generally not
reported in the literature. The size of streams and
rivers that could be dammed by the bear-sized
Pleistocene beaver is a matter of interesting specu-
lation (Mai 1978). We are not aware of any pub-
lished literature that has identified prehistoric bea-
ver dams.
Effects of BeaEffects of Bea
Effects of BeaEffects of Bea
Effects of Beavv
ver Dams oner Dams on
er Dams oner Dams on
er Dams on
Stream HydrologyStream Hydrology
Stream HydrologyStream Hydrology
Stream Hydrology
Even the casual observer notices that beaver dams
retain water and thus slow its downstream move-
ment. What is surprising is how few studies have
attempted to quantify what happens to the water
retained by beaver dams and what the hydro-
logic effects are. The limited available evidence
suggests that key hydrologic functions of beaver
dams are to dissipate stream energy, attenuate
peak flows, and increase groundwater recharge
and retention, which increases summer low flows
and elevates groundwater levels in stream val-
leys, thus expanding the extent of riparian veg-
etation (Stabler 1985; Lowry 1993).
Beaver dams slow the velocity of water,
thereby reducing energy and increasing retention
time. During floods, energy is dissipated as the
water works its way through a tortuous path of
small branches on the downstream side of the
dam. Finally, energy is further dissipated by flood-
plain vegetation below the dam that must be over-
come as the water works its way back to the
stream channel (Li and Shen 1973; Woo and
Waddington 1990; Dunaway et al. 1994).
Because beaver dams slow stream velocity,
they should also attenuate flood peaks. Research
on the effects of wood dams in small (third-
order) streams suggests that they can retain wa-
ter at least 50% longer than streams where such
dams are absent (Ehrman and Lamberti 1992).
Given the much lower permeability of beaver
dams compared to large wood jams, it is reason-
able to expect them to retain water for much longer
periods of time (Hering et al. 2001).
Beedle (1991) used the dimensional charac-
teristics of beaver dams in the glacially carved
valleys on a southeastern Alaska island to evalu-
ate the hydrologic effects of these structures on
peak flows. Using simulated peak-flow routing,
Beedle estimated that a single full beaver pond
reduced peak flows by more than 5%, but that a
series of five large ponds in series could reduce
peak flows of a 2-year event by 14% and peak
flows of a 50-year event by 4%. The simulation
also suggested that beaver dams did not greatly
alter the shape of outflow hydrographs, resulting
in a 10–15-min delay in the peak and a slight in-
crease in flood duration. Beedle’s simulation as-
sumed that all ponds were at full storage capacity
before the storm and that water pouring over
dams instantly returned to the channel. Although
these assumptions were necessary for the simu-
lation, under natural conditions, where dams are
not filled to capacity and flood water pouring over
dams is spread out across the floodplain, peak
flows could likely be further reduced. Because
many streams may have dozens of dams, expect-
ing stronger cumulative effects is reasonable
(Scheffer 1938; Smith 1950; Naiman et al. 1986).
Effects of BeaEffects of Bea
Effects of BeaEffects of Bea
Effects of Beavv
ver Dams oner Dams on
er Dams oner Dams on
er Dams on
Aquifer Recharge and LowAquifer Recharge and Low
Aquifer Recharge and LowAquifer Recharge and Low
Aquifer Recharge and Low
Flow DischargesFlow Discharges
Flow DischargesFlow Discharges
Flow Discharges
Some researchers have suggested that beaver
dams and other instream obstructions that retain
water contribute to groundwater recharge and
thus help to increase summer low flows. Anec-
dotal reports in the literature support this conten-
tion, though few quantitative data exist. In Mas-
sachusetts, Wilen et al. (1975) compared changes
in summer low flows caused by beaver dams
when beaver colonized one of their study sites. In
a paired stream study, beaver dams and large
wood were removed from one stream, and the
other stream was retained as a control. During
the first year of the experiment, both streams re-
mained perennial, but the undisturbed stream had
more flow. In spring of the next year, beaver re-
colonized the experimental reach, building a se-
ries of dams. That summer, the experimental
stream remained perennial below the newly built
dams, but the “control” stream with no beaver
dams went dry.
Tappe (1942) noted summer flows in several
streams in northern California increased after bea-
ver colonized upstream reaches. In Missouri,
Dalke (1947, cited in Stabler 1985) reported that
the return of beaver to small streams restored
perennially flowing water that could support fish.
Likewise, Finley (1937) reported that stream flow
in Silver Creek, in the Ochoco National Forest of
eastern Oregon, decreased substantially after the
loss of beaver dams. Trappers removed about 600
beaver from the headwaters of Silver Creek, and
several creeks dried up in following years as wa-
ter levels dropped. According to Finley, ground-
water levels dropped sufficiently that pastures
near streams could no longer support grass, re-
sulting in an annual loss of approximately 15,000
tons of pasturage. Further downstream, water was
no longer available to irrigate farmland, resulting
in additional economic losses.
In eastern Oregon, groundwater levels in the
floodplain near a beaver dam substantially in-
creased as compared to a reach lacking any dams,
suggesting that dams do help increase water avail-
ability to riparian areas at least 50 m from the
pond (Lowry 1993). Additionally, Lowry estimated
that about 90 m3 of groundwater could be ob-
tained if the beaver dam was breached.
Human-built structures have yielded analo-
gous results. Brown (1963), studying a conserva-
tion program in Flat Top Ranch, Texas noted that
small lakes and ponds created in sandy soils al-
lowed for rapid infiltration of stormwater and
helped to convert all major drainages from inter-
mittent to perennial streams. In coastal British
Columbia, a small check dam no larger than a
beaver dam was used to retain stormwater in a
27-ha pond and ensure that the stream flowed
throughout the summer (Wood 1997).
In Alkali Creek, Colorado, Heede and
DeBano (1984) observed that perennial flow de-
veloped in downstream reaches 7 years after in-
stallation of 132 small check dams in ephemeral,
gullied streams. Old beaver dams were observed
in the gully walls while the check dams were be-
ing built, and 14 years after the project began,
beaver have begun recolonizing some of the
reaches with perennial flow (Stabler 1985). Sta-
bler also noted the work of Jester and McKirdy
(1966) who installed check dams in Taos Creek,
New Mexico. Within 10 years, they obtained pe-
rennial flow that could sustain trout populations.
Similarly, Ponce and Lindquist (1990) noted that
perennial flow was an unexpected result of a small
(5-m) dam built to retain sediment on Sheep
Creek, Utah.
Lowry’s (1993) study suggests that the water
storage value of beaver dams does not lie solely
in the water stored behind the dam because 90 m3
is only enough water to keep even a small stream
flowing for a very short period. If, as the previ-
ous examples suggest, beaver dams change
stream hydrology so streams that formerly went
dry in the summer now flow year round, their
ability to recharge groundwater must be substan-
tial. Simple calculations indicate that recharge from
the dams of even a few beaver colonies can con-
tribute substantially to summer low flows. As an
example, a small stream flowing at a modest 0.1
m3/s over a period of 100 d (the approximate
length of summer in much of the western United
States) will need about 8.64 × 105 m
3 of water
above and beyond what is lost to evapotranspi-
ration. Assuming that a series of beaver dams pro-
vides 1 × 105 m2 of pond surface area (20 dams,
assuming that a typical pond has 5 × 104 m2 of
surface area), aquifer hydraulic conductivities of
about 4 × 10-7 m/s are needed to accept this much
water during the other 265 d of the year. Because
4 × 10-7 m/s is well within the range of hydraulic
conductivities typical of the silts and sands that
would be found at the bottom of beaver ponds
and fine-grained alluvial deposits (Dunne and
Leopold 1978; Lowry 1993), even a small number
of beaver impoundments should be able to aug-
ment low flow discharges during long periods
without rain, provided an aquifer is present to
accept the recharge.
A stream may remain perennial if the amount
of valley alluvium is sufficient to store waters in-
filtrating from beaver ponds. Assuming a specific
yield of 20%, typical of fine-grained alluvial de-
posits (Dunne and Leopold 1978; Lowry 1993), an
aquifer volume of 4.32 × 106 m3 is needed to store
the 8.64 × 105 m3 of water needed to maintain
flow at 0.01 m3/s for 100 d. As an example, sus-
taining this flow would require valley alluvium
150 m wide by 2.4 km long by 12 m deep. By
comparison, to retain this much water behind typi-
cal beaver dams (0.5 m high on a 1% stream gra-
dient and a 50-m-wide dam, and assuming they
are not storing any sediment) would require the
impoundment of 35 km of low-gradient streams.
Together, these numbers suggest that if beaver
impoundments augment summer low flows in
small streams, they do so primarily by recharg-
ing aquifers, and the direct storage of water be-
hind dams probably plays a minor role in low-
flow augmentation.
Whether a series of beaver dams is actually
able to recharge an alluvial aquifer, and whether
that water is ultimately available to augment
streamflows in the summer, depends upon the
geometry and hydraulic properties of the aqui-
fer. Aquifer geometry determines the size of the
aquifer. Hydraulic properties, such as hydraulic
conductivity and the coefficient of storage, con-
trol the rate of recharge and discharge, which de-
termines whether an aquifer will drain through-
out the summer. For example, a highly porous
aquifer may quickly recharge but also quickly dis-
charge, thereby precluding the slow release of
water throughout the summer.
Effects of BeaEffects of Bea
Effects of BeaEffects of Bea
Effects of Beavv
ver Dams oner Dams on
er Dams oner Dams on
er Dams on
Valley Floor Morphologyalley Floor Morphology
alley Floor Morphologyalley Floor Morphology
alley Floor Morphology
The slow velocity of water behind beaver dams
creates extensive depositional areas for sediment
and organic material transported from upstream
reaches. The shallow waters behind the dams al-
low highly productive emergent vegetation to
grow, allowing in situ development of organic
material, much of which is ultimately deposited
on the (submerged) valley floor. Together, the
onsite creation of organic material and the depo-
sition of sediment and organic material from up-
stream raise the surface of the stream bed and
inundate the adjacent valley floor. Sediment stor-
age behind beaver dams can be substantial.
In a boreal forest ecosystem, Naiman et al.
(1986) found that sediment storage behind bea-
ver dams ranged from 35 to 6,500 m3, yet they
found no relation between dam size and sediment
retained. Butler and Malanson (1995) studied sedi-
ment deposition behind five young (<6 years) and
three “old” (>10 years) beaver dams in Glacier
National Park, Montana. They found more sedi-
ment stored behind the older ponds. Sediment
volumes behind the more recent dams ranged
from 9 to 165 m3 (average = 56 m3); while behind
the older dams, sediment storage ranged from 77
to 267 m3 (average = 203 m3). In parts of the arid
western United States, beaver dams have been
used to trap sediment in eroding streams with
good results (Scheffer 1938; Apple 1985). In Mis-
sion Creek, Washington, beaver were successfully
“employed” to control sediment losses resulting
from poor land-use practices. In just 2 years, 4,863
m3 of sediment were trapped behind 22 dams
along a 622-m reach for an average of 7.8 m3 of
sediment stored per linear meter of stream
(Scheffer 1938). The average sediment retention
behind beaver dams was 221 m3 and ranged from
34 m3 to 586 m3. A reanalysis of Scheffer’s data
shows a significant relationship between the sur-
face area of the dam face and the amount of sedi-
ment stored (r2 = 0.30, p < 0.001, n = 22). Remov-
ing the three statistical outliers greatly increases
the strength of the correlation (r2 = 0.82, p < 0.001,
n = 19; Figure 3). These data suggest a general
trend of increasing sediment storage with increas-
ing dam size, with some notable exceptions. These
exceptions could be explained by the age of the
dams or the recent failure of a dam upstream.
These studies indicate that sedimentation
rates behind beaver dams vary widely. Factors
influencing sedimentation rates include growth
rates of the emergent vegetation (which itself is
determined by species composition, climate, and
site productivity), upstream sediment loads
(which is determined by geology, watershed
land-use history, and disturbance history), the
number of beaver dams upstream, and the fre-
quency of dam failure for both the dam in ques-
tion and any upstream dams. Given the high
spatial and temporal variation for all of these
factors, general predictions of sedimentation
rates will be inaccurate and site-specific measure-
ments will be required.
Sediment behind beaver dams is often a com-
bination of fluvially transported material and or-
ganic material produced by the emergent vegeta-
tion growing on the pond edge. As ponds fill in
with sediment, emergent vegetation is able to
grow towards the pond center, helping to trap
more sediment and producing more organic ma-
terial, thereby accelerating the rate of filling. Be-
fore being completely filled, most ponds are aban-
doned, and dams often breach (Pastor et al. 1993),
thus ending the accumulation of sediment. Al-
though inorganic sediment accumulation may
cease, however, wetland graminoids (sedges,
rushes, grasses) colonize the exposed sediments
on the pond edges, typically turning the former
pond into a highly productive wet meadow and
creating and depositing yet more organic matter
on the former pond floor.
The observation that beaver dams trap sedi-
ment have led some scientists to conclude that
the accumulation of such sediment over long pe-
riods can cause permanent changes in valley floor
morphology. Rudemann and Schoonmaker
(1938), Ives (1942), and Rutten (1967) argued that
the numerous small, broad, relatively level sur-
faces at the bottom of recently deglaciated, U-
shaped valleys were the result of sediment accu-
mulation behind the numerous beaver dams built
and maintained since the last glacial recession.
Ives observed that a competing hypothesis, that
postglacial lake deposits formed the bottoms,
was unlikely because the valleys were, in fact,
gently sloping over long distances. Ives also dis-
missed the hypothesis that the flatness of the
valleys was the result of the meandering of post-
glacial braided rivers on the grounds that such
action would have left a more uneven topogra-
phy because braided reaches differentially incise
and because the valley fill was fine-textured, sug-
gesting a depositional environment rather than
the coarse-textured sediment that would be typi-
cal of a braided river transporting glacial
outwash. Finally, Ives observed slight steps in
some valley floors coincidental with the pres-
ence of abundant buried wood similar in size and
shape to the wood of beaver dams.
Others, such as Rutherford (1964), have ar-
gued that although beaver dams have contrib-
uted to the accumulation of fine sediments on val-
ley floors, the presence of underlying geologic
features, such as glacial moraine or dykes, are the
underlying control that allows flat valleys to form.
We believe that both hypotheses have merit and
are not mutually exclusive. However, the extent
to which beaver dams control valley floor mor-
phology (e.g., slope) over long periods is still
largely unknown.
Over short periods in small reaches, beaver
dams create a stair-step valley and stream profile
consisting of flat areas with abrupt gradient
changes at dam sites. We suggest that, over long
periods, beaver dams can change the longitudi-
nal profile of streams such that they effectively
change valley slope over long distances. This
change could be accomplished if beaver dams at
the mouth of a valley were consistently built
higher or were more closely spaced to act as a
series of step dams. Although the total gradient
would stay the same, most of the valley would be
gently sloping, with a steeply graded but stepped
gradient near the mouth. Simple calculations sug-
gest that, over long periods, significant morpho-
logical changes are possible.
Geometric relations can be used to estimate
maximum potential sediment storage behind bea-
ver dams:
0.5 /
where Vm = maximum sediment storage volume,
H = dam height, W = dam width, and S = stream
slope. Thus, as an example, a dam rising 1.0 m
above a 50-m-wide valley floor with a slope of
0.01 could store a maximum of 2,500 m3 of sedi-
ment, or about 25 m3/m of stream length, which
on average would elevate the valley floor 0.5 m.
Assuming that on average beaver build 1.0-m-
high dams and that they are half filled every 50
years, a valley floor could be raised at a rate of 0.5
cm/year, or accumulate about 12 m (vertical) of
sediment and organic material, over a period of
10,000 years. Measured sediment accumulation
rates behind dams range from 4 to 30 cm/year
over short time frames and from 0.25 to 6.5 cm/
FIGURE 3. The relation between sediment stored and vertical dam face area is weak but significant (
2 = 0.30,
< 0.01,
= 22). With three statistical outliers removed (large square symbols), the relation becomes much more
apparent (
2 = 0.83,
< 0.01,
= 19) (adapted from Scheffer 1938).
0 100 200 300 400 500 600 700
Stored sediment (m )
Vertical surface area of dam (m )
year over decadal time scales (Scheffer 1938; Devito
and Dillon 1993; Butler and Malanson 1995), sug-
gesting that our calculations are conservative. On
the other hand, sediment will compact under the
weight of additional sediment, and a certain amount
of sediment behind abandoned dams is exported if
the stream incises into the deposits behind the
former dam. Also, the cycle of beaver habitat colo-
nization and abandonment can be at shorter inter-
vals than 50 years (Warren 1932; Neff 1959). As a
rough approximation, however, these calculations
illustrate the theoretical capacity of beaver dams
to substantially raise valley floor elevations over
long periods. Studies of the cumulative geomor-
phic effects of beaver dams (and their widespread
removal) would be worthwhile. Observations of
meters-thick accumulations of compressed organic
material exposed on the valley walls of incised
streams, where beaver dams are known to have
existed until recently, provide additional physical
evidence to support the idea that a series of beaver
dams built over long periods can raise valley floors
(Pattie 1833, Fredlake 1997; Pollock, author’s per-
sonal observation).
Other researchers have observed streams that
historically contained beaver and well-vegetated
floodplains and are now incised in many places
(Winegar 1977; Parker et al. 1985; Elmore and
Beschta 1987). The rapid incision that has occurred
throughout much of the western United States
and other parts of the world in recent centuries
suggests that base elevations were not controlled
by hard geological features, such as bedrock, but
by “soft” grade-control structures like beaver
dams, large wood, or the dense roots of stream-
side vegetation (e.g., Zierholz et al. 2001). In the
western United States, the coincidence of wide-
spread incision with the arrival of the first white
settlers suggests that one of their early land-use
activities may have been the causal agent. Removal
of beaver by trappers was one of the first major
land-use changes to occur in the United States fol-
lowing European settlement (Mackie 1997). Rap-
idly removing millions of beaver resulted in the
loss of millions of functioning dams throughout
the West. For small streams, this loss must have
resulted in a series of catastrophic floods, as dams
ruptured during high flows and destroyed down-
stream dams that were acting as “roughness ele-
ments” (sensu Lisle 1982) slowing stream veloci-
ties and dissipating energy.
The effects of catastrophic beaver dam fail-
ures are documented (Butler 1989; Stock and
Schlosser 1991). Often channel incision occurs just
downstream of the dam and fine sediment is de-
posited farther downstream, along with uprooted
plant material and organic matter. Large wood
from streams has also resulted in the loss of sedi-
ment and the rapid downcutting of channel beds
(Smith et al. 1993). Recent restoration efforts us-
ing large wood have elevated the base of stream-
beds (Slaney and Zaldokas 1997).
These studies taken as a whole suggest that
beaver dams can substantially elevate and main-
tain wide, low-gradient valley floors over long
periods and that these geomorphic features
should persist as long as healthy populations of
beaver are available to maintain the dams. The
available evidence also suggests that the repeated
building of beaver dams, the subsequent backfill-
ing with sediment and organic matter, and the
building of new dams on top of old fill can create
valley and stream-slope elevational profiles that
differ from the underlying slope.
Whether the widespread elimination of bea-
ver dams is the ultimate cause of the recent inci-
sion that has occurred in many streams in the
western United States has not been determined.
But this hypothesis is consistent with available in-
formation: indirect support for it comes from the
work of land managers who are using beaver
dams—or small artificial dams—as a tool to ag-
grade incised streams.
In Wyoming, Apple et al. (1983) provided
cut cottonwood branches to recently relocated
beaver who subsequently built dams that ag-
graded incised channels. The initial dams quickly
backfilled with sediment, and the beaver contin-
ued to build additional structures upstream. In
Mission Creek, Washington, relocated beaver
built dams and trapped thousands of cubic meters
of sediment in just a few years (Scheffer 1938).
Similar human-built structures have also yielded
good results. In Sheep Creek, Utah, a 5-m-high
retaining dam with a storage volume of 1.09 ×
105 m3 of sediment was built in 1961 and filled up
in just 1 year. The dam continues to cause aggra-
dation upstream of the storage area and, by 1984,
had retained a total volume of 4.39 × 105 m3 of
sediment (Haveren et al. 1987). A formerly in-
cised xeric valley is now supporting abundant
riparian vegetation on a gently sloping valley
On a smaller scale, in Alkali Creek, Colorado,
Heede (1978) built small concrete “check” dams
on small ephemeral streams containing no ripar-
ian vegetation. The barriers quickly filled with
sediment and now support substantial amounts
of riparian vegetation because the dams create
localized areas of groundwater recharge. The veg-
etation itself helps to trap additional sediment,
thus creating a positive feedback of valley aggra-
dation. Together, these studies suggest that, where
small dams exist, whether built by beavers or
people, they can cause localized aggradation of
valley floors and infilling of incised valleys, pro-
vided a sufficient supply of sediment is upstream.
Additional studies to determine the feasibility of
using beaver dams to initiate aggradation of in-
cised valleys would be useful.
ver Dams and Fer Dams and F
er Dams and Fer Dams and F
er Dams and Fishish
Effect of dams on communities and
When beaver impound streams by building
dams, they substantially alter stream hydraulics
that may benefit many fish species (Murphy et
al. 1989; Snodgrass and Meffe 1998). More than
80 North American fishes have been documented
in beaver ponds, with 48 species commonly us-
ing them (Table 3). Fish use of beaver ponds in
Eurasia is not as well documented. In Sweden,
Hagglund and Sjoberg (1999) observed that min-
now Phoxinus phoxinus and brown trout Salmo
trutta used beaver ponds, while burbot Lota lota
and northern pike Esox lucius were found in the
ponds occasionally. In general however, studies
of the effects of the Eurasian beaver on fish popu-
lations are not common, and data are often ex-
trapolated from studies of the North American
beaver (e.g., see Collen and Gibson 2001). Be-
cause beaver ponds usually have slow current
velocities and large edge-to-surface-area ratios,
they provide extensive cover to fish and a pro-
ductive environment for both vegetation and
aquatic invertebrates, affording fish foraging
opportunities not found in unimpounded stream
habitat (Hanson and Campbell 1963; Keast and
Fox 1990). Fish expend less energy when forag-
ing in slow water. Thus sections of streams im-
pounded by beaver dams are often more pro-
ductive than unim-pounded reaches in both the
number and size of fish (e.g., Gard 1961b; Hanson
and Campbell 1963; Murphy et al. 1989; Leidholt
Bruner et al. 1992; Schlosser 1995). Fish are not
the only beneficiaries of beaver dams. Relative
to unimpounded reaches, increases in either bio-
mass or diversity have also been observed for a
wide range of taxa, including birds, mammals,
amphibians, plants, and insects in areas affected
by beaver (see reviews in Naiman et al. 1988;
Pollock et al. 1994).
The physical components of beaver dams af-
fect a variety of fish species. Much of the research
on beaver–fish interactions has been concerned
with how salmonids are affected by beaver ponds,
primarily brook trout Salvelinus fontinalis and ju-
venile coho salmon Oncorhynchus kisutch (how-
ever, see Hanson and Campbell 1963; Keast and
Fox 1990; Schlosser 1995; Snodgrass and Meffe
1999). Some early observations of the increased
siltation in beaver ponds, along with increased
temperatures caused by the loss of shade-produc-
ing vegetation, led some to conclude that beaver
ponds could be detrimental to salmonid popula-
tions (Salyer 1935; Reid 1952; Knudsen 1962). These
opinions were based on subjective observations
and assumptions that fine sediment deposited be-
hind beaver dams smothered redds and decreased
benthic invertebrate production, that higher sum-
mer temperatures found in ponds were detrimen-
tal, and that the flooding of vegetation led to wa-
ter quality problems such as low dissolved oxygen
Although these researchers expressed con-
cern that beaver dams are harmful to trout, no
study has ever demonstrated a detrimental popu-
lation-level effect of dams on salmonids, nor has
a study shown that beaver dams are more than a
seasonal barrier to fish movement. On the con-
trary, most studies support the contention that
the pond habitat formed by beaver dams is highly
beneficial to many fishes and that species regu-
larly cross dams in both upstream and down-
stream directions (Rupp 1954; Huey and Wolfrum
1956; Gard 1961b; Hanson and Campbell 1963;
Call 1970; Bustard and Narver 1975; Swales et al.
1988; Murphy et al. 1989; Keast and Fox 1990;
Leidholt Bruner et al. 1992; Schlosser 1995;
Snodgrass and Meffe 1998, 1999).
Comparing salmonid productivity in stream
reaches above beaver dams with reaches where
beaver dams are absent generally demonstrate
that the reaches above beaver dams produce ei-
ther more fish or larger fish or both. In a detailed
study of the effects of beaver dams on trout in
Sagehen Creek in the Sierra Nevada of Califor-
nia, Gard (1961b) found that the number of trout
(brook, rainbow O. mykiss, and brown) were
about the same over a 3-year period for a dam-
affected reach and a “control” reach of a similar
length (67 m), but that the size of the trout in the
reach above the beaver dam were much larger.
TABLE 3. Common and scientific names of fish species known to use beaver ponds. The relevant scientific papers
are also noted.
Species Common name AbundanceaReferencesb
Aphredoderus sayanus Pirate perch C 1
Campostomus anomalum Central stoneroller C 2
Catostomus commersoni White sucker C 2,7,9
Centrarchus macropterus Flier C 1
Phoxinus eos Northern redbelly dace C 3,4,7,15
Culaea inconstans Brook stickleback C 3,4
Enneacanthus chaetodon Blackbanded sunfish C 1
Enneacanthus gloriosus Bluespotted sunfish C 1
Erimyzon oblongus Creek chubsucker C 1
Esox americanus americanus Redfin pickerel C 1
Esox niger Chain pickerel C 1
Etheostoma serrifer Sawcheek darter C 1
Fundulus diaphanus Banded killifish C 3
Fundulus lineolatus Lined topminnow C 1
Gambusia holbrooki Eastern mosquitofish C 1
Hybognathus hankinsoni Brassy minnow C 4
Ameirus melas Black bullhead C 2,4,5
Lepomis auritus Redbreast sunfish C 1
Lepomis cyanellus Green sunfish C 2
Lepomis gibbosus Pumpkinseed sunfish C 3,4
Lepomis gulosus Warmouth C 1
Lepomis marginatus Dollar sunfish C 1
Lepomis punctatus Spotted sunfish C 1
Notropis cummingsae Dusky shiner C 1
Notropis heterodon Blackchin shiner C 3,4
Notropis lutipinnis Yellowfin shiner C 1
Luxilus umbratilis Redfin shiner C 2
Oncorhynchus clarkii Cutthroat trout C 6, 9
Oncorhynchus kisutch Coho salmon C 6, 10,12,13c
Oncorhynchus nerka Sockeye salmon C 6
Phoxinus neogaeus Finescale dace C 4
Phoxinus phoxinus Minnow C 16
Pimephales promelas Fathead minnow C 2,3,4,16
Pungitius pungitius Ninespine stickleback C 7,15
Salmo trutta Brown trout C 8,17
Salvelinus fontanilus Brook trout C 8,9,12,16c
Salvelinus malma Dolly Varden char C 6,10,12
Semotilus atromaculatus Creek chub C 1,2,4,5,7,16
Semotilus coporalis Fallfish C 7
Cottus spp. Sculpins LC 6
Esox lucius Northern pike LC 4,14,17
Gasterosteus aculeatus Threespine stickleback LC 6, 12
Micropterus salmoides Largemouth bass LC 1,2,4,5
Notemigonus crysoleucas Golden shiner LC 1,2,3,16
Luxilus cornutus Common shiner LC 2,4
Notropis heterolepis Blacknose shiner LC 4
Notropis topeka Topeka shiner LC 2
Perca flavescens Yellow perch LC 3,4,16
Prosopium spp. Whitefish LC 6
Ameiurus natalis Yellow bullhead U 1,2
TABLE 3. Common and scientific names of fish species known to use beaver ponds. The relevant scientific papers
are also noted.
Species Common name AbundanceaReferencesb
Ameiurus platycephalus Flat bullhead U 1
Acantharchus pomotis Mud sunfish U 1
Ictalurus nebulosus Brown bullhead U 3
Lepomis humilis Orangespotted sunfish U 2
Lota lota Burbot C 16
Cyprinella lutrensis Red shiner U 2
Noturus leptacanthus Speckled madtom U 1
Notropis petersoni Coastal shiner U 1
Oncorhynchus mykiss Steelhead U 6, 10
Oncorhynchus mykiss Rainbow trout U 8, 9
Oncorhynchus tshawytscha Chinook salmon U 6, 10
Pimephales notatus Bluntnose minnow U 2,3
Prosopium williamsoni Mountain whitefish U 10
Margariscus margarita Pearl dace O 16
Amia calva Bowfin O 1
Anguilla rostrata American eel O 1
Elassoma zonatum Banded pygmy sunfish O 1
Etheostoma exile Iowa darter O 3
Etheostoma fricksium Savannah darter O 1
Etheostoma fusiforme Swamp darter O 1
Etheostoma nigrum Johnny darter O 2
Etheostoma olmstedi Tessellated darter O 1
Etheostoma spectabile Orangethroat darter O 2
Lepomis macrochirus Bluegill O 2
Minytrema melanops Spotted sucker O 1
Nocomis leptocephalus Bluehead chub O 1
Notropis dorsalis Bigmouth shiner O 2
Noturus gyrinus Tadpole madtom O 1
Oncorhynchus gorbuscha Chum salmon O 6
Percina caprodes Logperch O 2
Umbra limi Central mudminnow O 3
Umbra pygmaea Eastern mudminnow O 1
a Species sorted by frequency of use: C = common, LC = locally common, U = uncommon, O =
b Reference key: 1 = Snodgrasss and Meffee 1998; 2 = Hanson and Campbell 1963; 3 = Keast and Fox;
1990, 4 = Schlosser 1995; 5 = Stock & Shlosser 1991; 6 = Murphy et al. 1989; 7 = Rupp 1954; 8 = Gard 1961a,
1961b; 9 = Huey and Wolfrum 1956; 10 = Swales et al. 1988; 11 = Bryant 1983; 12 = Call 1970; 13 = Bustard
and Narver 1975; 14 = Knudsen 1962; 15 = Rupp 1954; 16 = Balon and Chadwick 1979; Hagglund and
Sjoberg 1999.
c For brook trout and coho salmon use, there is an abundance of additional literature, referenced in the
The average length in the dam-affected reach was
147 mm, and the average weight was 117 g; trout
lengths in the control reach averaged 110 mm,
and their average weight was 22 g. The brook
trout and brown trout generally benefited more
from the beaver ponds because they could feed
on the pond’s bottom fauna. After floods de-
stroyed the dams, the brown trout population
crashed and rainbow trout became the dominant
species (Gard and Seegrist 1972).
Similarly, Huey and Wolfrum (1956) com-
pared summer trout populations in three stream
reaches upstream of beaver dams with three
control reaches in the upper Red River, New
Mexico and found that the dam reaches contained
many more trout (mostly cutthroat O. clarki and
brook trout, but some rainbow). Average trout
populations in the three control reaches ranged
from 12 to 80 (mean = 40) and, in the dam reaches,
ranged from 131 to 218 (mean = 167). All the
reaches were about 46 m long. Other studies have
noted the abundance of trout upstream of beaver
dams but did not directly compare them with
unimpounded reaches. Call (1970), studying the
effects of beaver on headwater streams in Wyo-
ming, noted that beaver created trout habitat,
where previously none existed, by damming very
small streams and seeps, substantially increasing
available brook trout habitat and allowing for the
development of a productive fishery. Similarly,
Gard (1961a) built small artificial dams to mimic
beaver dams in the headwaters of Sagehen Creek,
California and created a productive brook trout
fishery where none had previously existed. Some
of the increased salmonid productivity may be a
result of the increased numbers of forage fish in
beaver ponds. Rupp (1954) observed in a beaver
pond in Maine that increased numbers of small
fish, primarily three-spine stickleback Gasterosteus
aculeatus and northern redbelly dace Phoxinus eos,
were used as forage by brook trout.
Beaver ponds may be particularly important
habitat for overwintering resident char and trout.
Both Chisholm et al. (1987) and Cunjak (1996)
observed that brook trout had a strong tendency
to move into the slow water habitat of beaver
ponds to overwinter. Likewise, Jakober (1995)
observed that, in Montana streams, bull trout
Salvelinus confluentus and cutthroat trout aggre-
gated in large numbers to overwinter in beaver
ponds. As temperatures drop in the autumn, other
fishes appear to aggregate and seek out slow wa-
ter habitat in which to overwinter, but it is not
clear whether or not they utilize or prefer beaver
ponds (Craig 1978; Paragamian 1981).
In anadromous-fish streams along the west
coast of North America, comparisons of the
growth and survival of juvenile coho salmon and
other salmonids between reaches upstream of
beaver dams and nonimpounded reaches have
yielded similar results. Swales et al. (1988) com-
pared populations of juvenile coho, chinook O.
tshawytscha, and steelhead O. mykiss in side-
channels impounded by beaver with the mainstem
of the Coldwater River in British Columbia and
found that the side channels upstream of beaver
dams were heavily used by overwintering coho,
but less so by chinook and steelhead. Over the
course of 3 years, these researchers captured and
measured coho in both habitats monthly and
found that the coho upstream of the beaver dam
were consistently larger, more abundant, and grew
faster than those downtream. Juvenile coho were
not only using the ponds for overwintering habi-
tat, but as important refuge and rearing areas
throughout the year. Murphy et al. (1989) studied
summer use of mainstem and off-channel habitat
in the valley floor of the Taku River, Alaska and
found the highest densities of juvenile coho in
reaches upstream of beaver dams (0.59 per m2)
and virtually all the larger coho were in beaver
ponds (Figure 4). These reaches accounted for just
0.7% of the total available habitat; yet, 34% of all
the juvenile coho were found there. Consistent
with other studies, the coho in these reaches were
longer than coho found in other habitat types.
Murphy et al. also found that juvenile sockeye
used reaches upstream of beaver dams, averag-
ing 0.48 per m2, and that these fish too were larger
and grew faster than fish using other instream
habitats. Similarly, Leidholt-Bruner et al. (1992)
found that summertime densities of juvenile coho
in “pools” upstream of beaver dams (0.34 per m2)
were higher than in pools formed by other ob-
structions (0.26 per m2). Studying three small
coastal streams in the islands of southeast Alaska,
Bryant (1983) found that populations of coho ju-
veniles in summer were significantly greater in
impounded reaches compared with reaches just
upstream and downstream, but that the densities
in the impounded reach were lower because the
beaver dams had greatly expanded the surface
area of the stream. The highest population of Dolly
Varden Salvelinus malma char was found in one of
the impounded reaches, but no consistent pattern
among all streams was found. In Carnation Creek,
British Columbia, Bustard and Narver (1975)
found that survival rate of overwintering juve-
nile coho in old beaver ponds was about twice as
high as the 35% estimated for the entire stream
Other studies have found that many fish spe-
cies, besides salmonids, are abundant in beaver
ponds. Hanson and Campbell (1963), in a study
of headwater streams in north Missouri, deter-
mined that the presence of beaver dams greatly
increased the productivity of fishes compared to
unimpounded reaches. They used rotenone to
sample three beaver ponds and one “natural”
pool to determine biomass and species composi-
tion for each of the habitats. The three beaver
ponds had an average biomass of 2.8 g/m2, and
the natural pool had a biomass of 1.1 g/m2.
Hanson and Campbell also found 21 species in
the beaver ponds, a 50% increase in number over
the 14 species they found in an unimpounded
reach nearby.
Snodgrass and Meffe (1998), in a study of
fish community use of beaver ponds in the south-
eastern United States, found that beaver dams in
first- and second-order streams had higher fish
diversity (32 and 38 species, respectively), than
in third-order streams (26 species). They con-
cluded that, by creating pool habitat in small head-
water streams where it is generally absent, bea-
ver dams increase fish diversity. They also
concluded that the generally positive relation
observed between species richness and drainage
area is a recent phenomenon resulting from the
extirpation of beavers from their historical range.
Finally, they found that headwater stream
reaches upstream of beaver dams tended to be
dominated by a few predators, suggesting that
the deeper, more open water created more pre-
dation opportunities.
Schlosser (1995) concluded that beaver
ponds in a headwater stream in northern Min-
nesota provided a source for fish populations,
while adjacent stream environments acted as
“sinks.” Keast and Fox (1990) found a high
amount of habitat and dietary specialization
among the common fish species observed in an
Ontario beaver pond. For example, Iowa darters
were found only over bare (unsilted) substrate;
one of the preferred habitats of the blackchin
shiner Notropis heterodon was the open, silted area
near the pond inlet; and fathead minnows
Pimephales promelas were generally found in the
old creek channel. They also determined that fish
species richness and body size were smaller in a
beaver pond relative to more permanent lentic
water bodies in the area.
FIGURE 4. Size-frequency distribution of juvenile coho in main channel and off-channel habitat in the Taku River,
southeast Alaska, showing that larger coho (age-1 light columns, age-0 dark columns) overwhelmingly prefer
beaver ponds over any other habitat. Beaver ponds account for just 0.7% of the total instream habitat area in the
Taku River floodplain (adapted from Murphy et al. 1989).
Juvenile coho, all habitats
except beaver ponds; n = 522
Juvenile coho, beaver
ponds; n = 272
Fork length (mm, interval midpoint)
Frequency (%)
37 47 57 67 77 87 97 107 117
Impact of beaver dams on spawning
A number of observers have speculated that bea-
ver dams can affect fish populations by covering
spawning sites with silt or deep, slow water or by
blocking access to upstream spawning grounds
(Knudsen 1962; Call 1970; Swanston 1991; Cunjak
and Therrien 1998). While beaver dams have led
to the siltation of spawning habitat and probably
restrict access to spawning grounds for some spe-
cies, there is little evidence of negative popula-
tion-level effects. Because beaver ponds trap sedi-
ments and dampen floods, siltation and scouring
of spawning gravels further downstream may be
reduced, making determination of an overall
negative population effect problematic (Scheffer
1938; Beedle 1991; Dunaway et al. 1994; Butler and
Malanson 1995; Hering et al. 2001). Further, some
species that require clean spawning gravels, such
as brook trout, cutthroat trout, and coho salmon,
extensively use beaver ponds as rearing habitat
(Gard 1961b; Call 1970; Murphy et al. 1989). How-
ever, where spawning habitat is limiting, beaver
ponds may affect fish populations. As an example,
Rabe (1970) found that beaver ponds with little
accessible spawning sites nearby contained fewer,
larger brook trout than those ponds where
nearby spawning habitat was abundant. Similarly,
Balon and Chadwick (1979) observed that three
lithophilic fish species, two Semotilus species, and
brook trout, creek chub Semotilus atromaculatus,
and Semotilus margarita, needing gravel or rock
substrate for their early development (e.g., spawn-
ing and egg incubation), did not reproduce in an
Ontario lake after construction of a beaver dam
that raised the lake level. Three other species, yel-
low perch Perca flavescens, fathead minnow, and
golden shiner Notemigonus crysoleucas, were not
lithophilic and were able to successfully repro-
Beaver dams as a barrier to fish
Studies examining the permeability of beaver
dams to fish passage suggested that many spe-
cies are able to gain passage, but they are season-
ally restricted by low flows. Gard (1961b) found
that all three species that they studied (brown,
brook, and rainbow trout) were able to move both
upstream and downstream across a series of 14
dams. Brown trout crossed dams more frequently
than the others; rainbow trout showed greater
ability to cross a series of dams, with some cross-
ing an entire series of 14 dams. Movement up-
stream and downstream was equal for all species.
In a less detailed study, Rupp (1954) monitored
the movement of brook trout in a series of five
beaver dams in the Sunkhaze Stream, Maine. Rupp
set up two monitoring stations, one above all the
dams and one below, and found substantial move-
ment across the entire dam series. In contrast to
Gard, Rupp found more downstream movement
than upstream movement, and he could not cor-
relate movement with streamflow. In Gould
Creek, Minnesota, Schlosser (1995) studied the
movement of a 12-species fish assemblage in bea-
ver ponds and observed that downstream move-
ment of fish was weakly correlated with elevated
stream flows that coincided with the approximate
stage at which water began moving over and
around a dam. Upstream movement, however,
was not correlated with streamflow, and fish
moved up over dams across a broad range of
flow conditions.
On the west coast of North America, the ques-
tion of whether beaver dams are a barrier to
anadromous salmonid movement has long been
debated, and even today, some natural resource
agencies remove beaver dams to “enhance”
salmonid habitat. Personal observations and pub-
lished literature suggest that both adults and ju-
veniles of coho, steelhead, sea-run cutthroat, Dolly
Varden, and sockeye are able to cross beaver dams
(Bryant 1983; Swales et al. 1988; Murphy et al.
1989). In a study of out-migration from a flood-
plain beaver pond on the Coldwater River, Brit-
ish Columbia, Swales et al. (1988) monitored 1,257
coho salmon smolts, 62 steelhead trout (mean
length = 129 mm), and 3 Dolly Varden char mov-
ing downstream over a period of 2.5 months in
the spring. Bryant (1983) found abundant popu-
lations of young-of-the-year and age-one coho
juveniles in the slow water areas behind beaver-
dammed streams in southeast Alaska. He con-
cluded that the abundance of young of the year
indicated that adult coho were able to cross the
dams (including one dam 2.1 m high), on the pre-
sumption that such small fish (<52 mm) do not
move upstream across beaver dams in great num-
bers. Little information has been published about
whether the adults of chum salmon are able to
cross beaver dams, but the general consensus
among salmon fisheries managers is that beaver
dams can be an obstacle to upstream chum salmon
movement. Anecdotal evidence also suggests that
beaver dams can be an obstacle to the upstream
movement of adult Atlantic salmon Salmo salar
(Cunjak and Therrien 1998).
The widespread removal of beaver dams across
small and medium-sized streams throughout most
of the Northern Hemisphere has caused substan-
tial changes to stream hydrology and geomor-
phology, resulting in dramatic shifts in the com-
position of communities. Because beaver build
dams on small streams, their presence creates slow
water pool habitat in a portion of the network
where such habitat is often uncommon or nonex-
istent. This allows fish depending on pool habitat
to move farther up the system, sometimes to
reaches previously inhospitable. The presence of
beaver dams often, though not always, creates
fish habitat with higher productivity or diversity.
Certain species, such as brook trout and juvenile
coho salmon, rely heavily on beaver ponds, and
the loss of such habitat has undoubtedly affected
Some studies suggest that beaver dams can
restore perennial flow to intermittent streams, but
the conditions under which such a transforma-
tion can be expected have not been well described.
Our calculations suggest that the ability of beaver
dams to increase aquifer recharge depends largely
on the hydraulic and geometric characteristics of
the aquifer. Manmade structures similar in size to
beaver dams that have been shown to improve
stream flow lend support to our contention that
beaver dams can also increase stream flows.
Based on our observations of stream systems in
the western United States, it is likely that many
intermittent streams could have perennial flow
restored if beaver colonize them.
Observations of extensive sediment accumu-
lation behind beaver dams lend support to the
hypothesis that beaver, acting over long periods,
can significantly change valley floor morphology.
The potential for sediment accumulation behind
beaver dams over the course of millennia is
tremendous, especially considering that, until re-
cently, few checks existed on their population out-
side of habitat or food limitations. There is some
evidence to suggest that much of the recent
stream incision throughout the western United
States is at least partially a result of the widespread
loss or removal of beaver dams. The historical
records and the banks of incised streams provide
evidence that beaver inhabited many of these
streams at one time. The cause of this recent inci-
sion is still unclear, but the hypothesis that it might
be the cumulative result of removing millions of
beaver dams is at least consistent with available
scientific information about the timing, location,
and extent of incision. Studies of incision and the
possible use of beaver in restoring incised streams
in other arid parts of the beaver’s former range,
such as in southern Europe and parts of Asia,
would be useful.
For all the scientific studies of beaver, very
few have addressed the cumulative effects of the
widespread dam-removal “experiment” conducted
across North America over the past few centuries
and across Eurasia over the past millennium. It is
likely that the hydrologic, geomorphologic, and
biological cumulative effects are, and continue to
be, substantial. Watershed-scale experiments to
assess the effects of restoring beaver dams to move
historical numbers would greatly aid our under-
standing of how stream systems historically func-
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stream-restoration strategies.
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... Beaver damming activities have different impacts on water systems depending on the number of dams, the size of the dams, and the surrounding habitat type (Naiman et al., 1988). In general, beaver dams alter the hydrology of a stream by creating wetlands, raising the water table, storing water, and slowing the flow of water (Collen & Gibson, 2000;Naiman et al., 1988;Pollock et al., 2003). During intense rainfall, beaver dams can attenuate flooding and reduce erosion by allowing excess rainfall to enter groundwater (Collen & Gibson, 2000;Pollock et al., 2003). ...
... In general, beaver dams alter the hydrology of a stream by creating wetlands, raising the water table, storing water, and slowing the flow of water (Collen & Gibson, 2000;Naiman et al., 1988;Pollock et al., 2003). During intense rainfall, beaver dams can attenuate flooding and reduce erosion by allowing excess rainfall to enter groundwater (Collen & Gibson, 2000;Pollock et al., 2003). This capacity to store water may even allow beaver damming to mitigate the impacts of drought events and convert intermittent streams into streams with a perennial flow (Collen & Gibson, 2000;Pollock et al., 2003). ...
... During intense rainfall, beaver dams can attenuate flooding and reduce erosion by allowing excess rainfall to enter groundwater (Collen & Gibson, 2000;Pollock et al., 2003). This capacity to store water may even allow beaver damming to mitigate the impacts of drought events and convert intermittent streams into streams with a perennial flow (Collen & Gibson, 2000;Pollock et al., 2003). Thus, aside from the initial flooding event caused by dam formation and the final flooding event when the dam bursts, beaver ponds have the capacity to regulate water levels in a manner that minimizes the impact of droughts and floods on the areas they occupy (Bylak et al., 2014;Gibson & Olden, 2014;G. ...
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Warming of the Arctic is leading to permafrost degradation, increased vegetation cover, earlier breakup of ice on rivers and lakes, and the arrival and northward expansion of new wildlife species. Beaver expansion into the Arctic has been attributed to shrubification and observed to impact the hydrology, permafrost, wildlife, and people of these regions. The objectives of this research were to 1) characterize changing beaver distribution and associated habitat characteristics in Nunavik, 2) document Inuit knowledge about beaver expansion and the impact on Inuit food security, and 3) identify adaptation strategies to minimize these impacts, while co-producing this knowledge through regional and local research partnerships and collaboration. A mixed methods and knowledge co-production research approach, which included Inuit knowledge interviews, helicopter survey of beaver lodges, dams, and food caches, community questionnaires, and habitat selection analysis, indicated that communities prioritized beaver management because of their concerns regarding the impact of beaver dams on Arctic char and associated impacts on food security. The earliest observations of beavers in the Ungava region of Nunavik occurred near Kangiqsualujjuaq in the late 1950s and near Kuujjuaq in the 1970s, with more recent observations confirming beaver presence much farther north near Aupaluk and Kangirsuk. A habitat selection analysis underlined the importance of dominant water body type as a predictor of beaver presence at both a landscape and local scale of analysis, with beaver sign most often observed along streams, then rivers, then small lakes and less commonly on large lakes. The findings demonstrate how species expansion can be better monitored by integrating western science and Inuit knowledge. Inuit observations can detect beaver impacts on other species, are sensitive to small changes, and can capture transient events, such as sightings of beavers unsuccessfully attempting to colonize a new habitat. Helicopter surveys cover larger areas than Inuit may be able to travel by land and provides systematic information on presence and absence at one point in time. Increased awareness of the distribution of beavers, associated habitat variables, and possible future colonization routes achieved through knowledge co-production can help Inuit policy makers mitigate and adapt to changing wildlife distributions and hydrological regimes.
... There are many similarities between the soil-water-carbon sinks resulting from different types of NIDS. Studies of the impacts of beaver dams, beaver dam analogs (BDAs), and rock detention structures allude to these likenesses Pollock et al., 2003;Silverman et al., 2019;Wheaton et al., 2019). NIDS store water and this attenuates floods, provides soil-moisture reservoirs that can be used by plants, and increases nutrient availability. ...
... Puttock et al. (2017) hypothesized that beaver-constructed features increase water storage within the landscape, with their creation of a stepped profile channel. Dams created by beavers result in ponds along the stream channel that raise the water table in the adjacent riparian zone Macfarlane et al., 2017;Naiman et al., 1988;Pollock et al., 2003Pollock et al., , 2014. Vanderhoof and Burt (2018) quantified increases in reachscale stream surface area upstream of multiple BDAs in the Upper Missouri River Headwaters Basin, as well as decreases in stream surface area for reaches just downstream (through 500 m). ...
... Wildfire NIDS promote fire resilient soil-water-carbon sinks; they create greener/wetter riparian areas with saturated soils that are harder to ignite (firebreaks), provide refugia for wildlife, and their increased biodiversity aids in quicker recovery post-fire. Davee et al., 2019;Geist and Hawkins, 2016;Gibbs, 2000;Gurnell, 1998;Naiman et al., 1988;Norman et al., 2014;Pollock et al., 2003;Sabo et al., 2005 Buckley and Nabhan, 2016;Fish, 1984, 2007;Fish et al., 2013;Gilbert, 2021;Howard and Griffiths, 1966;Leopold, 1937;Wohl et al., 2019. Concurrently, beaver populations have declined drastically in the United States and elsewhere, eliminating their cumulative and substantial hydrologic, geomorphic, and biological wetland development Pollock et al., 2003;Wohl, 2021). ...
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In this article we describe the natural hydrogeomorphological and biogeochemical cycles of dryland fluvial ecosystems that make them unique, yet vulnerable to land use activities and climate change. We introduce Natural Infrastructure in Dryland Streams (NIDS), which are structures naturally or anthropogenically created from earth, wood, debris, or rock that can restore implicit function of these systems. This manuscript further discusses the capability of and functional similarities between beaver dams and anthropogenic NIDS, documented by decades of scientific study. In addition, we present the novel, evidence-based finding that NIDS can create wetlands in water-scarce riparian zones, with soil organic carbon stock as much as 200 to 1400 Mg C/ha in the top meter of soil. We identify the key restorative action of NIDS, which is to slow the drainage of water from the landscape such that more of it can infiltrate and be used to facilitate natural physical, chemical, and biological processes in fluvial environments. Specifically, we assert that the rapid drainage of water from such environments can be reversed through the restoration of natural infrastructure that once existed. We then explore how NIDS can be used to restore the natural biogeochemical feedback loops in these systems. We provide examples of how NIDS have been used to restore such feedback loops, the lessons learned from installation of NIDS in the dryland streams of the southwestern United States, how such efforts might be scaled up, and what the implications are for mitigating climate change effects. Our synthesis portrays how restoration using NIDS can support adaptation to and protection from climate-related disturbances and stressors such as drought, water shortages, flooding, heatwaves, dust storms, wildfire, biodiversity losses, and food insecurity.
... While the number of dams built by humans is impressive, there are actually fewer dams in North America now than prior to European colonization, albeit of a different size and materials. Historic estimates of North American beaver (Castor canadensis) populations range from 60-400 million, suggesting that across their 1.5 x10 7 km 2 range, there was anywhere from 10-60 million beaver dams, mostly made of sticks and mud [5][6][7]. In addition, large wood formed millions of jams, dams and other obstructions that dammed and diverted sediment and water across streams, rivers and even entire valleys [8][9][10]. ...
... mykiss), Atlantic salmon (Salmo salar), cutthroat trout (O. clarkii) and brook trout (Salvelinus fontinalus) [6]. Many fishes use the structurally complex, deep, slow water and emergent wetlands created upstream of beaver dams [26,28]. ...
... different life-history stages and under all flow conditions. Upon review we found that most studies concluded that fishes, and in particular salmonids, benefit from natural obstructions such as beaver dams [6,26,38], while studies arguing that beaver dams are detrimental to fish are uncommon, and typically indicate a temporally intermittent negative impact, with no indication of a population-level effect [28]. For example, over a period of 12 years in Nova Scotia, it was observed that in years with low flow, adult Atlantic salmon were unable to pass over some beaver dams and thus spawned lower in the system, but in most years, beaver dams had no detectable effect on the distribution of spawning redds [57]. ...
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Across Eurasia and North America, beaver ( Castor spp), their dams and their human-built analogues are becoming increasingly common restoration tools to facilitate recovery of streams and wetlands, providing a natural and cost-effective means of restoring dynamic fluvial ecosystems. Although the use of beaver ponds by numerous fish and wildlife species is well documented, debate continues as to the benefits of beaver dams, primarily because dams are perceived as barriers to fish movement, particularly migratory species such as salmonids. In this study, through a series of field experiments, we tested the ability of juvenile salmonids to cross constructed beaver dams (aka beaver dam analogues). Two species, coho salmon ( Oncorhynchus kisutch ) and steelhead trout ( O . mykiss ), were tracked using passive integrated transponder tags (PIT tags) as they crossed constructed beaver dam analogues. We found that when we tagged and moved these fishes from immediately upstream of the dams to immediately downstream of them, most were detected upstream within 36 hours of displacement. By the end of a 21-day field experiment, 91% of the displaced juvenile coho and 54% of the juvenile steelhead trout were detected on antennas upstream of the dams. In contrast, during the final week of the 21-day experiment, just 1 of 158 coho salmon and 6 of 40 (15%) of the steelhead trout were still detected on antennas in the release pool below the dams. A similar but shorter 4-day pilot experiment with only steelhead trout produced similar results. In contrast, in a non-displacement experiment, juveniles of both species that were captured, tagged and released in a pool 50 m below the dams showed little inclination to move upstream. Further, by measuring hydraulic conditions at the major flowpaths over and around the dams, we provide insight into low-flow conditions under which juvenile salmonids are able to cross these constructed beaver dams, and that multiple types of flowpaths may be beneficial towards assisting fish movement past instream restoration structures. Finally, we compared estimates of the number of juvenile salmonids using the pond habitat upstream of the dam relative to the number that the dam may have prevented from moving upstream. Upstream of the dams we found an abundance of juvenile salmonids and a several orders of magnitude difference in favor of the number of juveniles using the pond habitat upstream of the dam. In sum, our study suggests beaver dams, BDAs, and other channel spanning habitat features should be preserved and restored rather than removed as perceived obstructions to fish passage.
... Recently, linkages between physical degradation, such as channel incision, and loss of ecosystem engineers have become a focal topic in river restoration ecology (Law et al., 2016). While channel incision, or lowered bed elevation, can occur naturally as climate changes (Cluer & Thorne, 2013), the accelerated rate and ubiquity of channel incision in the western United States mostly stems from land-use practices and the loss of plant and animal ecosystem engineers (Jones et al., 1994;Pollock et al., 2003). Perhaps the most recognized ecosystem engineer in streams is the North American beaver (Castor canadensis), which is widely distributed throughout the continent (Naiman et al., 1988). ...
... Dam building by beavers impounds water, aggrades stream beds, buffers stream temperatures, and creates diverse aquatic and terrestrial habitats; these changes can alter resource availability for macroinvertebrates that then extends to higher trophic levels and across ecosystem boundaries (Burchsted et al., 2010;McCaffery & Eby, 2016). Over the last two centuries, extensive trapping and removal of beaver have occurred, decreasing their populations by an order of magnitude and eliminating their important effect on the landscape (Naiman et al., 1988;Pollock et al., 2003). As a result, stream incision is much more prevalent when these important biotic components are absent. ...
... Because of declines in beaver population size and range, beaver mimicry structures (BMSs) have gained popularity in recent years as a tool to address channel incision and stream degradation (Pollock et al., 2003). BMSs are in situ structures designed to mimic the hydrologic and geomorphic effects of beavers on rivers and riparian corridors by raising water levels, modifying stream discharge, and increasing sediment standing stock (Castro et al., 2015;Pollock et al., 2014). ...
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Abstract Rising levels of stream degradation have motivated a boom in restoration projects across the globe. However, postrestoration monitoring is still frequently lacking and does not always incorporate biotic responses to changes in the physical template. Beaver mimicry structures (BMSs) are becoming a popular tool to restore degraded streams throughout the American West, but relatively little is known about how these installations influence both biotic and abiotic factors, with consequences for ecosystem functioning. We monitored basal resources, organic and inorganic material standing stocks, and macroinvertebrate density, biomass, and production to quantify functional responses to BMS installation. We compared conditions at BMS sites to naturally occurring beaver dam and reference riffle sites in a low‐gradient stream in southwest Montana. Thermal ranges were contracted, and daily maximum temperatures were higher, in the BMS treatment compared to the reference riffle treatment. Fine sediment standing stock and basal resources were similar in Beaver and BMS treatments, and both treatments were higher than reference riffles. All treatments differed in macroinvertebrate density, which was highest in the Beaver treatment, followed by Mimic and then Reference treatment. Biomass and secondary production were higher in Beaver and BMS treatments compared to the Reference treatment, but only Beaver and Reference treatments differed significantly, likely due to differences in physical habitat and basal resource availability. Consequently, production of collector–gatherers in the BMS treatment and shredders in the Beaver treatment was higher than in reference riffles. Changes to local hydrology and sediment dynamics resulting from BMS influence biotic functional responses like organic material standing stock and secondary production, creating habitat and ecosystem function distinct from riffles and similar to target conditions of natural beaver dams. To continue to improve BMS as a standard restoration practice, future research could consider the extent of degradation, increasing temporal scale of monitoring. Alterations to aquatic–terrestrial subsidies and impacts to fishes.
... A single dam and pond may create limited attenuation during peak flow (Burns & McDonnell, 1998), but numerous dams in a beaver wetland can effectively attenuate even the largest peak flows and serve as a sink for nitrates and organic carbon (Wegener et al., 2017). Beaver wetlands also provide diverse habitat for vegetation (Westbrook et al., 2011), fish (Pollock et al., 2003), aquatic insects and their riparian predators (Fuller & Peckarsky, 2011;McCaffery & Eby, 2016;McDowell & Naiman, 1986), frogs and other amphibians (Anderson et al., 2015;Arkle & Pilliod, 2015), and other semiaquatic mammals such as mink and otter (Rosell et al., 2005). Ponded water and high riparian water tables associated with beaver dams can also reduce the effects of climatic extremes such as drought (Hood & Bayley, 2008) and make the river corridor more resistant to wildfire (Fairfax & Whittle, 2020). ...
... Sediment storage volumes are more difficult to predict for beaver dams, as they can be dependent on variables that are difficult to measure such as suspended sediment concentration and time since dam establishment. Technically, the maximum sediment storage for each beaver pond would be equal to the total volume of the pond, assuming complete filling (Pollock et al., 2003). However, complete filling of all dams across the region is unlikely. ...
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The loss of beaver populations has commonly been accompanied by the failure of beaver dams, leading to stream incision, water table lowering, and the eventual transition from a beaver meadow to a drier riparian corridor. Widespread decline in North American beaver populations (Castor canadensis) has been documented from pre‐European settlement to the current day, representing an estimated 80% to 98% loss of historical populations. While individual case studies have investigated the ecosystem impacts of local beaver population loss, few studies have quantified large‐scale changes associated with widespread population decline. Here, we use the Beaver Restoration Assessment Tool to model landscape‐scale habitat suitability and beaver dam capacity in Colorado, USA, in order to determine whether a widespread loss in beaver population corresponds to a similar scale decline in the capacity to sustain beaver on the landscape and declines in physical benefits associated with beaver, such as surface water and sediment storage. Currently, the statewide stream network (298,119 stream kilometers) can support approximately 1.36 million beaver dams, compared with 2.39 million dams historically. All regions of Colorado have seen a decline in beaver dam capacity from historical conditions, likely due to agriculture, urbanization, and loss of vegetation necessary to beaver. Beaver dam capacity loss is accompanied by an approximate 40% decline in beaver‐mediated surface water and sediment storage potential across the state. Regions with high percent loss in storage potentials also had a high percentage of drainage network that had experienced beaver dam capacity losses of 15 or more dams per kilometer, which highlights the disproportionate impacts of losing high dam density reaches (i.e., beaver meadows). Extreme dam density declines were rare, and instead, most reaches have undergone a shift from high to moderate capacity. Statewide shifts in beaver dam capacity highlight the opportunity for using beaver‐related restoration in Colorado and across the American West.
... Mit dem Eintrag von Totholz, der Entstehung von Umgehungsgerinnen und der Anlage von langsam fliessenden Staubereichen bieten sich in Dammrevieren vielfältige dynamische Fischlebensräume (Fig. 3a und b). [7,10,21,[24][25][26][27][28]. Zudem helfen Biberdammreviere nachweislich, Hochwasserspitzen zu dämpfen [29], sogar in Bergbächen [30]. ...
Conference Paper
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Addendum zur Präsentation "Beaver Dam Analogs" am 3. Forum Gewässerrevitalisierung vom 17.11.2022 in Luzern -Literaturquellen -Normalien -Schemata
This case study synthesizes strategies that electric power utilities can implement to reduce surface water risks to infrastructure, operations, and regulatory compliance as climate change impacts hydrologic regimes over the next century. The strategies range from the reach scale to watershed scale. A reach-scale example would be evaluating relocation alternatives for a transmission tower along an eroding streambank versus a streambank stabilization strategy. A watershed-scale strategy would involve the value engineering of stormwater management strategies that could be implemented across a catchment that is restorative of a more natural flow regime such as prolonged baseflows and reduced flooding and erosion. The cost-effective watershed-scale strategies highlighted herein include retrofits of existing detention ponds, beaver reintroductions (or discontinued extirpation), riparian reforestation, adding wood to headwater streams, and the removal of postsettlement alluvium from floodplains coinciding with restoration of floodplain wetlands. Many of these strategies are management approaches that could be implemented on utilities' own property for relatively little cost while appealing to broader societal goals such as environmental restoration. Although costs will vary by setting and program goals, we hope that this article is a launching point for infrastructure managers to consider holistic, watershed-scale approaches to provide durable infrastructure resilience in the face of increased extreme events while contributing to long-term economic, social, and environmental sustainability. © 2023 This work is made available under the terms of the Creative Commons Attribution 4.0 International license,.
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Wildlife translocation facilitates conservation efforts, including recovering imperiled species, reducing human–wildlife conflict, and restoring degraded ecosystems. Beaver (American, Castor canadensis; Eurasian, C. fiber) translocation may mitigate human–wildlife conflict and facilitate ecosystem restoration. However, few projects measure outcomes of translocations by monitoring beaver postrelease, and translocation to desert streams is relatively rare. We captured, tagged, and monitored 47 American beavers (hereafter, beavers) which we then translocated to two desert rivers in Utah, USA, to assist in passive river restoration. We compared translocated beaver site fidelity, survival, and dam‐building behavior to 24 resident beavers. We observed high apparent survival (i.e., survived and stayed in the study site) for eight weeks postrelease of resident adult beavers (0.88 ± 0.08; standard error) and lower but similar apparent survival rates between resident subadult (0.15 ± 0.15), translocated adult (0.26 ± 0.12), and translocated subadult beavers (0.09 ± 0.08). Neither the pre‐ nor the post‐translocation count of river reaches with beaver dams were predicted well by the Beaver Restoration Assessment Tool, which estimates maximum beaver dam capacity by river reach, suggesting beaver‐related restoration is not maximized in these rivers. Translocated beavers exhibited similar characteristics as resident subadult beavers during dispersal; they were more vulnerable to predation and many emigrated from the study sites. High mortality and low site fidelity should be anticipated when translocating beavers, but even so, translocation may have contributed to additional beaver dams in the restoration sites, which is the common goal of beaver‐assisted river restoration. Multiple releases at targeted restoration sites may eventually result in establishment and meet conservation objectives for desert rivers. A translocated American beaver was released into the Price River, Utah, after it was fitted with a VHF tag on its tail.
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The unique biogeography of the Madrean Archipelago facilitates the cohabitation of species that otherwise rarely overlap in their spatial distributions. As part of a long-term study at Cuenca Los Ojos, Sonora, we deployed 25 cameras along washes within the property from October 2018 to April 2019, adding to the camera-trap information collected at the site since 2016. Ocelot (Leopardus pardalis) was recorded once in 2018, twice in 2016 and 2020, but not during the 2018–2019 study period. Black bear (Ursus americanus) was recorded regularly throughout the season with several images including cubs. One picture and several signs of beaver (Castor canadensis) were discovered in 2018 and 2019. Lastly, the first record of jaguar (Panthera onca) in Cuenca Los Ojos was recorded in February and March 2019 at four different sites. All species are considered endangered in Mexico and species of conservation concern in the United States and were recorded within 5 km south of the USA–Mexico border wall. Our addition of the new ocelot sighting in this region marks the only known location with records of all four species overlapping in space and time despite historical distribution ranges of black bears, jaguars, and beavers overlapping. Given the current border wall construction and highway development, which both affect the natural connectivity of the region, it will be necessary to incorporate the presence of the four species in all future mitigation efforts.
Variation in sediment yield may reflect a signal of disturbances in the upstream landscape, modified by sediment routing. This study, conducted in a forested drainage basin in the inland Pacific Northwest, USA, sought to generate a better insight into the interdecadal variability of sediment yield in mountain landscapes in response to environmental change during the last century. To this end, we examined: (1) sediment yield fluctuations; and (2) their association with streamflow and land use changes; as well as (3) streamflow links to climate variability modes; and (4) the influence of sediment delivery from hillslope sources to streams (lateral connectivity) and its downstream routing through the stream network (longitudinal connectivity) on land use signal at the basin's outlet. Sediment yield between 1910 and 2017, estimated based on reconstructed fluvial delta growth, displayed an order of magnitude variability, which indicates a substantial geomorphic sensitivity. The interpretation of temporal patterns and an exploratory statistical analysis pointed to land use-related sediment supply changes as the primary driver of these fluctuations, dominating system behavior before changes in environmental regulations and practices in the mid-1970s. Hydroclimatically controlled streamflow variability appeared to be more prominent in the subsequent period. Our connectivity analysis suggested that a considerable portion of coarse sediment mobilized by harvest and road construction may still reside within the channel network. In light of previous research in this landscape system, we speculate that, despite limited anthropogenic pressures in the recent decades, its characteristics and behavior continue to be conditioned by land use legacies. Overall, this study contributes to the growing understanding of profound anthropogenic transformation of the earth surface. Specifically, it demonstrates that historical resource extraction may have left a lasting imprint even in relatively remote mountain landscapes. Given the ongoing rapid environmental change, such understanding is crucial for watershed management, conservation, and restoration.
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A biological model was developed to calculate annual survival between life stages of juvenile Atlantic salmon, Salmo salar L., in Catamaran Brook, a small stream basin (52 km2) in the Miramichi River catchment in New Brunswick, Canada. Seven years' (1990-1996) were used in the model. Input variables included: daily fish counts and measurements of parr (3-4 age classes), smolts, and adult salmon at a fish-counting fence near the stream mouth; biennial quantification of all habitat types along the watercourse; fish density estimated by electric fishing at 30 sites; and estimates of young-of-the-year emigration via s tream drift. Continuous recording of stream discharge provided data to assist in interpretation of survival estimates. Annual survival of juvenile salmon in their first 3 years of life in the stream average between 31% and 34%. The greatest annual variation (CV = 0.699) occurred at the egg to 0+ (summer) stage with a low of 9.2% survival recorded for a winter with an atypical midwinter flood event; parr and pre-smolt survival were similarly affected. Survival from egg deposition (after correction for losses caused by predation and retention/non-fertilization) to smolt emigration was between 0.16% and 0.52%, which is low relative to estimates from many other studies. Survival of smolts to return 1-sea-winter adults (grilse) averaged 8.5%. Potential errors in the computation of the model are discussed, e.g. inaccurate counts of spawning adults during high autumn stream flow. A possible explanation for the low egg to smolt survival was the environmental conditions experienced during various winters. Mean egg survival was 1.3 times higher (39.3%) and egg to smolt survival increased to 1.03% when the two winters characterized by extremely low discharge or midwinter freshets were excluded from the calculation. Density-dependent factors related to a beaver dam, which limited spawning distribution, may also have contributed to poor survival and increased fry emigration in one year. Environmental factors, particularly winter conditions, in streams such as Catamaran Brook may act as bottlenecks to natural production of Atlantic salmon.
Between 1952 and 1961 standing crops of brook (Salvelinus fontinalis), rainbow (Salmo gairdneri), and brown (Salmo trutta) trout were determined in August for a 5.7 mile section of Sagehen Creek in east-central California. In 1953 a creel census was initiated which continued through 1961. The purpose of the study was to determine if a moderately productive wild trout stream could support a substantial angler harvest over an extended period without augmentation with hatchery-reared trout. The 10-year average standing crop of all trout was 1, 578 (37 pounds) per acre. Standing crops declined somewhat during the study period, largely due to habitat deterioration rather than to fishing. Floods and abandonment of beaver impoundments were the primary adverse influences on trout habitat. Catch did not decline significantly over the period. Fishermen annually removed 23-47% (average, 33%) of all trout over 99 mm in length, but recruitment replaced the loss. Although the natural fishery has proven to be viable, the trout population is not characterized by many large fish. We recommend an experimental management program aimed at (1) restoring some of the brown trout habitat by construction of low dams similar to the beaver dams that formerly were so productive, and (2) increasing the number of larger trout in the stream by regulation of the take. If each iarge trout is subject to multiple capture and release, additional high-quality sport would be provided. Future studies could evaluate the effect on the fishery of these changes in management.
In the NW USSR reintroduction of beavers began in 1934. In the early 1950s Canadian beavers penetrated the territory of Karelia and the Karelian Isthmus from Finland. Moving swiftly S and E they occupied a considerable part of the territory. At present there are >300 settlements of Canadian beavers totalling 1700-2000 animals. European and Canadian beavers have similar feeding habits: aspen, willow and birch are the main diet. The Canadian beaver is highly active in building shelters and dams. Lodges are found in 104 (74.8%), settlements and dams in 93: 160 (32.8%) and 261, respectively for the European beaver. The most substantial differences are traced in comparing reproductive characteristics. In the European beaver only 7-8% of females reproduce at the age of 1.5-2yr, in the Canadian up to 20%; and 50-60% of adult European beaver females are involved in reproduction against 70-80% of the Canadian beaver. Average fertility (number of kits in a litter) is 1.9 for the European beaver and 3.2 for the Canadian beaver. - Authors