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

January 2003

Authors:

National Oceanic and Atmospheric Administration

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.

. Beaver dam density reported in the literature for pristine, remote, or protected areas.

…

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

…

The relation between sediment stored and vertical dam face area is weak but significant (r 2 = 0.30, p < 0.01, n = 22). With three statistical outliers removed (large square symbols), the relation becomes much more apparent (r 2 = 0.83, p < 0.01, n = 19) (adapted from Scheffer 1938).

…

. Common and scientific names of fish species known to use beaver ponds. The relevant scientific papers are also noted.

…

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).

…

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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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MICHAEL M. POLLOCK, MORGAN HEIM, AND DANIELLE W ERNER

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.

IntroductionIntroduction

Introduction

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

1979).

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-

2POLLOCK ET AL.

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 ).

HYDROLOGIC AND GEOMORPHIC EFFECTS OF BEAVER DAMS AND THEIR INFLUENCE ON FISHES 3

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

BeaBea

Beavv

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).

4POLLOCK ET AL.

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

population

(Scheffer 1938) 35.4 n/a 0.6 Washington low Transplanted

population

(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

population

(Hering et al. 2001) 27.8 n/a 0.9 W. Germany n/a Recovering

population

(Hering et al. 2001) 12.3 n/a 1.3 S. Germany n/a Recovering

population

(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

HYDROLOGIC AND GEOMORPHIC EFFECTS OF BEAVER DAMS AND THEIR INFLUENCE ON FISHES 5

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 in

Watershedsatersheds

atershedsatersheds

atersheds

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

90%

80%

70%

60%

50%

40%

30%

20%

10%

Amount of surveyed habitat currently or formerly

used (%)

6POLLOCK ET AL.

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 Beavv

ver Dams oner Dams on

er Dams oner Dams on

er Dams on

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 Beavv

ver Dams oner Dams on

er Dams oner Dams on

er Dams on

Aquifer Recharge and LowAquifer Recharge and Low

Aquifer Recharge and Low

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,

HYDROLOGIC AND GEOMORPHIC EFFECTS OF BEAVER DAMS AND THEIR INFLUENCE ON FISHES 7

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

8POLLOCK ET AL.

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 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.

HYDROLOGIC AND GEOMORPHIC EFFECTS OF BEAVER DAMS AND THEIR INFLUENCE ON FISHES 9

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 /

VHWS ,

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 )

10 POLLOCK ET AL.

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

floor.

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

HYDROLOGIC AND GEOMORPHIC EFFECTS OF BEAVER DAMS AND THEIR INFLUENCE ON FISHES 11

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.

BeaBea

Beavv

ver Dams and Fer Dams and F

er Dams and Fer Dams and F

er Dams and Fishish

ishish

ish

Effect of dams on communities and

populations

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

levels.

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.

12 POLLOCK ET AL.

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

HYDROLOGIC AND GEOMORPHIC EFFECTS OF BEAVER DAMS AND THEIR INFLUENCE ON FISHES 13

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 =

occasional.

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

text.

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

14 POLLOCK ET AL.

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

system.

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

HYDROLOGIC AND GEOMORPHIC EFFECTS OF BEAVER DAMS AND THEIR INFLUENCE ON FISHES 15

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

16 POLLOCK ET AL.

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-

duce.

Beaver dams as a barrier to fish

movement

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

HYDROLOGIC AND GEOMORPHIC EFFECTS OF BEAVER DAMS AND THEIR INFLUENCE ON FISHES 17

movement of adult Atlantic salmon Salmo salar

(Cunjak and Therrien 1998).

ConclusionsConclusions

Conclusions

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

populations.

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-

tioned and perhaps lead to innovative large-scale

stream-restoration strategies.

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Using Earth observation to support the predictive accuracy of species distribution models in ecological restoration: a case study of Poland beavers (Castor fiber)

Article

Full-text available

Nov 2024

The ability of Earth observation and remote sensing to identify the potential of species to restore ecosystems is a critical element in combating habitat degradation and biodiversity loss. Historical climate data and current species occurrence, coupled with geospatial modelling methods, can significantly aid in decision-making to support ecosystem restoration. We chose Eurasian beavers (spp. Castor fiber) as our case species, which are known ecosystem engineers that create wetland habitats and significantly alter/reinforce the morphology, dynamics, and hydraulics of riverine landscapes. To investigate their ecosystem restoration potential, we applied species distribution modelling (SDM) to determine their current and future distribution scenarios. Our SDMs used current and simulated future environmental data along with different classes of rasters such as elevation, inland swamps, water bodies, natural seasonally or permanently wet grasslands, open mires, riparian mixed forests, riparian swamp broadleaved forests, riparian swamp coniferous forests, watercourses, protected areas, and urban and built-up areas derived from Google Earth Engine to predict areas suitable for beaver habitats. Further, by using the zonation method, we were able to develop a stepping stone model to prioritize habitat conservation efforts. We provide habitat projections according to time periods of "2041–2060", "2061–2080" and "2081–2100", and to different climate change scenarios (SSP126, SSP245, SSP370, and SSP585). Thus, using a potential nature-based solution, we have employed remote sensing data and tools to develop a holistic framework for not only quantifying the ecosystem restoration potential of beavers but will also provide a new direction for understanding species-specific importance in climate change mitigation.

Beavers (Castor canadensis) in Nunavik: integrating multiple ways of knowing to address climate change concerns related to the expansion of boreal species into tundra regions

Thesis

Full-text available

Dec 2021

Mikhaela Neelin

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.

Natural infrastructure in dryland streams (NIDS) can establish regenerative wetland sinks that reverse desertification and strengthen climate resilience

Article

Full-text available

Aug 2022
SCI TOTAL ENVIRON

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.

Time for dam rebuilding by the Eurasian beaver

Article

Full-text available

Aug 2024
AQUAT ECOL

The European beaver, the largest rodent in Europe, has enormous skills in transforming and adapting its habitat. It chooses a place for its habitat that provides it with food and a high degree of security. He builds dams to regulate water levels. It is assumed that beaver dams can survive for several dozen years, depending on the continuity of use. When a damaged dam is reused, beavers are able to quickly restore the structure to a suitable condition. By monitoring one of the dams for several years, we managed to record this interesting process. In this case, it was determined that the time needed to rebuild the dam and restore the water level was approximately 8 h. This, of course, depends on local conditions, but the data obtained allows for a better understanding of this process.

Beaver Dam Analogs - Klimaresilienz und Biodiversität für unsere Bäche - facts & figures

Conference Paper

Full-text available

Nov 2022

Addendum zur Präsentation "Beaver Dam Analogs" am 3. Forum Gewässerrevitalisierung vom 17.11.2022 in Luzern -Literaturquellen -Normalien -Schemata

Human disturbance and habitat structure drive eurasian otter habitat selection in heavily anthropized river basins

Article

Full-text available

Mar 2024
BIODIVERS CONSERV

Assessing habitat selection is essential to protecting threatened species but also to understand what factors influence species that, although globally not currently in decline, act as flagships of their ecosystems and remain highly vulnerable to human impacts, such as the Eurasian otter. This paper examines otter habitat selection at the river reach scale in two heavily anthropized river basins. Both river basins encompass a wide spectrum of human pressures and biogeographic units, which offers an excellent opportunity to assess otter responses to anthropogenic activities in different scenarios. Through two modelling approaches (structure-agnostic way and a priori hypothesized habitat factors) we demonstrate that otters currently inhabiting these human-dominated landscapes show a trade-off between a preference for highly productive areas and for well-structured and safe areas. We suggest that habitat simplification and human disturbance, which were of minor relevance to the dramatic decline of otter populations in the 20th century, are emerging as potential threats in the context of worldwide increasing land use intensification. Furthermore, we found that otter habitat requirements were remarkably more stringent for breeding site selection than for occurrence, particularly concerning variables related to human disturbance. The results of this work provide tools for integrating ecological criteria oriented to effective otter conservation into river management in human-dominated landscapes, as well as serving as methodological support for lowland river restorations. Our results suggest that long-term otter conservation in anthropized rivers will depend on ensuring the availability of habitat patches that maintain sufficient structural complexity away from intensely outdoor recreational activities.

Water Is Life: Importance and Management of Riparian Areas for Rangeland Wildlife

Chapter

Full-text available

Sep 2023

Water scarcity and climatic variability shape human settlement patterns and wildlife distribution and abundance on arid and semi-arid rangelands. Riparian areas-the transition between water and land-are rare but disproportionately important habitats covering just a fraction of the land surface (commonly < 2% in the western U.S.). Riparian areas provide critical habitat for fish and other aquatic species, while also supporting the vast majority (70-80%) of terrestrial wildlife during some portion of their life cycle. Diverse riparian types serve as vital sources of water and late summer productivity as surrounding uplands dry during seasonal drought. The health and function of rangeland riparian systems are closely tied to hydrology, geomorphology, and ecology. Riparian areas have attracted intense human use resulting in their widespread degradation. Conservation actions, including improved livestock grazing management and restoration, can help maintain and enhance riparian resilience to drought, wildfire, and flooding. This chapter provides readers with an introduction to the importance of riparian areas in rangelands, their nature and ecology, functions for wildlife, and prevailing management and restoration approaches.

Resetting the baseline: using machine learning to find lost meadows

Article

Full-text available

Aug 2023
LANDSCAPE ECOL

Context Mountain meadows occur in specific geomorphological conditions where low-gradient topography promotes fine sediment accumulation and high groundwater tables. Over 150 years of human-caused hydrological degradation of meadows along with fire suppression has resulted in decreased groundwater elevations and encroachment of upland vegetation, greatly diminishing the ecological value of meadows for water storage, baseflow, sediment capture, wildfire resistance, wildlife habitat, and carbon storage. Objectives We aimed to understand where and how frequently meadows historically occurred to reset the baseline condition and provide insight into their restoration potential. We trained machine learning algorithms to identify potential meadow areas with similar hydrogeomorphic conditions to extant meadows while ignoring their unique vegetative characteristics because we hypothesized that vegetation would change but geomorphology would remain. Methods We used a publicly available dataset of over 11,000 hand-digitized meadow polygons occurring within a 25,300 km², 60-watershed region in the Sierra Nevada, California USA to train random forest models to detect meadow-like hydrogeomorphic conditions. Predictor variables represented topographical position, flow accumulation, snowpack, and topographical relief at differing spatial scales. We assessed model performance and produced maps delineating high probability meadow polygons. Results Our findings showed that there is nearly three times more potential meadow habitat than currently documented. The predicted area includes a mixture of existing but undocumented meadows, non-meadowlands that may have converted from meadows due to lost function and forest encroachment, and areas with meadow-like geomorphology that may never have been meadow. The polygons encompassing predicted meadows often expanded existing meadows habitats into adjacent areas with continuous topography, but with upland vegetation and incised channels. Conclusions Using readily available data and accessible statistical techniques, we demonstrated the accuracy of a tool to detect about three times more historical meadows than currently recognized within a complex, mountainous landscape. This “found” area greatly increased the potential area that could be subject to meadow restoration with benefits for biodiversity, wildfire management, carbon sequestration, and water storage.

Watershed-Scale Strategies to Increase Resilience to Climate-Driven Changes to Surface Waters: North American Electric Power Sector Case Study

Article

May 2023

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,.

Comparing translocated beavers used as passive restoration tools to resident beavers in degraded desert rivers

Article

Full-text available

Dec 2022
ANIM CONSERV

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.

Inter-stage survival of wild juvenile Atlantic salmon, Salmo salar L.

Article

Full-text available

Jan 1998

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.

Once a River: Bird Life and Habitat Changes on the Middle Gila

Book

Nov 2021

AMADEO M. REA

Flat‐bottomed glacial valleys, braided rivers and the beaver

Article

Jan 1967

M.G. Rutten

Riparian habitat restoration and beavers

Article

Jan 1985

L.L. Apple

Abundance and Harvest of Trout in Sagehen Creek, California

Article

Jul 1972

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.

The state of populations and ecological characteristics of European ( Castor fiber L.) and Canadian ( Castor canadensis Kuhl.) beavers in the northwestern USSR.

Article

Jan 1983

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

Markovian analysis of the spatially dependent dynamics of beaver ponds

Article