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Restoring Floods to Floodplains: Riparian and Floodplain Restoration at the Cosumnes River Preserve

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Riparian and floodplain ecosystems are shaped and sustained by the river's hydrologic processes, such as flooding. The Cosumnes River Preserve is a multi-partner effort to protect and restore natural habitats within the floodplain of the Cosumnes River. This landscape-scale project protects over 40,000 acres of riparian forest, wetlands, and vernal pool grasslands. Early restoration efforts relied on active measures, such as hand planting of oaks and willows. This method, however, was expensive and labor intensive, and sometimes plantings failed. In the last several years we have focused on restoring natural processes that sustain and create habitat, such as flooding. Natural process restoration is now an integral part of the restoration program and central to our planning for property acquisition. Intentional levee breaches have restored the hydrologic connection between the lower Cosumnes River and its floodplain. Floods passing through levee breaches created in 1995 and 1997 have deposited sediment, seeds and plant cuttings on former farm fields, and stimulated natural recruitment of cottonwood and willow riparian forests. This method also provided valuable habitat for chinook salmon and Sacramento splittail. Creating seasonally flooded habitat rather than permanent ponds benefits native fishes more than non-native fishes. Monitoring by the Cosumnes Research Group and others is providing feedback for adaptive management.
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Restoring Floods to Floodplains: Riparian and Floodplain Restoration
at the Cosumnes River Preserve
R
AMONA
O. S
WENSON
, K
EITH
W
HITENER
,
AND
M
IKE
E
ATON
The Nature Conservancy, Cosumnes River Preserve, Galt, CA 95616
A
BSTRACT
. Riparian and floodplain ecosystems are shaped and sustained by the river’s hydrologic
processes, such as flooding. The Cosumnes River Preserve is a multi-partner effort to protect and
restore natural habitats within the floodplain of the Cosumnes River. This landscape-scale project
protects over 40,000 acres of riparian forest, wetlands, and vernal pool grasslands. Early
restoration efforts relied on active measures, such as hand planting of oaks and willows. This
method, however, was expensive and labor intensive, and sometimes plantings failed. In the last
several years we have focused on restoring natural processes that sustain and create habitat, such as
flooding. Natural process restoration is now an integral part of the restoration program and central
to our planning for property acquisition. Intentional levee breaches have restored the hydrologic
connection between the lower Cosumnes River and its floodplain. Floods passing through levee
breaches created in 1995 and 1997 have deposited sediment, seeds and plant cuttings on former
farm fields, and stimulated natural recruitment of cottonwood and willow riparian forests. This
method also provided valuable habitat for chinook salmon and Sacramento splittail. Creating
seasonally flooded habitat rather than permanent ponds benefits native fishes more than non-native
fishes. Monitoring by the Cosumnes Research Group and others is providing feedback for adaptive
management.
I
NTRODUCTION
California’s riparian and floodplain ecosystems
have been greatly modified and impaired.
Historically, rivers would overtop their banks in
the winter and spring to create extensive flooded
plains. Many of the fishes native to the Central
Valley, including chinook salmon and
Sacramento splittail, evolved to take advantage
of these productive seasonally-flooded shallow
water habitats. Riparian forests were once the
predominant floodplain vegetation in the
Sacramento Valley prior to extensive settlement
(Hunter et al., 1999). Currently on most rivers,
however, the natural hydrologic regime has been
drastically altered by dams and levees that
impound runoff, straitjacket stream channels, and
alter the timing and magnitude of flows (Mount
1995). Floodwaters cannot spread across the
floodplain to deposit sediment, distribute plant
seeds and cuttings, or seep into the ground to
recharge groundwater aquifers. Valley oaks grew
best in the deep alluvial soils preferred by early
farmers, who cleared the forests for agriculture
(Reiner 1986). By the late 1980s less than four
percent of the floodplain remained covered by
forest (Hunter et al., 1999).
Floodplain restoration will require restoring the
flooding process. The Cosumnes River is the
only unregulated Sierran river in the Central
Valley and, as such, it retains much of its natural
hydrology. This represents a major opportunity
to restore a functional ecosystem by providing
the river access to its floodplain. Successful
experiments with breaching and removing levees
on the preserve have made the Cosumnes a
natural laboratory for restoring riparian forests
and native fishes, as well as developing non-
structural flood management. In this paper we
review the planning, implementation and
outcomes of levee breaches at the Preserve, and
discuss future directions for natural process
restoration. An overview of the extensive
research being carried out here by the Cosumnes
Research Group (U.C. Davis) is provided in
Mount et al. (this volume).
Cosumnes River Watershed. The Cosumnes
River is the last free-flowing river in the Central
Valley, draining the western slope of the Sierra
Nevada (ESA, 1991). No large dams have been
constructed on the river. A small reservoir, Sly
Park Reservoir, is located on a tributary with a
capacity of 40,000 acre-ft, or 10% of the average
annual yield. The watershed (1,265 square miles)
reaches over 7,600 feet elevation in the Sierra
Nevada. Little snow accumulates because of the
watershed’s low elevation. Thus, the river’s
flow is derived principally from winter rain
instead of spring snowmelt.
The Cosumnes River historically wandered back
and forth across the valley floor and occupied
several channels. Tidal fluctuation is observed
in the lower river up to Twin Cities Road (river
mile 5). During the summer, streamflow is
intermittent upstream of Twin Cities Road. The
lower Cosumnes River is extensively leveed
(Philip Williams and Associates, 1997). This
has contributed to incision of the channel
upstream of Highway 99. Surface water
diversion rights exceed the river’s average
monthly flow in some months. Groundwater
pumping for agriculture and urban development
has created a dramatic decline of ground water
levels, up to 60 feet near Highway 99
(Montgomery Watson 1995). Increased water
use has delayed streamflow in the fall, which
adversely affects chinook salmon attempting to
migrate upstream to spawn.
The Cosumnes River supports a variety of
natural communities (ESA, 1991): coniferous
forests in the upper watershed, oak woodlands in
the foothills, and vernal pool grasslands and
riparian forests on the valley floor. Bottomland
riparian forests consist of cottonwood, willow,
valley oak and Oregon ash, which support many
species of migratory birds (PRBO, 2000; Estep,
1989). In the lower watershed and east Delta,
seasonal marshes and agricultural fields provide
habitat for waterfowl and sandhill cranes
(Littlefield and Ivey, 2000). Badger Creek
provides wetland habitat for giant garter snakes.
The upper Cosumnes River hosts native rainbow
trout, as well as non-native brown and brook
trout. The middle reaches of the river were
historically dominated by native minnows such
as hardhead and roach, but are now invaded by
redeye bass and smallmouth bass (Moyle et al.
manuscript). The lower reaches contain native
fishes, such as chinook salmon, Sacramento
splittail, and Sacramento suckers, as well as
many non-native species, such as largemouth
bass, carp, and silversides (Harris, 1986; Moyle,
Crain and Whitener, unpublished data). Chinook
salmon migrate through the lower reaches to
spawn along the middle reaches of the river
below a natural passage barrier at Latrobe Falls,
41.5 miles upstream from the confluence with
the Mokelumne River.
Cosumnes River Preserve. In 1984, the Nature
Conservancy acquired a large valley oak grove
on the Cosumnes River. The Cosumnes River
Preserve was established in 1987 and currently
protects over 40,000 acres of riparian forest,
wetlands, vernal pool grasslands and agriculture.
The Preserve is a showcase for the
Conservancy’s strategies of public and private
conservation partnerships, habitat restoration,
development and demonstration of wildlife-
friendly land management methods, community
involvement, and compatible economic
activities. The Cosumnes River is also a flagship
project of the California Riparian Habitat Joint
Venture.
The Preserve is managed under an cooperative
agreement among The Nature Conservancy, U.S.
Bureau of Land Management, California
Department of Fish and Game, California
Department of Water Resources, Ducks
Unlimited, and Sacramento County. About
2,400 acres are private lands that are protected
with conservation easements that restrict
development and intensive agricultural practices
that are incompatible with wildlife, such as
vineyards and orchards. Almost 90 percent of the
Preserve is maintained in active agricultural
production, including annual crops, grazing, and
a 1,000-acre organic rice farm managed by
Living Farms.
R
ESTORATION
P
ROJECTS
A
T
T
HE
C
OSUMNES
R
IVER
P
RESERVE
Active Restoration
Early efforts to enhance and restore habitats
along the lower Cosumnes focused on active
measures, such as wetlands construction and
hand planting of trees (Eaton, 1998).
Approximately 725 acres of ponds are managed
for waterfowl and sandhill cranes. In 1988, the
Preserve initiated the first large-scale replanting
of a valley oak forest (Griggs, 1991; Reiner,
1996). Since then, a total of 500 acres have been
planted with oaks, willows and other trees by
volunteers and school children. Planting remains
an integral part of the Preserve's education
program. However, a 1994 study found that
hand planting was expensive and some plantings
failed or grew slowly (Reiner 1996).
Furthermore, natural regeneration of oaks was
occurring in many areas, particularly where
natural flooding and sediment deposition still
occurred. The Nature Conservancy reoriented the
forest restoration program in 1995 to identify
areas where natural regeneration could be
encouraged by reestablishing natural flooding
(Reiner 1996).
Restoration of Natural Flooding Processes
Alteration or removal of levees, such as
breaching or creation of setback levees, provides
the opportunity to reconnect the river to its
floodplain.
A Lesson From Nature: The Accidental Forest.
In early 1985, a levee protecting a farm field
adjacent to the Preserve failed, and much of the
river's flow was diverted through a gap in the
levee for a brief period. The floodwaters
dropped a substantial amount of sand on about
15 acres. The farmer repaired the levee, but left
the deposited sediment and farmed the remainder
of the field.
Cottonwood and willows rapidly colonized the
depositional area, and by late 1985 the
"accidental forest" was well established (Figure
2). Within a few years the area contained a thick
mosaic of 15- to 20-foot high cottonwood trees,
Oregon ash, and willow thickets. Sixteen years
later, many of the cottonwoods are over 40 feet
tall and the "accidental forest" provides habitat
for songbirds, raptors, deer, beaver, and otter.
Researchers from the Point Reyes Bird
Observatory have found a variety of migratory
songbirds nesting in the forest (PRBO 2000).
PRBO has ranked the site well above other areas
they have studied in the Central Valley. In the
understory of the forest and on its edges,
naturally regenerated valley oak trees have taken
hold (Tu 2000). As the shorter-lived softwood
trees die or fall victim to beavers, the oaks will
fill in the forest canopy and ultimately succeed
the cottonwoods and willows (Tu 2000).
This farm property was acquired by the Nature
Conservancy in 1987. The rapidly growing
"accidental forest" inspired us to explore how
natural flooding processes could be enlisted to
expand the riparian corridor.
1995 Intentional Levee Breach. In 1994, a
hydrologic assessment was conducted of the
lower preserve, with funding from the US EPA.
The goal was to determine whether levees could
be intentionally breached without creating
flooding problems for the Preserve’s neighbors.
The HEC-2 modeling demonstrated that water
surface elevations in the river would be reduced
upstream of a levee breach because waters would
spread out on the expanded floodplain (Swanson
and Hart, 1994). Thus, a levee breach would
reduce flood levels elsewhere on the river.
In October 1995, a 50-foot gap in the levee was
created and a shallow channel was cut through
the field (Figure 2). This reopened about 200
acres of bottomland to natural flooding. The
project cost approximately $10,000-$15,000. In
December, the rising river flooded the field for
the first time. By March, high flows had scoured
the channel and deposited sand in bars parallel to
the channel, along with natural "cuttings" of
willow and cottonwood. In May, receding
floodwaters exposed mudflats, where
cottonwood seeds settled. By early fall 1996, the
cottonwood trees were about three feet tall (Tu
2000). A three-year study found that
cottonwood trees that grew from cuttings were
much taller and survived better than those that
grew from seed (Tu 2000). Beavers have
browsed many trees, but the trees have often
resprouted. By late 2000, some trees were over
10-12 feet tall. The success of the 1995 breach
has attracted much attention from resource and
regulatory agencies.
1997 Levee Breach and Floodproofing. The
massive floods of January 1997 caused many
levee breaks along the Cosumnes River. Given
the success of the 1995 project, the Preserve and
local farmers reached an agreement on an “un-
leveeing” project and convinced the Army Corps
of Engineers to fund a non-structural flood
management project in lieu of traditional levee
repairs. The project cost approximately $1.55
million, with $900,000 from the Corps
(nonstructural flood control program) and
$650,000 from a developer (mitigation grant to
create wetlands).
The project site was upstream of the “accidental
forest” and the 1995 levee breach (Figure 2). A
major component of the project involved
breaching and abandoning 5.5 miles of levees.
The main levee along the Cosumnes was
breached to let the river flow onto the floodplain.
An internal levee to the south was also breached
to allow the floodwaters to drain south into the
“accidental forest” and adjacent floodplain. A
waterfowl pond was excavated as part of the
wetlands mitigation. To protect adjacent farm
fields, a low setback levee was constructed.
Irrigation pumps were either elevated or replaced
with submersible pumps. Construction started in
the fall of 1997 with the levee breaches and
setback levee. This added about 100 acres to the
floodway. Floodproofing of infrastructure was
completed by winter 1998-1999.
Floodwaters again created a large sand splay on
this restored floodplain, and willows and
cottonwoods quickly established (Mount et al.,
this volume). After 2-1/2 years, these trees were
about 6-7 feet tall (W. Trowbridge, UC Davis,
unpublished data).
Reconnecting the river with its floodplain did
more than just start forest recruitment: it created
valuable habitat for native fishes. Significant
numbers of juvenile chinook salmon and
Sacramento splittail use the seasonal wetlands
(Whitener and Kennedy, 1999; Kennedy and
Whitener, 2000). The warm, shallow waters
produce enormous blooms of algae and
invertebrates (Mount et al., this volume), which
provide food for fishes (Sommer et al. 2001).
Habitat quality for native fish is influenced by
timing and depth of inundation, and habitat
complexity (P.B. Moyle, P. Crain, and K.
Whitener, unpublished data). Native fishes have
adapted to spawn early in the spring, when
floods inundate the floodplain and water
temperatures are colder. In contrast, the majority
of non-native fishes that dominate the Cosumnes
spawn later in the season, when water
temperatures are warmer. Thus, inundation of
the floodplain by the rising river provides
shallow water habitat that corresponds with the
life histories of native fishes. Late in the spring,
river flows decline, waters start draining from
the floodplain, water temperatures rise, and the
connection between the river and floodplain is
severed, thereby eliminating habitat for non-
native fishes.
Depth of water within the floodplain is also a
critical element. Preliminary data suggests that
native minnows such as Sacramento splittail
adults spawn on the submerged vegetation at
about 4-6 feet deep, while juvenile chinook
salmon and splittail feed in shallow waters about
1-3 feet deep. Areas deeper than six feet do not
appear to be as productive, possibly because it is
too deep for photosynthesis to occur. These
deep waters may also harbor more predators.
Finally, habitat complexity is important. A
mosaic of microhabitat types, with various flows,
depths and vegetation will ensure habitat for
different life stages of fishes and invertebrates.
D
ISCUSSION
The Cosumnes River Preserve is a proving
ground for innovative approaches to restoring
riparian forest and floodplain habitats. Its
history mirrors changes in restoration practices
and reflects the growing sophistication of
conservation biology (Reiner, 1996; Poiani et al.,
2000). The preserve has evolved from
opportunistic acquisition of the best remaining
groves of riparian valley oak forest in the Central
Valley, to active restoration via hand-planting of
trees, and finally to restoration of natural
processes that encourage natural forest
regeneration. Current scientific theory
emphasizes conserving biodiversity at multiple
scales within an ecosystem or landscape context,
along with the ecological processes that sustain it
(Poiani et al., 2000).
The levee breach projects are a living laboratory
for investigating floodplain dynamics and
restoration ecology. The Cosumnes Research
Group, a coalition of more than 30 university
and agency researchers, has been monitoring
patterns and processes of the physical and
biological environments, including surface and
groundwater hydrology, geomorphology,
primary productivity, aquatic invertebrates, fish,
and vegetation (Mount et al., this volume). This
interdisciplinary effort is providing information
for further restoration and adaptive management
in the lower Cosumnes River and other stream
systems.
Altering or removing levees has several
consequences for ecosystem restoration as well
as flood management and groundwater recharge.
These two projects have added about 300 acres
to the active floodway, and about 1,200 acres
overall to the Cosumnes floodplain. Floodwaters
have deposited fresh sediment, seeds and other
plant materials on the floodplain, resulting in
natural recruitment of cottonwood and willows,
which will lead to the establishment of riparian
valley oak forest (Tu 2000). Sediment deposition
has created diverse topography on the floodplain,
thereby increasing habitat diversity. The
inundated floodplain provides valuable habitat
for wintering waterfowl and sandhill cranes, as
well as for native fishes, such as Sacramento
splittail and juvenile chinook salmon, that are
adapted to take advantage of these seasonally
flooded areas. From a farm management
perspective, future levee maintenance and
emergency repair costs have been reduced.
From a flood management perspective, the
floodplain’s holding capacity has been increased.
This attenuates the flood peak and reduces flow
velocity in the river, thereby decreasing the risk
of flooding to surrounding areas (Philip Williams
and Associates, 1997). Finally, groundwater
recharge is enhanced when floodwaters have a
longer residence time on the floodplain. The
regional groundwater table has declined
dramatically since 1940, resulting in reduced or
absent flows in the Cosumnes River during the
fall when chinook salmon attempt to migrate
upstream to spawn (Phil Williams and
Associates, 1997).
Monitoring is providing valuable information for
adaptive management. A few of the lessons
learned and possible applications are discussed
below.
Multiple breaches could foster increased forest
recruitment – Regeneration of cottonwoods and
willows was most successful on the freshly
deposited sand splay that formed immediately
inside the levee breach (Mount et al., this
volume). In order to foster more extensive
riparian restoration in a field, it may be worth
considering filling old breaches once recruitment
has occurred, and creating new breaches to get
more sand and plant materials spread across the
field at a new location (J. Mount and J.
Florsheim, unpublished data).
Seasonal spring flooding creates habitat for
native fishes –The timing of flooding can have a
large effect on fish use of the floodplain,
depending on the species’ life history (P.B.
Moyle, P. Crain and K Whitener unpublished
data). Inundation that corresponds to the historic
natural hydrograph will allow native species to
utilize the habitat while excluding many of the
non-native species. Ponds and ditches that hold
water year-round tend to favor non-native fishes.
Therefore, creation of seasonally flooded habitat
will enhance conditions for native fishes.
Floodplain topography can affect fish stranding
Stranding of fishes can be a potential problem.
Preliminary data suggests that the decreasing
flows and rising temperatures cue the native
species to leave the floodplain and avoid
stranding. The project design should consider
drainage patterns from the floodplain, with an
eye toward creating exit points for fish.
Stranding seems to be associated with man-made
structures such as roads, fences, ponds, ditches
and levees. Removal or adjustment of such
structures will alleviate potential stranding. For
example, a waterfowl pond excavated as part of
the 1997 project may be a stranding site for fish.
It is worth noting, however, that overall
production of fishes on the inundated floodplain
is greater than in the river channel alone, even
with some stranding.
Placement of breaches and setback levees
-
Consider carefully the placement of the levee
breach and the resulting flow of water. If you’re
going to breach a river levee and put in a setback
levee, leave enough room for flows to work the
new floodplain. In some cases, the river could
abandon its channel in favor of a new channel
through the restoration site. At the Preserve, one
end of the low setback levee was constructed too
close the levee breach, in order to maximize
farmed acres. High flood flows overtop this low
levee, resulting in some erosion of the levee and
sand deposition in the adjacent farm field, which
has required ongoing maintenance.
Floodproof farm infrastructure in the floodplain
–Incorporate floodproofing of farm infrastructure
in the restoration project if you want to continue
farming on adjacent lands. Remove or replace
equipment that cannot withstand inundation,
raise and improve necessary roads so they can
withstand some wave action from temporary
floodwaters.
So what does the future hold for natural process
restoration of riparian habitat along the
Cosumnes River? We are looking for other
opportunities to allow the river to inundate its
floodplain. Areas that receive regular flooding
(i.e. 2-5 year recurrence interval) would be the
highest priority for land acquisition and
restoration. A hydrologic study indicated that
the best opportunities for restoring a functional
floodplain exist downstream of Highway 99 (11
miles above the confluence of the Cosumnes and
Mokelumne Rivers) (Philip Williams and
Associates 1997). Upstream of Highway 99, the
channel is incised and consequently the river is
isolated from its floodplain. As the channel
deepens and increases its capacity, larger flows
are required to overtop the banks, resulting in
less frequent floodplain inundation. Restoring
functional floodplains will not only benefit
riparian ecosystems, it can also minimize flood
damages and improve groundwater conditions.
The Nature Conservancy is discussing initiating
a feasibility study with the Army Corps of
Engineers along the lower Cosumnes and
Mokelumne Rivers. The objective is to identify
and evaluate ecosystem restoration projects that
will also reduce flood damages via non-structural
measures. The study would include hydrologic
and hydraulic modeling of the rivers and
measures such as controlled levee breaches,
setback levees, and flood bypasses. The Corps
process also presents the opportunity for
leveraging federal funds for implementation, and
provides a vehicle for engineering and
environmental review of such projects. We hope
this partnership will be a model for innovative
riparian restoration and flood management, both
in the Cosumnes River basin and beyond.
A
CKNOWLEDGEMENTS
The Cosumnes River Preserve is a collaborative
project that receives funding and support from
several partners, including the U.S. Bureau of
Land Management, California Department of
Fish and Game, California Department of Water
Resources, Sacramento County, Ducks
Unlimited, and Living Farms. Additional
funding for acquisition and restoration has been
generously provided by CALFED, the David and
Lucille Packard Foundation, Natural Resources
Conservation Service, U.S. Bureau of
Reclamation, U.S. Army Corps of Engineers,
U.S. Environmental Protection Agency, U.S.
Fish and Wildlife Service, and the Wildlife
Conservation Board.
L
ITERATURE
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F
IGURE
1. Cosumnes River Preserve.
F
IGURE
2. Location of levee breaches on the Cosumnes River Preserve.
... floodplains through the breaching of riverside levees an emerging floodplain restoration practice (Swenson, Reiner, Reynolds, & Marty, 2012;Swenson, Whitener, & Eaton, 2003). ...
... Despite increasing urban pressures on the eco-agricultural matrix found in the Cosumnes River basin, the lack of flood control beyond riverside levees has resulted in comparatively low human population densities, which in turn has allowed for a unique approach to floodplain management. Throughout the 1990s, levee breaches along the lowest 3.5 km of the Cosumnes River immediately upstream from its confluence with the Mokelumne River were excavated to promote the development of sand splays and the establishment of riparian forests on former tomato fields Swenson et al., 2003;Swenson et al., 2012). The depth of channel incision along the lower Cosumnes River has largely precluded the possibility of channel avulsion through the breaches. ...
... In some localities, the channel incised more than 2 m, often extending through both sandy, alluvial bedforms and underlying, erosionally resistant duripan layers generally thought to limit channel bed incision throughout the lower Cosumnes River (Constantine et al., 2003). Longitudinal distribution of topographic changes at levee breach sites U and L. Distances are measured from the bank of the Cosumnes River levee breaches to initiate process-based restoration of floodplain riparian and wetland ecosystems , in concert with, or in lieu of, labor-intensive horticultural approaches (Reiner, 1996;Swenson et al., 2003;Swenson et al., 2012). However, this existing model of floodplain topography change in response to intentional levee breaching was derived from a largely uniform starting point: narrow levee breaches (<75 m) opening onto graded floodplain areas. ...
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The transport of water and sediment from rivers to adjacent floodplains helps generate complex floodplain, wetland, and riparian ecosystems. However, riverside levees restrict lateral connectivity of water and sediment during flood pulses, making the re-introduction of floodplain hydrogeomorphic processes through intentional levee breaching and removal an emerging floodplain restoration practice. Repeated topographic observations from levee breach sites along the lower Cosumnes River (USA) indicated that breach architecture influences floodplain and channel hydrogeomorphic processes. Where narrow breaches (<75 m) open onto graded floodplains, archetypal crevasse splays developed along a single dominant flowpath, with floodplain erosion in near-bank areas and lobate splay deposition in distal floodplain regions. Narrow breaches opening into excavated floodplain channels promoted both transverse advection and turbulent diffusion of sediment into the floodplain channel, facilitating near-bank deposition and potential breach closure. Wide breaches (>250 m) enabled multiple modes of water and sediment transport onto graded floodplains. Advective sediment transport along multiple flow paths generated overlapping crevasse splays, while turbulent diffusion promoted the formation of lateral levees through large wood and sediment accumulation in near-bank areas. Channel incision (>2 m) upstream from a wide levee breach suggests that large flow diversions through such breaches can generate water surface drawdown during flooding, resulting in localized flow acceleration and upstream channel incision. Understanding variable hydrogeomorphic responses to levee breach architecture will help restoration managers design breaches that maximize desired floodplain topographic change while also minimizing potential undesirable consequences such as levee breach closure or channel incision.
... Ultimately, as water moves through rehabilitated habitat its ecological value or potential cumulatively improves [44,45] It follows that the restoration-laden Cosumnes River carries with it the potential to boost restoration value within the MWT restoration: the last "pearl" in the string. Understanding the spatiotemporal patterns of SWF allows us to identify regions that retain source waters from the Cosumnes, and thus water that has a higher likelihood to contain nutrients or productivity from upstream restoration sites [31,46,47]. Being downstream of completed and ongoing restoration projects is not a unique trait of the MWT within the Delta. ...
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Tributary source water provenance is a primary control on water quality and ecological characteristics in branching tidal river systems. Source water provenance can be estimated both from field observations of chemical characteristics of water and from numerical modeling approaches. This paper highlights the strengths and shortcomings of two methods. One method uses stable isotope compositions of oxygen and hydrogen from water in field-collected samples to build a mixing model. The second method uses a calibrated hydrodynamic model with numerical tracers released from upstream reaches to estimate source-water fraction throughout the model domain. Both methods were applied to our study area in the eastern Sacramento–San Joaquin Delta, a freshwater tidal system which is dominated by fluvial processes during the flood season. In this paper, we show that both methods produce similar source water fraction values, implying the usefulness of both despite their shortcomings, and fortifying the use of hydrodynamic tracers to model transport in a natural system.
... Here, we found that, on average, planted riparian forest in study areas with a relatively dry climate would have a somewhat lower K for biomass carbon stock than their naturally regenerating counterparts (Figure 2), but we note that these estimates represent predicted averages. The successful establishment and growth of planted riparian forests may vary widely due to initial conditions and financial constraints (Chazdon, 2008), and particularly in areas with a relatively dry climate, whether there is hydrologic connectivity or irrigation (Friedman, Scott, & Lewis, 1995;Stromberg, 2001;Swenson, Whitener, & Eaton, 2003). While the naturally regenerating riparian forests in our data set were, by definition, in suitable locations that were capable of recruiting riparian forest vegetation, the planted riparian forests in our data set are more likely to include locations facing more challenging conditions (Reid, Fagan, & Zahawi, 2018). ...
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Restoration of deforested and degraded landscapes is a globally recognized strategy to sequester carbon, improve ecological integrity, conserve biodiversity, and provide additional benefits to human health and well‐being. Investment in riparian forest restoration has received relatively little attention, in part due to their relatively small spatial extent. Yet, riparian forest restoration may be a particularly valuable strategy because riparian forests have the potential for rapid carbon sequestration, are hotspots of biodiversity, and provide numerous valuable ecosystem services. To inform this strategy, we conducted a global synthesis and meta‐analysis to identify general patterns of carbon stock accumulation in riparian forests. We compiled riparian biomass and soil carbon stock data from 117 publications, reports, and unpublished data sets. We then modeled the change in carbon stock as a function of vegetation age, considering effects of climate and whether or not the riparian forest had been actively planted. On average, our models predicted that the establishment of riparian forest will more than triple the baseline, unforested soil carbon stock, and that riparian forests hold on average 68–158 Mg C/ha in biomass at maturity, with the highest values in relatively warm and wet climates. We also found that actively planting riparian forest substantially jump‐starts the biomass carbon accumulation, with initial growth rates more than double those of naturally regenerating riparian forest. Our results demonstrate that carbon sequestration should be considered a strong co‐benefit of riparian restoration, and that increasing the pace and scale of riparian forest restoration may be a valuable investment providing both immediate carbon sequestration value and long‐term ecosystem service returns. https://rdcu.be/bbkFV
... Another reforestation was initiated when an unintentional levee breach and flood event in 1985 resulted in the recruitment and establishment of riparian vegetation, including a grove of cottonwood trees (Populus fremontii). This observation inspired a second, intentional levee breach 10 years later to improve floodplain connectivity and promote natural forest regeneration in the area (Swenson, Whitener, & Eaton, 2003). Thus, much of this study area ("naturally regenerating") was 22 years old in 2017, but the original cottonwood grove was 32 years old. ...
Article
Climate change and biodiversity loss are two global challenges that can be addressed simultaneously through reforestation of previously cleared land. However, carbon markets can encourage reforestations that focus on maximizing carbon storage, potentially at the expense of biodiversity conservation. To identify opportunities to optimize reforestation design and management to meet both goals, we examined the forest stand features associated with carbon stocks in biomass and soil, as well as bird abundance and diversity, in remnant and restored riparian forest stands in central California, U.S.A. Within three decades of reforestation, both planted and naturally regenerating riparian forest stands provided significantly greater carbon storage and avian biodiversity benefits compared to baseline conditions. They were also similar to a remnant riparian forest stand. We identified a synergy between carbon storage and biodiversity benefits in their positive associations with understorey cover, but we also identified a trade‐off in their relationships to forest stand density. Biomass carbon stocks were strongly positively related to stand density, while bird density and diversity suffered at the highest stand densities. The variability in understorey cover across forest stands indicates an opportunity for further enhancement of carbon and biodiversity benefits in areas where understorey cover is low, while the variability in stand density suggests an opportunity to re‐examine reforestation goals and consider thinning to achieve those goals. Synthesis and applications. We identified synergies and trade‐offs between carbon storage and biodiversity in their relationships to forest stand features, indicating opportunities to optimize reforestation design and management to achieve multiple goals. Our approach can be adapted to other reforestation efforts intended to simultaneously address the global challenges of climate change and biodiversity loss. El cambio climatico y la pérdida de biodiversidad son dos problemas globales que se pueden enfrentar con la reforestación de tierras previamente degradadas. Sin embargo, los mercados de carbon promueven reforestaciones enfocadas en maximizar el almacenamiento de carbon, posiblemente a cuestas de la conservación de la biodiversidad. Para identificar oportunidades de diseño y gestión de programas de reforestación enfocados en estos dos objetivos, evaluamos las características de rodales de bosques que determinan las medias de carbon en la biomasa y en los suelos, y que también determinan la abundancia y diversidad de aves en rodales de bosques riparios remanentes y restaurados en el centro de California, Estados Unidos. En tres décadas de reforestación, los rodales de bosques riparios sembrados y regenerados naturalmente incrementaron de manera significativa el almacenamiento de carbono y los beneficios para la biodiversidad de aves comparado con condiciones de referencia. Los rodales de bosques riparios también mostraron características similares a un rodal de bosque ripario remanente. Hemos identificado una sinergia entre el almacenamiento de carbono y beneficios a la biodiversidad porque estan positivamente asociadas con la cobertura del sotobosque, pero también identificamos una compensación con la densidad del rodal de bosque. Las reservas de carbono en la biomasa demostraron una fuerte relación positiva con la densidad del rodal, mientras que la densidad y la diversidad de las aves sufrieron en las densidades más altas del rodal. La variabilidad en la cobertura del sotobosque en los rodales de bosque indica una oportunidad para augmentar beneficios de carbon y de biodiversidad en áreas donde la coberatura esté baja, mientras que la variabilidad en la densidad de los rodales sugiere una oportunidad para reexaminar las metas de reforestación posiblemente considerando el raleo para alcanzar las metas. Síntesis y aplicaciones. Identificamos sinergias y compensaciones entre el almacenamiento de carbono y la biodiversidad con las características de rodales de bosque, indicando oportunidades para optimizar el diseño y la gestión de programas de reforestación para lograr objetivos múltiples. Nuestro enfoque se puede adaptar a otros esfuerzos de reforestación dedicados a enfrentar simultáneamente los problemas globales de cambio climatico y la pérdida de biodiversidad. https://rdcu.be/8S76
... As an example of how these guidelines can be applied in the field, we describe our efforts to 345 establish a restoration experiment in the Central Valley of California. Thirty years ago, the 346 Cosumnes River was identified as a conservation and restoration priority in the Central Valley of 347 California, USA, due to the presence of large blocks of remnant riparian forest and grassland in 348 an otherwise agricultural and rapidly developing landscape ( Swenson et al. 2003). Since then, 349 nongovernmental conservation organizations and public agencies have worked together to 350 protect and restore riparian forest. ...
Preprint
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Experimenting with new and unconventional restoration methods and designs is critical to advancing the field of ecological restoration. Conventional methods cannot be considered reliable in a future with climate change-induced shifts in weather conditions, species distributions, and ecosystem processes. It is crucial that researchers and practitioners collaborate to identify the most effective restoration methods, yet there remains a disturbing lack of restoration experiments at the spatial or temporal scales relevant for evaluating wildlife responses. We suspect that willingness to attempt such experiments is hampered by the perceived difficulty of conducting these experiments combined with a fear of failure. However, we argue that failure to experiment with new methods guarantees learning nothing new. Here, we address many of the major challenges of designing an experiment to evaluate wildlife responses to restoration, including (1) distinguishing between the goals and objectives of the restoration project and the key uncertainties the experiment will address, (2) designing the experiment itself, including optimizing plot size and replication, and (3) determining how and when the results will be evaluated. We then illustrate how we designed an experiment to evaluate riparian bird responses to restoration along the lower Cosumnes River in the Central Valley of California, USA. Researchers and practitioners working together from the start of the objectives-setting process, through experimental design, implementation, and evaluation can proactively address the challenges of conducting a restoration experiment and maximize the chances of successfully identifying effective restoration methods, adding to the practitioners’ toolbox, and accelerating the rate of successful habitat restoration.
... Within the Lower Cosumnes River watershed, the adjacent riparian corridors and grasslands are subject to annual flooding, creating an interspersion of gallery riparian forest, oak woodland, permanent and seasonal wetlands, and aquatic habitats within the floodplain (Vahti and Greco 2007). High winter flows are a critical factor in the creation of the floodplains through the channelization process (Moyle et al. 2003), and several floodplains are hydrologically connected to the Cosumnes River in its lower region (Swenson et al. 2003). While the mean annual average flow is 529 cubic feet per second , peak winter discharges of as high as 54,000 cubic feet per second have been recorded for the Cosumnes (USGS 1979). ...
Conference Paper
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Background/Question/Methods The lower Cosumnes River watershed, located in the northeastern portion of the Sacramento-San Joaquin Delta of Central California, has been home to the Plains Miwok for over 4,000 years. Ecological studies suggest that a symbiotic relationship may have existed between indigenous communities, floodplain biodiversity, and native fish fecundity prior to Euro-American settlement within the watershed. This question was explored by reconstructing Plains Miwok traditional management practices and subsistence patterns through ethnographic and archaeological data. Ethnographic data was gathered from historical literature and local Miwok informants, while archaeological data was generated from zooarchaeological analysis of four sites dating to the Late Period (1200 to 100 B.P.). These data sets were then compared to current floodplain conditions, modern day fish populations, and native fish life histories. Results/Conclusions Study results suggest that traditional management practices may have optimized the productivity of floodplain rearing habitats through the removal of senescent vegetation and soil rejuvenation. Robust floodplains provide ideal conditions for increased native fish fecundity and reduced juvenile mortality. The presence of large healthy fish populations is supported by the zooarchaeological data which indicates that native fish species provided one-third of the local Plains Miwok diet for at least 1,100 years prior to 1850. In contrast, today, 49 percent of these fish are endangered, extirpated, or extinct as a result of floodplain habitat loss and the introduction of exotic species. It is hoped that the interdisciplinary approach presented here can provide a template for the integration of ethnographic and archaeological data into ecological restoration and conservation efforts.
Chapter
Floodplains support rich ecosystems and provide critically important benefits to people. Despite their diverse benefits, floodplains rise to the attention of decision makers and the public most frequently during times of disaster, such as large flood events that displace people and cause losses of life and property. Due to levees and water regulation from dams, floodplains are among the most converted and threatened ecosystems on Earth. In this article, we review how maintaining or restoring river-floodplain connectivity can serve as a Nature-based Solution (NbS) to promote the full spectrum of societal benefits from floodplains, including increasing resilience to climate change. Floodplains support the largest freshwater fisheries in the world and provide the primary source of protein to hundreds of millions of people worldwide. Particularly in arid or semi-arid regions, floodplains are important sites for agriculture with crops planted in floodplain soils as floodwaters recede. By storing and conveying floodwaters, floodplains can also reduce flood risk for other areas.
Article
Despite the recognition of floodplain importance in the scientific community, floodplains are not afforded the same legal protection as river channels. In the United States alone, flood-related economic losses were much higher in the second half of the 20th century than the first half despite the expenditure of billions of dollars on flood defenses. Partially to blame are the low appraisal and understanding of human impacts to floodplain functions. Here, we explore the impacts of levees on floodplain functions and analyze case studies of floodplain restoration through levee removal. Floodplain functions include (1) fluxes of water, solutes, and particulate materials; (2) enhanced spatial heterogeneity of hydrology and biogeochemistry; (3) enhanced habitat abundance and diversity; (4) enhanced biomass and biodiversity; and (5) hazard mitigation. Case studies of floodplain restoration involving artificial levee adjustment are heavily concentrated in North America, Europe, and Japan, and those case studies assess floodplain functions within 30 years of restoration. In the United States, restoration through levee removal comprises less than 1% of artificial levee length and 1–2% of disconnected floodplains. In Europe, restoration effectiveness was severely limited by upstream flow regulation. Most case studies were impacted by stressors outside the study site and took place in lowland alluvial rivers. Reconfiguration was successful at achieving limited aims while reconnection set floodplains on a trajectory to more fully restore floodplain functions. Case studies illustrated the tension between restoration scale and study resolution in time and space as well as the role of site-specific characteristics in determining restoration outcomes. Numerous knowledge gaps surrounding the integrative relationships between floodplain functions must be addressed in future studies. The ubiquity of flow regulation demands that future floodplain restoration occur in a whole-of-basin manner. Monitoring of restoration must take place for longer periods of time and include multiple functions.
Preprint
Full-text available
Experimenting with new and unconventional restoration methods and designs is critical to advancing the field of ecological restoration. Conventional methods cannot be considered reliable in a future with climate change-induced shifts in weather conditions, species distributions, and ecosystem processes. It is crucial that researchers and practitioners collaborate to identify the most effective restoration methods, yet there remains a disturbing lack of restoration experiments at the spatial or temporal scales relevant for evaluating wildlife responses. We suspect that willingness to attempt such experiments is hampered by the perceived difficulty of conducting these experiments combined with a fear of failure. However, we argue that failure to experiment with new methods guarantees learning nothing new. Here, we address many of the major challenges of designing an experiment to evaluate wildlife responses to restoration, including (1) distinguishing between the goals and objectives of the restoration project and the key uncertainties the experiment will address, (2) designing the experiment itself, including optimizing plot size and replication, and (3) determining how and when the results will be evaluated. We then illustrate how we designed an experiment to evaluate riparian bird responses to restoration along the lower Cosumnes River in the Central Valley of California, USA. Researchers and practitioners working together from the start of the objectives-setting process, through experimental design, implementation, and evaluation can proactively address the challenges of conducting a restoration experiment and maximize the chances of successfully identifying effective restoration methods, adding to the practitioners’ toolbox, and accelerating the rate of successful habitat restoration.
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Approaches to conservation and natural resource management are maturing rapidly in response to changing perceptions of biodiversity and ecological systems. In past decades, biodiversity was viewed largely in terms of species richness, and the ecosystems supporting them were seen as static adn predictable(Fielder et.al. 1997). Conservation activities were often aimed at hotspots rich in total species or in rare species(Noss 1987). Consequently, relatively small nature preserves proliferated throughout the 1970's and 1980's, as did endangered species management and recovery plans on more extensive public lands. More recently, biodiversity is being viewed more expansively, to include genes, species, populations, communities, ecosystems and landscapes, with each exhibiting characteristic and complex composition, structure, and function (Noss 1990). As a result, current recommendations for biodiversity conservation focus on the need to conserve dynamic, multiscale ecological patterns and processes that sustain the full complement of biota and their supporting natural systems(e.g. Angermeier & Karr 1994; Turner et.al. 1995; Harris et.al. 1996; Poff et.al. 1997). Translating expanding perceptions into pragmatic guidelines and appropriate action is a challenge for conservation organisations and natural resource agencies. In this article, we describe an imperfect but practical framework that can help practitioners transition from biodiversity conservation based on rare or endangered species to conservation based on ecosystem-and landscape-level concepts. We begin by providing a brief overview of the scientific concepts from which the framework has evolved. We then describe a convenient way to catagorise ecosystems and species based on spatial pattern and sale. Next, we describe three types of "functional conservation areas"--sites, landscapes and networks--defined by the scale of ecosystems and species they are designed to conserve. We then present a suite of ecological attributes that can be used to evaluate the functionality or integrity of a conservation area at any scale. Finally, we discuss the challenges of implementing these ideas in applied settings. We illustrate concepts with examples and a case study from the work of the Nature Conservancy(TNC), a nongovernmental organisation dedicated to conserving biodiversity throughout the United States and in selected other countries worldwide.
Article
In this study, we provide evidence that the Yolo Bypass, the primary floodplain of the lower Sacramento River (California, U.S.A.), provides better rearing and migration habitat for juvenile chinook salmon (Oncorhynchus tshawytscha) than adjacent river channels. During 1998 and 1999, salmon increased in size substantially faster in the seasonally inundated agricultural floodplain than in the river, suggesting better growth rates. Similarly, coded-wire-tagged juveniles released in the floodplain were significantly larger at recapture and had higher apparent growth rates than those concurrently released in the river. Improved growth rates in the floodplain were in part a result of significantly higher prey consumption, reflecting greater availability of drift invertebrates. Bioenergetic modeling suggested that feeding success was greater in the floodplain than in the river, despite increased metabolic costs of rearing in the significantly warmer floodplain. Survival indices for coded-wire-tagged groups were somewhat higher for those released in the floodplain than for those released in the river, but the differences were not statistically significant. Growth, survival, feeding success, and prey availability were higher in 1998 than in 1999, a year in which flow was more moderate, indicating that hydrology affects the quality of floodplain rearing habitat. These findings support the predictions of the flood pulse concept and provide new insight into the importance of the floodplain for salmon.
Article
/ This GIS-based study analyzes the distribution and management of woody riparian vegetation in California's Sacramento Valley and discusses the prospects for its conservation. Although forests were the predominant floodplain vegetation prior to extensive settlement, only 3.3% of floodplain was covered by forest in the late 1980s. This remaining forest was fragmented into 2607 patches with an average area of 3.1 ha. Only 180 patches were >10 ha, with three patches >100 ha. Despite over two decades of conservation efforts, these forests are essentially unpreserved: Only 14.5% of extant forests are in public ownership or on land managed primarily for biological conservation. Some privately owned forests represent opportunities for preservation, but owing to their small size and scattered distribution, reforestation would be necessary to obtain a high cover of forest over large areas. Additionally, high property values, existing land uses, and regulated hydrology constrain conservation efforts. As a consequence of these constraints, and current distribution and ownership patterns, preservation or restoration of substantial areas of riparian forest would be extremely expensive and would divert conservation resources from other habitats in this rapidly developing state. Therefore, efforts to conserve these forests should satisfy two criteria: (1) that the specific goals are attainable with available funding and existing human uses, and (2) funding the effort will result in more effective regional conservation than would funding the conservation of other habitats.KEY WORDS: Central Valley; Conservation; Floodplains; Geographic information systems; Riparian vegetationhttp://link.springer-ny.com/link/service/journals/00267/bibs/24n1p65.html
Restoration of natural flooding process on the Cosumnes River Preserve. Overview and status report. The Nature Conservancy
  • M R Eaton
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