Restoring Floods to Floodplains: Riparian and Floodplain Restoration
at the Cosumnes River Preserve
The Nature Conservancy, Cosumnes River Preserve, Galt, CA 95616
. 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
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
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
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
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
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
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
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,
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-
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.
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
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
Monitoring is providing valuable information for
adaptive management. A few of the lessons
learned and possible applications are discussed
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
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.
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
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1. Cosumnes River Preserve.
2. Location of levee breaches on the Cosumnes River Preserve.