Cosumnes Floodplain Mitigation Bank

The Sacramento-San Joaquin Delta is the largest freshwater tidal estuary on the west coast of both North and South America. A maze of twisting and winding waterways, the Delta creates over 60 leveed tracts and islands. Two of the largest rivers in California converge in the Delta - the Sacramento and San Joaquin River. The water that enters the Sacramento-San Joaquin Delta is essential for supplying almost half of California's population with water and supporting the central valley's agricultural industry. 

The Delta is also home to a wide variety of wildlife species. Salmon migrate through the Delta moving upriver to lay their eggs and spawn. Each year millions of migratory birds follow the wetlands along the river as they make their way north in the summer and south in the winter.

The Delta was once covered in an almost continuous freshwater marsh. In the dry season, rivers formed low natural levees and riparian forests that were primarily covered willows. Wetlands beyond the levees were filled with tule and reed marshes. In the rainy season, the rivers would flood, and marshes would again be inundated with water. The brackish and freshwater habitats were teeming with an abundance of fish, waterfowl, and elk.

Around the end of the 1800s, farmers and pioneers began creating levees to reduce marshland and drain the Delta. Today, over 1,115 miles of levees have been constructed just in the Sacramento-San Joaquin Delta, protecting critical farmland and cities.

The Delta has been drastically changed since the mid-1800s and the California gold rush. The dynamic, historic floodplains that provided alcoves and backwater channels for juvenile salmon and steelhead when seasonally flooded are primarily gone. Levees and farmland now replace the riparian habitat that once used to dominate the landscape.

In 2019, the American Society of Civil Engineers gave California's levee and flood management system a "D" grade. Throughout the state, many dams are over 60 years old. Levees in the central valley are often over 100 years old. These older dams and levees are not necessarily up to today's design standards. They require more maintenance to prevent catastrophic flooding. The responsibility for flood-related maintenance is spread across local, state, and federal agencies, often making improvements a complicated bureaucratic process. 

Current investments in infrastructure fall short of their requirements. A 2017 report from the Legislative Analyst's Office, "Managing Floods in California," reported an average of 2 billion dollars a year is spent on flood maintenance improvements. Still, the budget needs an additional 1-2 billion dollars to keep up with maintenance and repairs. Other reports have suggested that even more money may be required.

Moving forward, California is looking to implement long term infrastructure improvements in and around the Sacramento San-Joaquin Delta. The improved infrastructure needs to protect human resources and interests as well as restore biological function.

Before WES purchased the property, the land was used to grow grapes. Since the 1800s, the site has been protected by levees. The property has not been influenced by natural flood regimes or daily tides. 

WES selected the site for floodplain habitat restoration based on several key factors:

• Ecological Value for Fish - From their spawning grounds, juvenile salmon and steelhead travel down the Sacramento River. The fish find refuge in the slower moving floodplain waters.

• Increased Flood Protection - The site will provide additional transition habitat and a buffer zone for water to inundate during significant flood events. 

• Protection for Local Farm Communities - The project exported excess soil from construction to support levee inspection roads, irrigation berms, and farm field access roads. Also, the project provides a comparable deed restriction on prime farmland.

When developing restoration plans, WES referenced the nearby Cosumnes River Preserve (CRP). The CRP is directly north of the Cosumnes Floodplain Mitigation Bank across the Cosumnes River. The preserve, established in 1987, consists of over 50,000 acres of wildlife habitat. The CRP supports examples of early-, mid-, and late-successional riparian habitats. The CRP also actively revegetates areas through direct planting of riparian tree species, making the CRP an excellent reference site. Peak flows and runoff in the Cosumnes River still routinely flood the Cosumnes River Preserve, providing abundant wetland services for dependent wildlife.

Since completion, the Cosumnes floodplain mitigation bank has won awards for exemplary restoration efforts. In 2011, WES was awarded the Tree Hero - Legacy Award by the Sacramento Tree Foundation for preserving a historic remnant oak grove on the bank property. The Legacy Award honors a tree, landscape, or woodland for its innate worthiness, uniqueness, or beauty. Additionally, In 2013, WES was one of two recipients of the Riparian Challenge Award by the American Fisheries Society Western Division for their excellent management of the Bank. The award recognizes efforts in managing, restoring, and improving riparian wetlands and ecosystems.

WES established a conservation easement and long-term stewardship endowment for the site. WES desires to carefully manage the protection and stewardship of the property in perpetuity.

Our land stewardship staff is on the front lines of conservation. WES believes that good stewardship is proactive. Land steward objectives include removing invasive plant and animal species, maintaining infrastructure, and providing appropriate site management over time. The land stewardship team works with the land and community to protect and restore native habitats, creeks, and wildlife corridors that are permanently protected under our care.

WES completed restoration at the Cosumnes Floodplain Mitigation bank in 2013. Over the years, WES staff has closely monitored the site noticing a resurgence in native foliage and an abundance of healthy juvenile salmonids. The project plays a vital role in bringing balance back to the Sacramento-San Joaquin Delta protecting ecological, agricultural, and human interests.