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Resilient landscape vision for Lower Walnut Creek

San Francisco Estuary Institute (SFEI) | November 30th, 2016

Summary

Lower Walnut Creek (Contra Costa County, CA) and its surrounding landscape have undergone considerable land reclamation and development since the mid-nineteenth century.

Lower Walnut Creek (Contra Costa County, CA) and its surrounding landscape have undergone considerable land reclamation and development since the mid-nineteenth century. In 1965, the lower 22 miles of Walnut Creek and the lower reaches of major tributaries were converted to flood control channels to protect the surrounding developed land. In the recent past, sediment was periodically removed from the lower Walnut Creek Flood Control Channel to provide flow capacity and necessary flood protection. Due to the wildlife impacts and costs associated with this practice, the Contra Costa County Flood Control and Water Conservation District (District) is now seeking a new channel management approach that works with natural processes and benefits people and wildlife in a cost-effective manner. Flood Control 2.0 project scientists and a Regional Science Advisory Team (RSAT) worked with the District to develop a long-term management Vision for lower Walnut Creek that could result in a multi-benefit landscape that restores lost habitat and is resilient under a changing climate. Development of the Vision began with technical analyses focused on understanding past and present landscape functioning and the changes to key processes and landscape features over the past 150 to 200 years. The key findings from these analyses include the following: • Landscape change – During the mid-19th century, lower Walnut Creek was surrounded by a continuous expanse of tidal wetlands occupying approximately 5,000 acres. Perennial freshwater marsh, willow thicket, and alkali meadow formed a large non-tidal wetland complex that adjoined the tidal marsh on its southern end. Over the past century and a half, the non-tidal wetland complex has been lost completely and the tidal wetland area has decreased by 40%. The remaining marsh areas are highly fragmented and cut-off from Walnut Creek by engineered levees. The loss of tidal wetland area has resulted in a substantial decrease in tidal prism and contributed to current in-channel sediment accumulation issues. Around the mouth of Walnut Creek, sediment accumulation has caused the position of the shoreline to expand into the Bay by up to half a mile. • Sediment accumulation – Since 1965, approximately 1.4M cubic yards of sediment have been removed from lower Walnut Creek, with approximately 70% coming from the tidal zone downstream of head of tide (i.e., the inland extent of tidal inundation at mean higher high water). Repeat channel cross-section surveys indicate that the channel quickly fills in with sediment following dredging events (due in large part to decreased tidal prism and associated channel scour capacity) and that the channel bed elevation is currently at or near “quasi-equilibrium.” A channel sediment budget for 1965-2007 indicates that approximately 80% of the watershed-derived sediment made it through lower Walnut Creek and out to the Bay, while the remaining 20% was likely deposited directly following channel construction and subsequent dredging events.