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Yolo Bypass

Floodplain inundation is one of the major factors that has shaped the geography, flora and fauna of California's Central Valley. As in most developed regions of the western United States, the majority of the historical floodplain has been lost to dam and levee construction. However, California was fortunate in that early flood engineers retained relatively large areas of floodplain as part of the flood management system. The largest contiguous floodplain area of the lower Sacramento River (the primary tributary of the San Francisco Estuary) is the Yolo Bypass, a 24,000 ha basin. The Bypass is a central feature of the Sacramento River Flood Control Project, which conveys floodwaters from the major valley rivers including the Sacramento, American, and Feather Rivers, and their tributary watersheds.

Seasonal inundation of the Yolo Bypass is one of the most dramatic seasonal events in California's Sacramento Valley. The Yolo Bypass typically floods in about 60 percent of years, when high winter and spring floodwaters enter from the Sacramento River and several small streams: Cache and Putah creeks; Willow Slough; and the Knight's Landing Ridge Cut from the Colusa Basin. This has a major physical effect on the San Francisco Estuary and its two component regions, the Sacramento-San Joaquin Delta and downstream Bays. Complete inundation of the floodplain approximately doubles the wetted area of the Delta and is equivalent to about one-third the area of San Francisco and San Pablo bays. The floodplain is seasonally dewatered in summer and fall, except for perennial ponds and a single tidal channel. During extended droughts such as 1987-1992, the floodplain is not inundated from its tributaries.

Although flood control is the major function of Yolo Bypass, the floodplain also supports agriculture, fisheries and wildlife. The Yolo Bypass floodplain is dominated by agricultural uses, but there are also substantial "natural" habitats such as seasonal wetlands, riparian and upland habitat. The largest contiguous area of non-agricultural floodplain habitat is the Yolo Basin Wildlife Area, managed by California Department of Fish and Game.

Click here for our Yolo Bypass research publications.

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