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66 STATE OF THE ESTUARY BENEFICIAL FLOODS Inundation of floodplains by high river flows drives important ecological processes such as sediment and nutrient transport and food production, and provides habitat for spawning and rearing for numerous fish species. Photo: Chris Austin. CONTEXT Following winter rainstorms and spring snowmelt in the Sacramento-San Joaquin watershed, the Estuary’s tributary rivers may flood, spilling over their banks to create ecologically important floodplain hab- itat and sending high volumes of fresh water into the Estuary. These seasonal high flows drive multiple ecological processes. They fuel primary and secondary production of micro- scopic plants and animals – important food for fish and wildlife – in inundated floodplains and the upper Estuary. They transport organisms, sediment, and nutrients downstream and signal anadromous species like salmon to 2015 come upstream. They create spawning and rearing habitat for numerous fish species. High flows also help mix ocean and river waters and create large areas of low-salinity habitat in Suisun and San Pablo Bays. They also help im- prove habitat conditions in riverine migration corridors for both adult fish moving upstream and young fish moving downstream. In sum, high flows, as well as rapid increases in flow, provide conditions favorable for many native fish, invertebrate and other wildlife species. Several factors have substantial impacts on the frequency, magnitude and duration of high volume flows into the Estuary and prevent ecologically important, regular seasonal flood events. These include: dams, many of which were built for the purpose of managing down- stream flooding; water extraction and diver- sion that can decrease flows to levels below important ecological thresholds for floodplain inundation, habitat creation, and sediment transport; and confinement of rivers between levees, which restricts inundation of adjacent floodplains during high flow events. In addi- tion, dams also physically block the flow of sediment, which starves riverine and estuarine wetlands and marshes of the materials they need to sustain (and restore) themselves. INDICATOR This analysis uses two indica- tors to measure and evaluate the frequency, magnitude and duration of ecologically im- portant high volume flow or flood events. The first indicator measures floodplain inundation in terms of seasonal inflows to the Delta from the Yolo Bypass, the large, partially managed floodplain immediately upstream of the