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California Water Today 103 power makes it particularly valuable for meeting peak summertime demands. This resource will diminish if California's climate becomes drier,as less stream flow means less fuel for hydroelectric power plants.29 Hydropower management also has major implications for ecosystem health, because of the disruptions caused by dams and flow alterations to the aquatic environment(Chapter 5). Flood Vulnerability and Flood Management Infrastructure Protecting people and businesses from flooding has been a long-standing con- cern of California water management(Chapter 1).The current system of flood management infrastructure includes surface reservoirs (many of which also provide water supply storage), levees, and flood bypasses (Figure 2.13). This infrastructure is used in conjunction with land use regulations,insurance,and warning systems(Chapter 6). Levees, the most common tool, attempt to limit the area of flooding by containing flows with embankments. Because levees are managed by many diverse public agencies and private individuals, no comprehensive statewide levee inventory exists. The Central Valley alone has as many as 6,000 miles of levees. The Sacramento-San Joaquin Delta and the federally authorized Sacramento-San Joaquin Flood Control projects together have about 2,700 miles of levees.In the Sacramento Valley,levees are supplemented by a system of flood bypasses established in the early 20th century. The bypasses are large areas of seasonal farmland and habitat, bounded by levees, which essentially create a second Sacramento River to accommodate large floods. Upstream reservoirs also help manage floods by storing water to reduce flood peaks that must be accommodated downstream by levees and bypasses. In 2000, almost 5 percent of California's households were living in what is known as the 100-year" floodplain—an area susceptible to more frequent floods, where land use is regulated by federal flood policy and where federal flood insurance is required(Chapter 6).30 Another 12.5 percent of households lived in the "500-year" floodplain, an area susceptible to larger, less frequent floods that have a 0.2 percent or more chance of occurring in any given year. 29. The adaptability of hydropower to changes in climate and water management purposes has been widely examined (Jacobs et al.1995;Madani and Lund 2009,2010;Tanaka et al.2006;Vicuna et al.2008). 30. Authors'calculations,using Census 2000 block data for household population and floodplain designations from the Federal Emergency Management Association.