The city of Sacramento is located northeast of San Francisco Bay in California at the confluence of the American and Sacramento Rivers. The city's location puts it among the U.S. cities most vulnerable to significant flooding. Potential flood losses grow as development in the area places more lives and properties in harm's way. As illustrated by disasters like Hurricane Katrina, flood damage reduction infrastructure cannot protect all areas, control all floods, and be completely reliable. To reduce flooding risks in Sacramento, local, state, and federal entities have built dams, levees, and other structures, including the federally constructed Folsom Dam on the American River. These entities currently are studying and pursuing ways to improve the reliability, capacity, and operations of the existing infrastructure as well as construction activities to modify and build flood damage reduction infrastructure. Whether and how to combine nonstructural methods (e.g., building restrictions and codes, insurance premiums) and structural methods (e.g., levee strengthening, dam modification, new dam construction) for managing flood risks is the subject of some dispute among stakeholders. Following a significant flood threat in 1986, Congress in 1987 authorized the U.S. Army Corps of Engineers (Corps) to study additional flood damage reduction measures (e.g., dam and levee improvements, construction of new structures, adoption of operational improvements). Since then, Congress has authorized and appropriated funding for studies and construction of specific flood damage reduction measures. This report outlines the status of these studies and measures, with particular attention to measures at Folsom Dam. Sacramento has historically been prone to flooding. As shown in Figure 1 , the American River descends the Sierra Nevada crest from the northeast down to the city of Sacramento, where it meets the largest river in California, the Sacramento River. On occasion, warm and wet West Coast storm patterns deliver rain in the nearby mountains, which can create very large flows on the American River; the American River water then combines with the formidable flows of the Sacramento River, producing a high flood threat to the greater Sacramento area. Sacramento historically suffered significant damage during these storms. Soon after the city's founding in 1839, local efforts were undertaken to reduce the city's flood damages. A complex set of levees, dams, and related facilities were built near and within the city on both the Sacramento and American Rivers. Levees were built to keep flood waters confined to the river, and out of the floodplain where the city is located. Figure 1 shows levees lining both sides of the American River from its intersection with the Sacramento River upstream for 17 miles. Levees also completely surround the Natomas Basin, a historically agricultural area just north of Sacramento and east of the Sacramento River. These levees work in combination with Folsom Dam, which operates to capture flood waters and for other purposes (e.g., hydropower, irrigation, and municipal/industrial uses). Congress authorized construction of Folsom Dam 29 miles northeast of Sacramento, at the confluence of the North and South Forks of the American River (shown in Figure 1 ), in the Flood Control Act of 1944 (P.L. 78-534). The Corps completed construction of the 340-foot high structure in 1956. The dam was designed to regulate floodwaters by capturing heavy inflows from the upper American River watershed in the dam's reservoir. After construction, dam operations were transferred to the Bureau of Reclamation (Bureau) as part of the Central Valley Project. Other major multipurpose dams considered along the American River included Auburn Dam on the American River (see Figure 1 ). After decades of study by state and federal agencies, a dam at the Auburn site and substantial distribution facilities (commonly known as the Auburn-Folsom South Unit) were authorized in 1965 (P.L. 89-161). Although the primary purpose of the dam as authorized was to provide new and supplemental water supply for irrigation and municipal and industrial needs, another long-sought purpose of the project was to provide flood control benefits for the lower American River. Construction on the dam began in 1965, and was halted in 1975 due to seismic safety concerns. Some stakeholders continue to promote discussion of a dam at the Auburn site as an attractive alternative for managing floodwaters on the American River. Efforts to authorize construction of such a dam were unsuccessful in 1992, 1996, and 1999. (For more information, see " Studies " and " Other Considerations ," below; for some history of Auburn Dam, see out-of-print CRS Report 96-447 ENR, Auburn Dam on the American River: Fact Sheet , by [author name scrubbed], Steve Hughes, and Shelley Price, available upon request from the authors.) Sacramento is facing a problem confronting communities nationwide as they update their flood hazard maps for the National Flood Insurance Program. Local agencies responsible for flood control have to demonstrate that their protection meets the 1% threshold (i.e., a greater than 1% annual probability of a flood). The 1% standard is used for imposing building restrictions and insurance requirements under the National Flood Insurance Program. Sacramento's flood risk has been periodically reevaluated as understanding and factors affecting its components change. Flood risk is the composite of three factors: threat of an event (e.g., probability of flood flows of different sizes affecting the region); consequence of an event (e.g., property damage, loss of life, economic loss, environmental damage, reduced health and safety); and vulnerability that allows a threat to cause consequences (e.g., level of protection provided by levees and dams, and their reliability). In designing Folsom Dam and other flood control projects to reduce Sacramento's vulnerability to flooding, Corps engineers used historic rainfall records, river flows, runoff data, land use information, and statistical tools available at the time. The initial design of Folsom Dam was for a dam with levees to protect against the threat of the largest documented flood in the watershed, which at the time was the flood of 1862; that design was soon adjusted to protect against a higher threat. Large storms in 1955, 1964, 1986, and 1996 produced rainfall in excess of any previous storm on record for the region. The floods in 1950 and 1955 and additional analysis following construction suggested that the dam and levees would provide less protection than originally estimated; in 1961, the Corps lowered its estimate of the city's protection level to protection from a 120-year event (i.e., a storm creating floodwaters that have a 0.83% annual probability of occurring). Then in 1986 (and again in 1996), the volume of flood waters came within 90% of Folsom Dam's flood operation capacity. The 1986 storm produced record inflows into Folsom Dam's reservoir, resulting in dam operators releasing floodwaters into the American River at a rate exceeding 115,000 cfs (cubic feet per second), which is the safe conveyance capacity for outflow on the river's channel below the dam. Portions of the city were nearly flooded as the American River came within inches of overtopping the levees; a major disaster for threatened areas was avoided only by abating storm conditions. A subsequent National Research Council report concluded that operational carelessness led to errors in dam operation during the 1986 flood that contributed to the flood threat that the city was exposed to. Recent studies using a more comprehensive picture of the city's flood risk place the city's flood protection at less than the 100-year level (i.e., a greater than 1% annual probability of a flood affecting the city). This revised estimate of protection is based on an improved understanding of the city's vulnerability that considers both the level of protection provided by levees and dams and the reliability of those structures. In particular, decreased confidence in levee reliability contributes to higher flood vulnerability estimates (i.e., lower estimates of the level of flood protection); confidence that levees can perform up to their full design capacity has decreased in the wake of levee failures in the region and weaknesses (using current standards) identified in levee construction and foundations. Some observers raise additional concerns about the city's level of protection; they note that storm and climate variability, as well as runoff patterns that can result from land use changes such as conversion of agricultural land to residential and urban land uses, may contribute to a higher flood threat than is currently assumed. Large storms could have a particularly catastrophic impact on Sacramento. A decade-old estimate of damages from an over 500,000 cfs peak inflow into Folsom Dam reservoir (400-year flood, or 0.25% modeled annual likelihood) indicated that Sacramento would suffer $16 billion in residential, commercial, industrial, and public property damage, in addition to the disruption of government and transportation networks, and the loss of lives. Recent growth in the Sacramento area may increase flooding damages. Following the 1986 storm, the Corps, the California Reclamation Board, and the Sacramento Area Flood Control Agency (SAFCA) formed a partnership to find ways to reduce flood vulnerability and losses. Since 1992, Congress has authorized construction of physical modification projects to improve flood protection around Sacramento and Folsom Dam, including (1) the Common Features Project that consists of levee improvements on the American and Sacramento Rivers; (2) the Folsom Dam Modification that entails changes to the flood gates and spillway of Folsom Dam; and (3) the Folsom Dam Raise that elevates the concrete and earth portions of the dam, provides for the construction of a permanent bridge, and authorizes other related measures. Construction of the Common Features, the Dam Modifications, and the Dam Raise as currently planned would raise the flood protection for Sacramento to a 1 in 233-year flood (0.4% annual chance of flooding). These projects would improve levee reliability and permit higher releases from Folsom Dam. The non-federal partners for these projects are the State of California and the Sacramento Area Flood Control Agency.