page_title: Droughts in California
section_title: Reservoir capacity reserved for flood control
hierarchical_section_title: Droughts in California / Causes / Reservoir capacity reserved for flood control
caption_attribution_description: Folsom Lake January 9th 2014
context_page_description: Throughout history, California has experienced many droughts, such as 1841, 1864, 1924, 1928–1935, 1947–1950, 1959–1960, 1976–1977, 1986–1992, 2006–2010, and 2011–2019. As the most populous state in the United States and a major agricultural producer, drought in California can have a severe economic as well as environmental impact. Drought may be due solely to, or found in combination with, weather conditions; economic or political actions; or population and farming.
context_section_description: Most of California's major reservoirs serve important flood control functions. Due to the limited capacity of river channels and dam spillways, reservoirs cannot be quickly drained before major storms. This limits how much of a reservoir's capacity can be used for long-term storage. Reservoirs in California are designed to control either rain floods, snowmelt floods or both.
  In the coastal and southern parts of the state, and much of the Sacramento River system, the primary threat is rain floods in the November–April wet season. Oceanic "atmospheric river" or Pineapple Express storms can generate massive precipitation in a short period (often up to 50 percent of the total annual rainfall in just a few storms). This requires a certain safety margin to be maintained in reservoirs, which are often not allowed to capacity until late April or May. Shasta Lake, California's largest reservoir, is limited to approximately 71 percent of capacity in the winter in order to control rain flooding. Levees along Northern California rivers, such as the Sacramento and American rivers, are quite generously sized in order to pass large volumes of floodwater.
  In the San Joaquin River basin (San Joaquin Valley) and other areas of the state where snowpack is the primary source of river flow, river channels are sized mainly to control snowmelt floods, which do not produce the huge peaks typical of rain floods, but are longer in duration and have a much higher total volume. As a result, reservoirs in this region have very strict restrictions on the amount of water that can be released. An example of a reservoir operated for snow floods is Pine Flat Lake near Fresno, which is restricted to about 53 percent capacity well into spring in order to capture summer snowmelt. However, Pine Flat and other San Joaquin reservoirs are frequently ineffectual in controlling rain floods, because they cannot release water fast enough between winter storms.
  Certain parts of the state, especially in the central Sierra Nevada, are prone to both rain and snow floods. Reservoirs such as Lake Oroville and Folsom Lake must respond to a wider range of runoff conditions. Lake Oroville is typically limited to 79–89 percent of capacity during the winter and Folsom Lake to 33–60 percent. These values are often adjusted up and down based on the amount of rain and snow forecast. At Folsom Lake, due to the small size of the reservoir, it is difficult to balance the need for winter flood-control space with the need to store water for the summer. This often results in a failure to fill the lake due to a lower than expected spring snowmelt. Water managers and hydrology experts have criticized the outdated, overly conservative operation criteria at Folsom Dam, citing improved weather forecasting and snowpack measurement technology.
  Progress in forecasting methods has allowed more efficient or "smart" operation at certain California reservoirs, such as Lake Mendocino. If dry weather is forecast, water is allowed to be stored above the legal flood control limit, rather than being wasted downstream. This program is known as "Forecast Informed Reservoir Operations". In addition, capital improvements such as the $900 million spillway project at Folsom Dam will allow greater flexibility in water releases, making it safer to maintain a high reservoir level during the wet season.
caption: Dry boat ramp at Folsom Lake, January 2014
exif: {"Image ImageDescription": "GEDSC DIGITAL CAMERA", "Image Make": "GENERAL IMAGING CO.", "Image Model": "X400", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Version 1.00", "Image DateTime": "2014:01:09 14:23:08", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "290", "Image PrintIM": "[80, 114, 105, 110, 116, 73, 77, 0, 48, 50, 53, 48, 0, 0, 2, 0, 2, 0, 1, 0, 0, 0, 1, 1, 0, 0, 0, 0]", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail Orientation": "Horizontal (normal)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "1332", "Thumbnail JPEGInterchangeFormatLength": "10065", "Thumbnail YCbCrPositioning": "Co-sited", "EXIF ExposureTime": "1/550", "EXIF FNumber": "33/5", "EXIF ExposureProgram": "Program Normal", "EXIF ISOSpeedRatings": "80", "EXIF ExifVersion": "0220", "EXIF DateTimeOriginal": "2014:01:09 14:23:08", "EXIF DateTimeDigitized": "2014:01:09 14:23:08", "EXIF ComponentsConfiguration": "YCbCr", "EXIF ShutterSpeedValue": "226/25", "EXIF ApertureValue": "11/2", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "317/100", "EXIF MeteringMode": "Average", "EXIF LightSource": "Unknown", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "49/10", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "4320", "EXIF ExifImageLength": "3240", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "EXIF InteroperabilityOffset": "1184", "EXIF FocalPlaneXResolution": "6129", "EXIF FocalPlaneYResolution": "6129", "EXIF SensingMethod": "One-chip color area", "EXIF FileSource": "Digital Camera", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF DigitalZoomRatio": "0", "EXIF SceneCaptureType": "Standard", "EXIF GainControl": "None", "EXIF Tag 0xA40D": "0", "EXIF Tag 0xA442": "b'tw`\\x80'"}