4.9 HYDROLOGY and WATER QUALITY Sacramento River
4.9 HYDROLOGY AND WATER QUALITY This section of the DEIR identifies the hydrological resources, the existing drainage conditions, and the surface and groundwater quality in the General Plan Planning Area. This section also evaluates the potential impacts with respect to flooding, drainage, erosion, water quality, and water supply, and identifies appropriate mitigation measures to lessen the identified impacts. 4.9.1 EXISTING SETTING REGIONAL HYDROLOGY Surface watersheds are those land areas that catch rain or snow and drain to specific marshes, streams, rivers, lakes, or the groundwater table. There are several watersheds in Sacramento County, including: the Lower Sacramento, the Lower American, the North Fork of the American, the Upper Butte, the Upper Yuba, the South Fork of the American, and the Upper Cosumnes. The largest watershed in the County is the Lower Sacramento Watershed, which covers approximately 27,000 square miles and drains the Sacramento Valley, the Modoc Plateau, and parts of the Cascade Range and Sierra Nevada Range. The Lower Sacramento Watershed is one of the largest watersheds in the United States and covers most of northern California. The main tributaries in the Lower Sacramento Watershed are the Feather, Yuba, Pit, and American Rivers. The Lower American, North Fork American, South Fork American, and Upper Cosumnes watersheds are adjacent to the Lower Sacramento Watershed. In addition to the natural hydrologic processes of rainfall runoff, snowmelt, and base flow from groundwater, the flows in the Sacramento River are greatly affected by reservoir releases, water diversions, irrigation return flows, and diversions through bypasses. Both the Sutter and Yolo bypasses have the capacity to carry larger volumes of water than the Sacramento River channel, and are used to prevent flooding during wetter years and higher flows.
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