A watershed, also called a drainage basin or catchment, is an area of land that channels rainfall and snowmelt to an outflow point, such as a bay or reservoir. Watersheds vary greatly in size and are categorized into several levels, called hydrologic units, which are nested within each other. This maps shows watersheds in and around Los Angeles County at the HU8 (sub-basin) level.
In addition to surface water (lakes, streams, wetlands, etc.), watersheds also include groundwater resources. Precipitation that is able to infiltrate the soil becomes groundwater instead of flowing out to the ocean or a reservoir. Groundwater may eventually join a nearby stream, but some of it may also seep deeper into underground reservoirs called aquifers. Aquifers are a valuable source of fresh water, especially in Los Angeles. However, they do require recharging, and in developed areas, impervious surfaces like concrete and asphalt prevent water from soaking into the ground and replenishing aquifers.
Data source: HU8 boundaries, streams, artificial paths, canals/ditches, and waterbodies from the National Hydrography Dataset