Hydrologic Setting

Figure 1: Map of the San Fernando Aquifer regional extent (ULARA 2021).

Regional Hydrogeology

The valley is drained by the Los Angeles River and its tributaries. Groundwater flows generally from the edges of the basin toward the middle of the basin, then beneath the Los Angeles River Narrows into the Central Subbasin of the Coastal Plain of Los Angeles Basin (CDWR, 2004). The water-bearing sediments consist of the lower Pleistocene Saugus Formation, Pleistocene and Holocene age alluvium (CSWRB, 1962).

Regional Geology

The San Fernando Aquifer (Figure 1) is below the San Fernando Valley (Figure 2), within the Transverse Ranges Geomorphic Province. This valley is bounded on the east and northeast by the San Rafael Hills, Verdugo Mountains, and San Gabriel Mountains; on the north by the San Gabriel Mountains and the eroded south limb of the Little Tujunga Syncline, which separates it from the Sylmar Basin; on the northwest and west by the Santa Susana Mountains and Simi Hills; and on the south by the Santa Monica Mountains. The San Fernando Valley is a thick alluvial, broad syncline with bedrock deeper than 500 feet bgs over a large portion of the valley. The San Fernando Valley is composed of primarily Miocene and Pliocene sediments, and the thickness of the Quaternary alluvial sediments increase to the south (DPW, 1934).

Figure 2: Approximate geographical location, image sourced from: https://www.britannica.com/place/San-Fernando-Valley

Regional Climate

The San Fernando Valley has a subtropical Mediterranean climate. The increase in the frequency and severity of droughts in the region have caused groundwater levels to fall over time. This can be seen across several sites across the San Fernando Valley (GE&R 2024). Figure 3 below shows the trends in temperature and precipitation over the last 30 years.

Figure 3: Average Temperature and Precipitation tables and graphs for the San Fernando Valley region. Data from NOAA 2025, image sources from: https://www.climatestotravel.com/climate/united-states/san-fernando

Recharge

As mentioned above, droughts are becoming more frequent and severe. The natural recharge from rainfall and the surrounding mountains is not enough to keep the aquifer full and so artificial recharge is needed (LADWP 2016). This artificial recharge is done through spreading grounds such as the Hansen Spreading Grounds where water is poured on permeable land (LADWP 2021). There are also programs such as the Los Angeles Groundwater Replenishment Program (LAGWR) that are aiming to advance water purification techniques so that we can better use our own groundwater. The cost of importing water is very high, so investments into treating our contaminated water are crucial (LADWP 2024).

Surface Water

Figure 4: Image showing general location of the Los Angeles River and its tributaries along the San Fernando Aquifer. Image sourced from: https://en.wikipedia.org/wiki/Los_Angeles_River#/media/File:LARmap.jpg

The man-made Los Angeles River is the only notable surface water feature above the San Fernando Groundwater Basin. The Pacoima and Tujunga Wash feed into the Los Angeles River (Figure 4). As the river is man-made, it is entirely made of concrete and with little precipitation as it is, there is not much that makes it back into the aquifer unintentionally.

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