Rainfall and Landslides in Northern California

Science Center Objects

A summary of recent and past landslides and debris flows caused by rainfall in northern California, and links to current shallow landslide monitoring.

A soil landslide scar above a neighborhood of houses.

A debris flow in Marin County triggered by intense rainfall destroyed several homes and injured one person during February 2019.  View is looking from the top of the landslide downslope. (Credit: Brian Collins, USGS. Public domain.)

California is well-known to be susceptible to landslides. These range from small, but potentially fatal, shallow landslides that may mobilize into rapidly moving debris flows, to larger, deep-seated landslides that are capable of moving entire houses downslope. Coastal cliff collapses and cliff erosion also present landslide hazards along the coast of northern California, and debris flows from burned areas are a particular concern following wildfires in the region.  Landslides in the state generally occur due to precipitation, and to a lesser extent, earthquakes. Historically, large winter storms have caused the most damage, and in the highly developed San Francisco Bay area in northern California these impacts have sometimes been quite severe, causing both fatalities and significant property damage (for example the January 1982 winter storm - see Landslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California - USGS Professional Paper 1434).  More recently, storms in January and February of 2017 caused widespread landsliding throughout the East Bay region of the San Francisco Bay area and elsewhere.  In Marin County, north of San Francisco, a debris flow destroyed several homes following a particularly intense storm on February 14, 2019 (see Data from the Sausalito, California debris flow).  Elsewhere, in the Sierra Nevada foothills of Tuolumne County, a small but intense storm on March 22, 2018 caused hundreds of landslides that led to severe erosion and sedimentation of the Tuolumne River and a downstream storage reservoir (Don Pedro Reservoir).

The USGS conducts active research on identifying the triggering mechanisms and hazards associated with landsliding. In northern California, research efforts have predominantly focused on the San Francisco Bay area, and a number of products are available that showcase the landslide effects of previous large winter storms on the region, as well as current efforts to identify the soil moisture conditions that make the region susceptible to shallow landslides and debris flows at particular times of the year.  The USGS also works on rock fall hazards in northern California with efforts primarily focused on assisting the National Park Service in Yosemite National Park.

Shallow Landslides and Debris Flows

A hillside showing landslides with debris from one landslide impacting a house at the bottom of the slope.

Shallow landslides damaged homes and closed many roads in the San Francisco East Bay region as a result of storms in January and February, 2017. (Credit: Brian Collins, USGS. Public domain.)

Shallow landslides are generally those less than 3-5 m (10-15 ft) in depth. When shallow landslides are sufficiently wet, they may move rapidly and can be highly mobile over long distances – these are termed debris flows. Whereas shallow landslides can cause property and infrastructure damage, and clog streams and drainages, debris flows are more likely to cause injuries and fatalities through their ability to move quickly and with large amounts of debris (soil, boulders, trees, etc.). Shallow landslides and debris flows are most often generated by intense rainfall, with rainfall rates measured in tens of millimeters per hour (few to several inches per hour).

Shallow landslides can occur at any point during a winter season in the San Francisco Bay area, but most often occur once the ground is nearly saturated – this typically occurs after the first few winter storms come in the November and December timeframe. These conditions, termed the “antecedent moisture conditions” have been correlated with seasonal rainfall and maps are available based on previous storms (see Landslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California - USGS Professional Paper 1434 Plate 1).

Generally, researchers have found that at least 25 cm (10 in) of rainfall is needed to nearly saturate the ground on San Francisco Bay area hillslopes. Because antecedent conditions do not account for drying and evaporation between storms, USGS researchers have been developing methods to monitor the soil moisture of landslide prone hillslopes directly. This research project aims to quantify the soil moisture conditions responsible for shallow landslides and consequent debris flows.

It is often hard to identify exactly where shallow landslides and debris flows will occur. Instead, researchers use models to develop debris flow susceptibility maps that are, in turn, based on measurements of soil type and depth, and topographic slope and shape. Debris flow source area maps based on measured topographic parameters are available for the entire San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part E – Map of debris-flow source areas in the San Francisco Bay Region, California - USGS Open File Report 97-745E) – these indicate areas with hazard potential for debris flows should seasonal cumulative rainfall and storm rainfall intensity thresholds be exceeded. Storm rainfall thresholds have also been calculated for the entire San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part F – Preliminary maps showing rainfall thresholds for debris-flow activity, San Francisco Bay Region, California - USGS Open File Report 97-745F).

Soil Moisture Monitoring for Shallow Landslide Initiation in the San Francisco Bay Area

The USGS conducts active monitoring of the soil moisture in several locations throughout the San Francisco Bay area.  These data provide an indication of the “soil wetness” (that is, the antecedent soil moisiture conditions) that may lead to widespread shallow landsliding and potentially debris flows if a storm of sufficient intensity impacts the region.  If the soil moisture is low, even high rainfall intensity may not lead to landslides.  On the other hand, high soil moisture and high rainfall intensity are more likely to cause widespread landsliding with consequences such as road closures, property damage, and potential fatalities.  Residents concerned with the potential for landslide hazards where they live can find helpful information here.

Note: Soil moisture monitoring sites provide highly site-specific information and do not necessarily exemplify the regional conditions.

Deep-seated Landslides

A deep-seated landslide in the Santa Cruz mountains of the San Francisco Bay area during April 1998.

A deep-seated landslide in the Santa Cruz mountains of the San Francisco Bay area during April 1998. Multiple scarps (sliding planes) are visible with the trees tilted backward and the house tilted forwards. (Credit: Robert Schuster, USGS. Public domain.)

Deep-seated landslides are generally those greater than 3-5 m (10-15 ft) in depth. These landslides are often generated by prolonged above-average rainfall which can occur during El Niño years, although even “normal” precipitation years in northern California can lead to landslide initiation. Typically, deep-seated landslides occur towards the end of the winter season (March, April, May) due to the time it takes for seasonal rainfall to reach the bottom “slip surface” of the landslide. However, heavy rain earlier in the season can also have this effect. Often, deep-seated landslides lay dormant for lengthy periods of time.

Many deep-seated landslides have been mapped in the San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part C – Summary distribution of slides and earth flows in the San Francisco Bay Region, California - USGS Open File Report 97-745C), and often a qualified geotechnical engineer or engineering geologist may be able to determine if particular properties are on a landslide or susceptible to landslide movement. In 1997, the USGS developed a simple way to determine the locations of landslides that have occurred in the past throughout the San Francisco Bay Area by referencing published topographic quadrangle maps (see San Francisco Bay Region Landslide Folio Part D – Index to detailed maps of landslides in the San Francisco Bay Region, California - USGS Open File Report 97-745D).  The California Geological Survey currently tracks the location of deep-seated landslides using an inventory.

Generally, deep-seated landslides do not cause injuries or fatalities; rather, they move slowly and can severely distort and damage buildings and infrastructure such as roads and pipelines. Sometimes, smaller shallow landslides can initiate from the bottom (or toe) of deep-seated landslides. In these cases, the hazard of the shallow landslides turning into debris flows should be assessed.

Recently Burned Areas

Debris flow from steep slopes in the Big Sur area of central California

Debris flow from steep slopes in the Big Sur area of central California that resulted from rains following the 2008 Basin Complex fire. With little vegetation to hold soil in place, channels may fill with sediment that can then be triggered into debris flows by heavy rainfall. (Credit: Kevin Schmidt, USGS. Public domain.)

Steep, recently burned areas are especially susceptible to debris flows. Even modest rain storms during normal, non-El Niño years can trigger post-wildfire debris flows. In many respects, the hazards associated with debris flows from burned areas are similar to those from shallow landslides, although the precipitation thresholds for initiation are often lower. In addition, there is typically no antecedent soil moisture condition required for initiation.  The USGS has conducted hazard assessments for post-wildfire debris flows for recent fires in northern California, as well as numerous fires across the Western U.S. including southern California.

Coastal Cliff Erosion

Many areas of coastal California are subject to cliff erosion and coastal landslides. Hazards from these types of landslides can occur both at the bottom of cliffs (from burial) and at the tops of cliffs (from falling over). During the winter season in California, beaches typically erode thereby allowing waves to reach further inland and to inundate the bottoms of coastal cliffs. Wave energy is also typically higher during the winter, and particularly during El Niño events, thereby exacerbating the potential for coastal erosion. Coastal cliff failures may also occur simply as a result of heightened precipitation – wave action makes cliffs inherently unstable, and rainfall may be the ultimate trigger for failure, even during times with little to no wave action.

During and just after storms, existing coastal landslides may become reactivated and seemingly stable coastal cliffs may erode and fail rapidly. Background rates of coastal cliff erosion are variable along the California coast (see National Assessment of Shoreline Change Part 4: Historical Coastal Cliff Retreat along the California Coast - USGS Open File Report 2007-1133) and tied to the rock or soil strength of the cliffs among other factors, but these measurements of historic coastal cliff retreat provide indications of places most susceptible to coastal landslides.

Coastal Cliffs are subject to wave action as well as precipitation induced seepage.

Coastal cliffs are subject to wave action as well as precipitation-induced seepage. These examples from northern California showcase several different styles of failure. (Credit: Brian Collins, USGS. Public domain.)

Mapping of Effects of Previous Landslide-Triggering Storms in the San Francisco Bay Area

(last updated June 16, 2020)