Land Subsidence in the Southwestern Mojave Desert, California, 1992–2009

A USGS study in cooperation with the Mojave Water Agency

Fissures on the southeastern edge of Lucerne Lake, San Bernardino County. Photo: Loren Metzger, USGS, May 2001.

Why the ground is sinking

Groundwater has supplied homes, farms and towns in the southwestern Mojave Desert since the early 1900s. Growing demand lowered water levels more than 100 feet in places between the 1950s and 1990s (Stamos and others, 2001; Sneed and others, 2003). As water levels fall, the aquifer system compacts, and the land subsides.

Uneven subsidence can reroute drainage and damage infrastructure. Fissures across State Route 247 at Lucerne Lake required repairs, and so did pipelines near Troy Lake.

How it was measured

The USGS, with the Mojave Water Agency (MWA), tracked subsidence with satellite radar (InSAR) plus ground measurements, geology and water-level analysis, for 1992–2009, the years with InSAR data. InSAR maps proved an economical way to catch small problems before they grow — across the MWA's nearly 5,000-square-mile management area.

National Agriculture Imagery Program imagery from 2012, with the dry lakebeds and the Mojave Water Agency's management area. USGS.

Five sinking lakebeds

InSAR and field work found subsidence in five localized areas near dry lakebeds:

Only Troy and Lucerne sped up, from the late 1990s or early 2000s — perhaps from changes in farming or a growing population using more water.

The study area, groundwater basins and InSAR subsidence maps of the five lakebeds, with a water-level record near Lucerne Lake. USGS.

What caused it

The subsidence came from falling water levels in fine-grained sediments — clay and silt. Changing climate, tectonics and other factors laid down these compressible old lakebed clays unevenly: wetter times built thicker clay, as at Lucerne Lake; drier times thinner, as at Troy Lake; and the centers of deposition shifted (Motts, 1969).

Lucerne Lake, with the most subsidence:

• water levels in shallow wells fell more than 100 feet to historic lows since the 1950s, while deep wells recovered during 1998–2009;
• 20 to 40 feet of loose clay lies in the shallow aquifer, but none at depth — so compaction is likely shallow and at least partly permanent;
• the fissures, trending northeast–southwest, likely formed from uneven subsidence — more to the southeast than the northwest.

A fissure near Lucerne Lake, along State Route 247. Photo: Michelle Sneed, USGS.

Repairs to State Route 247 after fissures damaged the road. Photo: Michelle Sneed, USGS.

Recoverable or permanent?

• Recoverable: nearly all alluvial aquifers compact and expand slightly as water levels rise and fall.
• Permanent: fine-grained aquifers whose water levels keep falling below historic lows can compact for good (Galloway and others, 1999).
• Concurrent or residual: compaction may stop soon after water levels stabilize, or continue afterward — the thicker the clay, the longer it goes on. The two call for different management.

The five areas:

• Harper, Troy and Lucerne: both kinds of compaction may have occurred, and at least some subsidence is permanent; with water levels still falling, their shares can't be separated. Measuring during periods of recovery would help — subsidence then would be residual.
• El Mirage: water levels likely at or near historic lows, so some compaction may be permanent.
• Coyote: too little historical data to say.

Knowing where water levels fell and clays lie tells managers where to target mitigation; clay thickness sets planning horizons; and the historical low water level marks the threshold for permanent subsidence.

Future monitoring

Water-level declines and subsidence are expected to continue, and have accelerated near Lucerne and Troy Lakes.

• InSAR can map subsidence seasonally, yearly and over several years, and find new subsidence anywhere in the MWA area.
• It can help place extensometers and GPS stations where detail is needed.
• As of 2017, InSAR satellites were routinely collecting data over the area, with more planned.
• Continuous GPS: 13 sites in the study area were collecting position data in 2016.

Sources

Based on Land Subsidence in the Southwestern Mojave Desert, California, 1992–2009, USGS Fact Sheet 2017–3053, U.S. Geological Survey, prepared with the Mojave Water Agency; a work of the United States government in the public domain. It draws on Sneed and others (2003), Stamos and others (2007) and Solt and Sneed (2014). The imagery, map and USGS photographs are taken from the fact sheet's PDF, which the import had left out; a GPS-station photograph by UNAVCO is not reproduced.