Geology and El Niño in the desert:

Landslides, debris-flows, and flash floods during El Niño events

In Arizona, the largest floods of this century have occurred during El Niño events.

Santa Cruz River from St. Mary's Bridge, Tucson; October 2, about 10 a.m. Caused by 1982 El Niño storm.	Bank erosion along Rillito Creek at Tucson, AZ, October 2. View shows the collapse of the bank at a townhouse development on Country Club Road. Direction of flow is from right to left.

Lakes with no outlet river can rise dramatically during El Niño events. At Great Salt Lake, the Bear River Migratory Bird Refuge first flooded in 1983 and was still under water when this photograph was taken on June 19, 1986. The rise in Great Salt Lake started during the El Niño of 1982-83 and peaked in 1987 during a second El Niño. The lake level rose over 12 feet, causing about $285 million in damage to lakeside industries, transportation, farming, and wildlife habitat by the inundation of freshwater marshes. It disrupted local and interstate transportation. Interstate 80 was intermittently flooded and the roadbed had to be raised. The Union Pacific Railroad elevated its routes across the lake three times to keep the tracks above water. The international airport escaped inundation by only 8 feet. The normally salty lake was diluted, hurting salt and metal extraction industries and reducing populations of brine shrimp and green brine algae while increasing those of blue-green algae and some fish.

Landslides and Debris Flows

Long, intense rainfall triggered thousands of landslides during a 32-hour rainstorm in the San Francisco Bay area in January, 1982. Intense rainfall is one of several triggers for landslides and debris flows. Infiltrating rainwater saturates the soil and fills pores, making the heavy soil slip more readily. As with floods, heavy precipitation may lead to landslides in the Southwest regardless of El Niño timing. Nevertheless, some of the most destructive landslides in this region over the past few decades have occurred during El Niño events.

An especially effective recipe for debris flows is intense rainfall over areas in which vegetation has been destroyed by wildfire. Such fires not only expose loose soil but may also produce a water-repellent (hydrophobic) soil layer; these effects together promote erosion and transport of sediment as debris flows. This pattern, termed the "fire-flood sequence", triggered a number of disastrous debris flows causing loss of life and severe property damage in southern California when large autumn fires were followed by heavy winter storms. (Click on the image for a higher resolution picture [101 kb]).

The Thistle Slide, Utah--April, 1983

The most expensive single landslide in U.S. history was caused by El Niño storms. Unusually heavy precipitation in central Utah during 1982 and 1983 saturated slopes in the Wasatch Mountains and nearby mountains, triggering numerous landslides and debris flows. The largest and most destructive of these was active for a two-week period in April, 1983, near the town of Thistle. Beginning as renewed movement on an old landslide, the Thistle landslide transported about four million cubic meters of rock and mud, filling part of Spanish Fork Canyon and blocking two creeks.
The resulting lake, nearly 5 km long and 60 m deep, inundated a town, three major highways, and an important route of the Denver & Rio Grande Western Railway, and it posed a flooding hazard to downstream communities. The threat was eliminated only after the lake was drained by tunneling through one of the canyon walls. Combined direct and indirect costs from the Thistle landslide exceeded about $400 million. (Click on the image for a higher resolution picture [132 kb]).

Information from USGS Climate Program web site

For further information, contact David Miller

http://geology.wr.usgs.gov/wgmt/elnino/deserten/what.html, 30 November 1999, Contact: El Niño Web Team