Preliminary Report; January 26, 2005
Recent flooding in southwestern Utah extended northward into the Escalante Desert in western Iron County, about 40 miles west of Cedar City. Erosion by floodwaters revealed three earth fissures (ground cracks) near Beryl Junction.
A likely cause of the earth fissures is ground subsidence due to ground-water withdrawal. Ground-water levels in the Escalante Desert have declined steadily and consistently since 1950 due to pumping of large wells for agricultural irrigation.
Monitoring wells in the valley show large declines (often in excess of 100 feet and in one instance 148 feet) with virtually no water-level recovery during periods of above average precipitation.
Pumping and removal of water from ground-water aquifers is a common cause of ground subsidence and earth fissures in the arid southwestern U.S. Removal of ground water from the pore spaces in the valley-fill alluvium (unconsolidated gravel, sand, silt, and clay) reduces pore-water pressure and allows the drained alluvium to compress and the ground surface to subside.
Earth fissures up to 10 miles long have been reported in Arizona. The Las Vegas Valley likewise has also experienced earth fissuring as a result of heavy ground-water pumping.
The earth fissures in the Escalante Desert were initially an inch or less wide, but they intercepted floodwaters that quickly drained into the fissures and eroded their walls.
In places the gullies eroded along the fissures were as much as 9 feet wide and more than 6 feet deep.
The fissures locally showed up to 8 inches of vertical displacement. Local residents reported seeing vortices develop in the floodwater above the fissures as water rapidly entered the fissures.
The three fissures are referred to as the Laub, Beryl Junction, and Holt fissures, of which the Beryl Junction fissure was the longest (approaching 1000 feet long) and the most affected by erosion and infiltration of floodwater.
The Beryl Junction fissure intercepted State Route 56 just east of Beryl Junction, and caused the road surface to displace several inches down on the east side.
Neither the Holt nor Laub fissures encountered roads or other infrastructure.
The location of the earth fissures appears to be related to areas of maximum lowering of the water table by pumping and variations in thickness of the unconsolidated alluvial-fill aquifer.
The Complete Bouguer Gravity Anomaly Map of Utah shows that the Holt and Beryl Junction fissures are located along a steep gravity gradient on the west edge of a northwest trending gravity low east of Beryl Junction. The gravity low indicates that bedrock in that area is particularly deep, and that the valley-fill alluvium there is correspondingly thick.
The orthophotograph shows the location of Laub, Beryl Junction, and Holt earth fissures (red lines) in the Escalante Desert near Beryl Junction. Green lines are contours in milligals from the Complete Bouguer Gravity Anomaly Map of Utah; larger numbers indicate a greater depth to bedrock.
Preliminarily it appears that the earth fissures, which also trend to the northeast, formed over a shallow bedrock high that bounds the edge of the deeper part of the bedrock basin east of Beryl Junction.
As ground water was withdrawn from the alluvium in the deeper part of the basin, the thick alluvium in that area compacted and subsided causing the earth fissures to form as tension cracks at the basin edge. We do not know how long the three fissures existed before being revealed by the floodwaters.
Ground subsidence and resulting earth fissures may cause many problems including (1) changes in elevation and slope of streams, canals, and drains, (2) damage to roads, buildings, railroads, and underground utilities, and (3) failure of well casings from forces generated by compaction of the drained valley-fill alluvium.
Diversion of surface water into earth fissures, which may extend to great depths and reach the water table, may also cause ground-water contamination.
The landowner at the southern end of the Beryl Junction fissure, which extends into an alfalfa field at its south end, noticed that slopes in his field have changed recently, altering surface-water flow directions and causing water to pond in areas where it formerly drained. He also reported that the casing in a well at the south end of the field sheared and failed during the summer of 2004.
The floodwaters that infiltrated and enlarged the Holt ground fissure flowed through a cattle feedlot prior to encountering the fissure.
The feedlot operator reported seeing a vortex above the fissure at the height of the flood, and it is likely that contaminated surface water reached the water table in that area.