LANDSLIDES IN THE LAKE IDAHO GLENNS FERRY FORMATION

The Hagerman Valley area is the landslide capitol of Idaho with over 25 landslides identified. Land-use change has coincided with the development of perched aquifer systems, which are the causative agent for six large slope failures since 1979 within the Hagerman Fossil Beds National Monument and three others on Bureau of Land Management property. They are damaging natural resources and private property. In 1987, a million-dollar water diversion facility (pump station) was obliterated by a landslide and two workers had to run for their lives, not having time to get into their vehicle and drive to safety. Numerous studies and monitoring programs have been implemented since 1984 to gain an understanding of the dynamic ground water flow systems and the related massive landslides within the Glenns Ferry Formation.

Background Snake River water is pumped onto the Bruneau Plateau and distributed by canal systems to crops. Irrigation water percolates down through the unconsolidated sediments from canals, ponds and fields to form shallow perched ground water systems. The perched aquifers and slope failures are directly related to the land-use change and development of Bell Rapids Irrigation District in 1970 on the plateau adjacent to Hagerman Fossil Beds National Monument (Young 1984; Summer 1986; Riedel 1992; Vector 1994). The slope failures are located on the hillside of the Bruneau Plateau along the Snake River. They typically range in size from 300 to 800 feet wide and up to 1000 feet long and have occurred about every two years since 1979. In 1987, a slope failure destroyed a million-dollar irrigation pumping facility and nearly killed two workers. In November 2003, a bridge across the Snake River was closed due to a landslide affecting its stability. A conceptual hydrostratigraphic model was developed in an attempt to define the aquifer systems in the Lake Idaho Glenns Ferry Formation. The six-layer model provides clarification for the spatial distribution of perched aquifer discharge patterns on the hillsides and for identification of variable recharge areas to each system; which is needed to aid a mitigative plan to abate canal leakage.