Description of the Problem

Perched ground water aquifers are causing slope stability problems within the Hagerman Fossil Beds National Monument located in northwestern Twin Falls County, Idaho. 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, workers were starting the Bell Rapids irrigation pumps in preparation for the irrigation season when a large slope failure occurred, completely destroying the station while the workers escaped on foot. Snake River water is pumped onto the Bruneau Plateau and distributed by canal systems to agricultural crops. Irrigation water percolates down through the unconsolidated sediments and forms shallow perched ground water systems. Historical photos from paleontological expeditions during the 1930's to present support initial aquifer development and discharge during the 1970's. The perched aquifers and slope failures are related to the development of Bell Rapids Irrigation District in 1970 on the plateau adjacent to monument property (Young 1984, Summer 1986, Riedel 1992, Vector 1994).

Many studies have addressed the slope stability problem (Table 1) and the following paragraphs provide a summary of the contribution from each study. In 1984 the first investigation of the slope stability problem was completed by Young (1984) with the United States Geological Survey (U.S.G.S.) in cooperation with the Bureau of Land Management (B.L.M.). This study provided quantitative data for canal leakage and perched aquifer discharge which set in motion a series of succeeding studies for the next eleven years. A detailed discussion of Young (1984) is provided in Appendix C.

Table 1. Chronological listing of studies.

In 1985 a second investigation was performed by a private consultant for the Bell Rapids Irrigation District. Worstell (1985) measured canal leakage using a different technique than the U.S.G.S.. This data shows an order of magnitude less water leaking from the canal than the previous study by Young (1984). A detailed discussion of Worstell (1985) is provided in Appendix C.

In 1986 one report was produced and eleven monitor wells were installed. The report by Summers (1986) discussed general geologic and ground water conditions related to the slope stability problems. Eleven monitor wells were installed by the U.S.G.S. which was contracted by the B.L.M.. This marked the first ground water data collection for the perched aquifers.

In October 1987 a third canal leakage study was conducted by Montgomery (1988) by constructing a water impoundment in the first one-half mile of the Fossil Gulch Canal. This study provided a canal leakage value similar to Worstells' (1985) reported value. A detailed discussion of Montgomery (1988) is provided in Appendix C.

In spring 1988 the first one-half mile of the Fossil Gulch Canal was straightened and lined with concrete. The "old' segment of canal, where previous leakage tests had been performed (Young, 1984; Worstell, 1985; Montgomery, 1988) was filled with excavated material from the newly aligned segment located just a few hundred feet to the north.

In 1992 a detailed site characterization report was produced by Riedel (1992) and a canal/pond leakage test was performed by Martin (1992). Martin also performed a leakage test on the Fossil Gulch Pond. The Fossil Gulch Canal leakage test was performed on the concrete lined segment as well as two unlined segments immediately downstream separated by earthen dams. The report by Martin is incomplete and there is a detailed discussion of these studies in Appendix C.

In 1993 two investigations were conducted, one by the U.S.G.S. and one by Pelton and Petteys (1993). The U.S.G.S. conducted a photogrammetric analysis that provided quantitative data on volumes of slope failures. Pelton and Petteys (1993) performed a preliminary seismic and magnetic field study used as a basis for an in-depth seismic study in 1994.

The National Park Service (N.P.S.) contracted U.S.G.S. to install six additional monitor wells in 1994. Michaels and Donaldson (1994) performed a seismic refraction investigation identifying the surface of the Shoestring Basalt flow at locations within the plateau. Vector Engineering, Inc. performed surface geoelectric and hydrologic studies that were compiled in a site characterization report, which is discussed in Appendices A and C.

In 1995 reports were produced by Pelton (1995), and Chleborad and Schuster (1995). Pelton analyzed the report produced by Vector Engineering (1994) for correctness and accepted standard methodologies. Chleborad and Schuster produced a report that describes specific procedures for monitoring slope movement and aquifer seepage.

Purpose and Objectives

The purpose of this report is to aid development of a mitigative plan for slope stability problems located at the Hagerman Fossil Beds National Monument. The general objective is to formulate a conceptual hydrostratigraphic model of the study area based on a review and compilation of results from existing studies, recently collected hydrogeologic data and focused field mapping. Specific objectives of the study include:

1. Use available geologic and geophysical data to develop a geologic conceptual model of the site.
2. Use available canal leakage and water level data to develop an understanding of the ground water flow systems.
3. Combine the geologic and hydrologic models to present a general hydrostratigraphic model of the site.
4. Use the hydrostratigraphic model to explain spatial distribution of perched aquifer discharge zones and aid mitigation efforts.

Geographic Setting

The Hagerman Fossil Beds National Monument and study site lie within the eastern portion of the Western Snake River Plain (Figure 1). The monument boundary is illustrated as slanted lines and the general study site area as a rectangle with horizontal lines. The monument is located about 90 miles southeast of Boise and 30 miles downstream of Twin Falls along the Snake River.

West of the town of Hagerman are fossil bearing sediments exposed in a 600 foot high bluff that forms the western wall of the Hagerman Valley. The bluff is part of the Bruneau Plateau with the Snake River located at the base. The river is held at a constant elevation of 2,800 feet by the Lower Salmon Falls hydroelectric dam.

The surface of the bluff has an average elevation of 3,400 feet (600 feet above the river) and it is the northeastern extent of the Bruneau Plateau. The monument boundary effectively traces the edge of the bluff (Figure 1). The deeply incised canyons range from a half to one mile in length and have slope angles commonly exceeding 35° with a few up to 70°. Native vegetation on the bluff is dominated by sagebrush, which grows better on lower slope angles. Non-native vegetation types grow at the perched aquifer discharge zones which include cattails, Tamarisk trees, rye grass, willows, Russian Olive trees and thistle.

The study site covers an area of approximately 10 square miles and includes parts or all of sections 4, 5, 8, 9, 16, 17, T.7S. R.13E., section 31 T.6S. R.13E., sections 35, 36, T.6S. R.12E. and sections 6, 7, T.7S. R.12E., Twin Falls County, Idaho. The general location of the study area is illustrated in Figure 1 and lies primarily on the plateau surface but extends partially down the bluff face where perched aquifers are discharging in the northwest area of the monument.

Figure 1 Location of study site (omitted from on-line edition)

The basis for the study area boundary is to include areas of recharge, discharge, monitor wells and geophysical studies. The northwest/southeast orientation of the study area correlates to the same orientation of the Fossil Gulch Canal. The southeast boundary is located at aquifer discharge areas and the northwest boundary extends to recharge areas ending where the Fossil Gulch Canal enters an aqueduct and crosses a small canyon. The northeast and southwest boundaries extend far enough to enclose locations of monitor wells and geophysical studies.

Table of Contents
Chapter 1 | 2 | 3 | 4 | 5
Appendix A | B | C | D

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