Assessment of Hazards Associated with the 

Bluegill Landslide, South-Central Idaho

By William L. Ellis, Robert L. Schuster, and William H. Schulz

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey 

editorial standards or with the North American Stratigraphic Code.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

U.S. Geological Survey Open-File Report 2004-1054

U.S. Department of the Interior

U.S. Geological Survey
Abstract flows from south to north and merges with the Snake River 

approximately 12 km downstream from the landslide site. 

The Bluegill landslide, located in south-central Idaho, is The landslide occurs predominantly on public property 

part of a larger landslide complex that forms an area in administered by the U.S. Bureau of Land Management 

the Salmon Falls Creek drainage named Sinking Canyon. (BLM), although portions of the landslide are on state and 

Recent movement of the Bluegill landslide, apparently private property. Recent movement of the landslide was 

beginning sometime in late 1998 or early 1999, has caused first brought to the attention of BLM officials in 1999 when 

a 4.5 ha area of the canyon rim to drop as much as 8 m and recreational rock climbers noted the formation and widening 

move horizontally several meters into the canyon. Upward of fractures in the basalt cliffs that make up the canyon rim. 

movement of the toe of the landslide in the bottom of the Since that time, the landslide has been monitored by BLM, 

canyon has created a dam that impounds a lake approximately with assistance provided by the U.S. Geological Survey 

2 km in length. The landslide is on public property (USGS) and by investigators from Idaho State University.

administered by the U.S. Bureau of Land Management 

(BLM). As part of ongoing efforts to address possible public Since late 1999, the landslide has moved several meters in 

safety concerns, the BLM requested that the U.S. Geological what appears to be a classic rotational type of slope failure 

Survey (USGS) conduct a preliminary hazard assessment (Varnes, 1978). The toe of the landslide has moved both 

of the landslide, examine possible mitigation options, and horizontally and upward about equal amounts, the central 

identify alternatives for further study and monitoring of the portion has moved primarily horizontally in the downslope 

landslide. This report presents the findings of that assessment direction, and the head of the slide, which includes a portion 

based on a field reconnaissance of the landslide on September of the canyon rim, has moved downward and horizontally 

24, 2003, a review of data and information provided by BLM about equal amounts. The upward component of motion at 

the toe of the slide has formed a landslide dam in Salmon and researchers from Idaho State University, and information Falls Creek, which has impounded a lake estimated to collected from other sources. contain about 325,000 m3

 of water. Other estimates place the 

lake volume as high as 1,000,000 m3 (Chadwick and others, 

Introduction in press).

In June 2003, the USGS was contacted by a representative The Bluegill landslide (fig. 1), also referred to as the Salmon of the BLM and requested to provide an evaluation and Falls landslide, is located approximately 35 km west of Twin assessment of the hazards posed by the Bluegill landslide. Falls, Idaho, and about 10 km west of the town of Buhl, The BLM was primarily concerned about the potential Idaho (fig. 2). The landslide is on the east rim of a 120-m- hazard posed by possible failure of the landslide dam. In deep canyon formed by Salmon Falls Creek (fig. 3), which response to this request, the authors of this report visited 

Figure 1. Oblique aerial photo of Sinking Canyon taken in March 2001, showing Sinking 

Canyon, Salmon Falls Creek, and Bluegill landslide. View is to the north. Approximate 

boundaries of the Bluegill landslide are shown by dashed line. Confluence of Salmon 

Falls Creek with the Snake River is approximately 10 km north of the landslide site. 

Photo courtesy of Karen Shilling, BLM.

2

the landslide site on September 

24, 2003. During the site visit, 

they were accompanied by Karen 

Shilling and Forrest Griggs of 

the BLM, and Glenn Thackray 

and Stephen Dorsch from Idaho 

State University. Investigators 

from Idaho State University have 

been conducting remote-sensing 

studies of the landslide over 

the past year. They have also 

taken over periodic surveying 

of landslide movement from the 

BLM and are in the process of 

doing detailed mapping of the 

landslide. The site inspection also 

included Dr. Robert Holtz from 

Washington State University 

who was retained by BLM to 

provide a separate assessment of 

the landslide situation. This report 

presents the findings of the USGS 

authors based on the site visit and 
review of other available data and information concerning 

the landslide.

Much of the information contained in this report was made 

available by the BLM, and is presented here in the same 

form as provided to the authors. Other information and 

data were obtained from World Wide Web sites. These 

diverse sources of information and the desire to present 

information in its original form have resulted in use of both 

metric and English units in this report. Metric conversions 

are provided where data plots are presented in English 

units.

Description of Landslide

The Bluegill landslide occurs on the southern end of a 

relatively wide section of the Salmon Falls Creek canyon, 

named Sinking Canyon (fig. 3). Sinking Canyon has 

been formed by progressive failure of the eastern wall 

of the canyon, which resulted in widening of the canyon, and 

deposition of pre-historic landslide deposits that now cover the 

canyon floor. The last major landslide movements in Sinking 

Canyon apparently occurred in the summer of 1937 when 

several acres of farmland were lost over a period of several days 

by “sinking” (i.e., landslide movement) into the existing canyon. 

An examination of early newspaper accounts and a scientific 

paper by Lee (1938) indicate that the area of the canyon rim 

lost to landslide activity was relatively small compared to the 

canyon width, and that landslide movement caused adjustments 

of the stream channel and at least one small landslide dam. 

The 1937 Sinking Canyon episode was likely a continuation of 

mass-wasting processes that have occurred in this area since the 

inception of the canyon. The Bluegill landslide is the southern 

extension of the Sinking Canyon landslide complex. Old 

fractures located behind the current Bluegill landslide headscarp 

indicate that some extensional deformation occurred in this area 

in the past. These fractures may have formed at the time of the 

1937 landsliding, but without the large amount of movement that 

took place just to the north of the present landslide 

30

91

20

26 33

26 26 20

93

93

75

21

95

55

95

12

95

95

200

93

2

N

15

15

86

84

84

90

84

Twin Falls

Buhl

Bluegill

landslide

43�N

47�N

area.

There has been some speculation that an earthquake 

felt in the Buhl area in 1934 (The Buhl Herald, 

March 15, 1934) may have produced fracturing 

behind the canyon rim that weakened the canyon 

wall and ultimately resulted in the 1937 landsliding 

episode. That earthquake is listed in the Utah historic 

earthquake catalogue (www.seis.utah.edu/Reports/

UtahHistoricEqCat1850-1962) as a magnitude 6.6 

with the epicenter in north central Utah. Although 

the earthquake was felt in the Buhl, Idaho, area, little 

damage was reported and there is no direct evidence 

to link the earthquake to later landsliding activity. 

The Bluegill landslide consists of an approximately 

4.5 ha area of the upper canyon rim, and a larger 

portion of uncertain size within the older landslide 

deposits that cover the lower part of the canyon 

(fig. 4). The southern lateral boundary of the active 

slide appears to be expressed as a series of stepping 

extensional fractures that extend from the base of 

the basalt cliff at the head of the slide to the bank of 

Salmon Falls Creek. The northern lateral boundary 

of the slide is not clearly discernible because basalt 

boulders and unconsolidated material cover the 

slope. The northern boundary depicted on figure 4 

43�N is speculative and is based on limited observation 

of disrupted and fractured slide debris in a few 

locations.

The overall movement of the slide has resulted in 

115�W 112�W down dropping and westward movement of the 

canyon rim, and westward and upward motion at the 200 km toe of the slide in the canyon bottom. The maximum 

downward movement of the canyon rim is at the Figure 2. Index map of Idaho. Star shows location the Bluegill landslide. north end of the landslide, with displacement 

3
Figure 3. Topographic map showing Sinking Canyon and Salmon Falls Creek (Balanced 

Rock 7.5-minute Quadrangle, USGS, 1987).

4
Figure 4. Map showing approximate boundaries of Bluegill landslide based on 24 September 2003 field reconnaissance and 

monitoring results. Base map consists of the USGS Balanced Rock topographic quadrangle dated 1 July 1987, superimposed 

on a vertical aerial photograph taken 16 May 1992. Note that Bluegill landslide dam and impounded lake were not present 

on date of the photograph.

decreasing to the south. Thus, the landslide headscarp exhibits 

a scissor type of motion. The upward movement of the 

landslide toe has resulted in the formation of a landslide dam 

that currently impounds a lake approximately 2 km long by 40 

m wide (Chadwick and others, in press). The toe of the slide 

has also encroached on Salmon Falls Creek downstream of 

the dam, and in places has caused realignment of the stream 

channel and minor impoundments of water.

According to the landslide classification system of Varnes 

(1978) and Cruden and Varnes (1996), the Bluegill landslide 

can probably best be classified as a rock slump or rock slide. 

The basalt cliffs in the area are also subject to local rock fall. 

Local Geology

The Sinking Canyon area is located within the Owyhee 

Plateau near its northeastern boundary with the eastern Snake 

River Plain. Malde (1991) describes the Owyhee Plateau as 

5

an upland volcanic terrain consisting of rhyolite that is 

thinly veneered with basalt. 

A reconnaissance geologic map of the west-central 

Snake River Plain (Malde and others, 1963) identifies 

the basalt unit that forms the canyon rim at the Bluegill 

landslide site as the upper part of the Banbury Basalt 

(fig. 5). The underlying geologic unit is mapped as the 

middle part of the Banbury Basalt, and is described as 

consisting largely of sand and pebble gravel in lenticular 

channel deposits, but including light-colored silt, clay, 

and diatomite in lake deposits. Just to the north of Sinking 

Canyon, these sediments are mapped as a thin unit that 

overlies the Idavada Volcanics, which are described as 

consisting chiefly of thick layers of devitrified welded tuff 

with some bedded vitric tuff and lava flows. Within the 

Sinking Canyon area, the thickness of the middle Banbury 

Basalt sediments is uncertain because the landslide debris 

obscures the underlying geologic units. It may be that the 

underlying sediments are thicker in the Sinking Canyon 

area than they are to the north, and that it is failure within 
landslide movements of less 

than a centimeter per month 

(Chadwick and others, in press). 

Thus, the available data and 

observations indicate that the 

slide probably began moving in 

late 1998 or early 1999, moved 

relatively rapidly (several 

meters) over a period of about 1 

year, began slowing in late 2001, 

and is now continuing to move 

at a rate of less than a centimeter 

per month.

Potential Causes of 

Landslide Activity

The cause of the recent landslide 

movement is unknown, but 

increasing ground-water 

levels are often responsible 

for initiating slope failures by 

Figure 5. Photo of the upper portion of the Bluegill landslide taken from the west side increasing pore-water pressures, 

of the canyon in November 2000. View is to the southeast. Length of the canyon rim that thus decreasing the shear 

is visible in photo is about 600 m. The upper part of the Banbury Basalt that forms the strength of soil and rock. The 

canyon rim and an outcrop of the underlying sediments of the middle part of the Banbury Bluegill landslide is located 

Basalt are labled. The headscarp of the landslide and Bluegill Lake are also identified. near the boundary of the Snake 

Photo courtesy of Karen Shilling, BLM. River Plain and the Owyhee

the thicker, weak sediments that has resulted in the past and 

current landslide activity in Sinking Canyon.

Recent Movement History

The timing of the most recent movement of the Bluegill 

landslide is uncertain, but based on public reports to the 

BLM it is believed that the movement probably began in 

late 1998 or early 1999, (K.F. Shilling, BLM, 2001, oral 

commun.). Initial movement during the recent activation was 

relatively rapid, with a 4.5 ha (11 acre) area of the canyon 

rim moving downward an estimated 6-8 m and moving 

laterally several meters over a period of several months to 

a year. Remote-reading extensometers placed across the 

headscarp of the landslide in January 2001 by the USGS 

indicated that the upper part of the slide was moving about 

10-15 cm per month during most of 2001. Total station 

surveys of prisms placed on the landslide (fig. 6), which 

were begun by the BLM in July 2001, indicated similar 

movement rates for the upper and lower parts of the slide 

through November 2001, with noticeable slowing of slide 

movement in late 2001 (fig. 7). From about November 2001 

through mid-2003 the prism survey data indicate average 

movement rates of 2-4 cm per month. GPS data obtained 

during the February – May 2003 time frame indicate 

Plateau, which comprises the 

southwest highlands of Idaho. 

Meteorological data available from the Western Region 

Climate Center (www.wrcc.dri.edu) for the southwest 

highlands of Idaho over the past 100 years indicate that 

annual precipitation in the area averaged about 300 mm (12 

in) per year (fig. 8). Beginning in the early 1980s, however, 

there are periods of much higher than normal precipitation, 

with some yearly precipitation exceeding 400 mm (16 in). 

The 4-year period from 1995 through 1998 was unusually 

wet, with annual precipitation ranging between 400 and 

530 mm (16 and 21 in). In contrast, the period from 1999 

to present has been dry, with annual precipitation less than 

the 100-yr average. A similar 100-yr precipitation pattern is 

also observed for the central plains of Idaho, just to the north 

of the study area, although precipitation amounts are less 

and periods of higher than normal precipitation began in the 

1960s. Therefore, it is likely that ground-water levels in the 

Bluegill landslide area were near their highest point in the 

past 100 years at about the time of the most recent landslide 

movement.

Whether or not the recent climatological change, specifically 

the unusually wet period in the latter part of the 1990s, was 

a factor in initiating the recent landslide movement cannot 

be stated for certain, but could well be a contributing factor. 

Annual precipitation during the 1930s was not unusually 

6
Assessment of Current 

Hazard

Landslide Dam Failure

Recent movement of the 

Bluegill landslide has created a 

landslide dam that forms a lake 

that is approximately 2 km long 

and averages about 40 m wide 

(Chadwick and others, in press). 

The dam (fig. 9) has been formed 

primarily by uplift of the canyon 

floor at the southern boundary of 

the slide, rather than by closing of 

the stream channel due to horizontal 

movement of the landslide. To 

the north, and just downstream 

from the dam, horizontal and 

upward movement of the toe of the 

landslide have narrowed the stream 

channel, and in some places have 

caused realignment of the stream 

and created small impoundments 

of water (fig. 10). The primary 

hazard currently associated with 

the landslide is a possible breach of 

the landslide dam that could cause 

flooding and outburst debris flows

Figure 6. Aerial photo of Bluegill landslide showing locations of survey prism reflectors downstream from the landslide. 

used in plots of landslide movement shown in figure 7. Area where uplift of toe of land- The worst-case scenario would be 

slide has created landslide dam is circled. Photo courtesy of Karen Shilling, BLM. a sudden and catastrophic failure of 

high, so climatological conditions would not appear to be an 

obvious factor in the 1937 Sinking Canyon landslide activity. 

Other factors, such as irrigation or ground-water withdrawal, 

could also contribute to ground-water fluctuations in the area. 

The land affected by the 1937 Sinking Canyon activity was 

under cultivation at the time, and was presumably irrigated 

by surface water. Irrigation practices, or any changes in 

the amount of land under irrigation in the vicinity of the 

landslide, are unknown to the authors. Annual fluctuations 

in the level of Bluegill Lake, located on the landslide, have 

been observed (K.F. Shilling, BLM, oral commun., 2001) but 

there have been no quantitative or long-range observations 

to indicate that recent water levels in the lake were unusual. 

Note that the Sinking Canyon and Bluegill landslides are on 

the eastern side of the canyon, and that it is predominantly 

land on the east side of the canyon that is subject to irrigation 

(fig. 1 and fig. 4). This observation suggests that irrigation 

may play a role in initiating and maintaining landslide 

activity. A correlation between irrigation and initiation of 

landslides has been noted in other similar localities (Schuster 

and others, 1989, Chleborad and Schuster, 1995). 

the landslide dam, resulting in a release of all or most of the 

impounded water over a short period of time.

The amount of water impounded by the landslide dam has 

been estimated by Chadwick and others (in press) to be as 

much as 1,000,000 m3. However, two depth measurement 

transects of the lake conducted in November 2001 (K. 

Weinner and J. Tharp, BLM, written commun., 2001), 

indicated that 50 m upstream of the dam the mean lake 

depth was 2.3 m, and that about 400 m upstream of the dam 

the mean lake depth was 4.5 m. Because the lake becomes 

shallower with greater distance upstream, the average depth 

of the lake over its entire 2 km length would probably be 

on the order of 2-3 m or less. Based on the two stream 

transects, the BLM estimated the lake volume in November 

2001, to be a little over 260,000 m3. From late 2001 to mid 

2003, the toe of the slide moved upward less than 0.6 m. 

(fig. 7b), or about 15 percent of the current dam height. 

Assuming that the lake depth increased by a corresponding 

amount during this time period and that the lake did not 

widen significantly in the narrow canyon, the volume of the 

lake probably did not increase by more than about 15-25 

7
8 

Figure 7. Plot of A, horizontal movement; and B, vertical movement; as determined from repeat surveys of reflector prisms 

begun on July 18, 2001. Locations of survey points are shown in figure 6. Data plots courtesy of Karen Shilling (BLM) and 

Stephen Dorsch (Idaho State University). 

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1 in = 25.4 mm

Figure 8. Plot of yearly precipitation for the southwest highlands of Idaho from 1900 through 2002. Red 

symbols indicate total precipitation for 12-month periods ending in September. Blue symbols are the 10-year 

running mean precipitation. Green lines represent the average precipitation and standard deviation for the 

period. Note the years of high precipitation in the later part of the 1990s, which produced the highest 10-

year running means of precipitation since 1900. (Data plot from Western Region Climate Center web site, 

www.wrcc.dri.edu/divisional.html).

percent. It is therefore inferred that the volume of the lake is 

actually about 325,000 m3.

Examination of the landslide dam indicates that it consists 

primarily of large boulders ranging in size from several 

centimeters to up to about a meter in diameter in a matrix of 

fine material (fig. 9). The fines are apparently derived from 

the sedimentary deposits underlying the upper basalt flow 

and are now intermixed with the basalt to form the bulk of 

the landslide deposit. Stream deposits in the canyon also 

contribute to the observed fines. Movement of the toe of the 

landslide has caused a portion of the old stream channel just 

below the dam to be uplifted several meters (fig. 9a). Uplift 

of the old streambed reveals fine-grained stream deposits 

about a meter thick on the east side of the current stream 

channel.

Where water from the impounded lake flows over the 

landslide dam, the streambed sediments and fine matrix 

material have been eroded away, leaving only the large 

boulders. As can be seen in figure 9, there has been some 

minor erosion of the dam, probably as a result of the 

removal of the fine matrix material by running water. 

The relatively small amount of downcutting into the dam 

indicates that boulders make up a sufficient volume of the 

dam for it to be “self armored” against erosion, and that 

sudden erosion and breaching of the dam is unlikely.

The total width and shape of the landslide dam parallel to 

the stream has not been determined. However, the BLM 

transects showed that 400 m upstream of the dam the lake 

was deeper (7.2 m) than at 50 m upstream of the dam (5.3 

m). This indicates that the dam is a relatively broad feature 

formed by uplift of the streambed. BLM field notes also 

indicate a transition from shallower to deeper water about 

175 m upstream from the dam, with the deeper water 

occurring on the upstream side of this boundary. This 

observation also supports the idea that the dam is a relatively 

broad feature and that the narrow part of the dam visible 

above water level is where the greatest amount of uplift has 

occurred. Because the total uplifted area that forms the dam 

is apparently much broader than the visible part of the dam, 

and because of the large amount of coarse material that forms 

the dam, a rapid dam failure with sudden release of all of 

the impounded water is considered unlikely by the authors. 

A longitudinal profile of depth to the bottom of the stream 

channel would help to confirm the width and cross-sectional 

shape of the dam.

Another area of concern is that continued movement 

of the toe of the landslide downstream from the current 

dam, especially at rates like occurred early in the recent 

movement, could potentially dam the stream channel 

and create a larger and deeper lake than currently exists. 

However, because the toe of the active landslide is quite 

9
A

Figure 9. Photos of landslide 

dam formed by uplift of the toe 

of the Bluegill landslide. A. View to the southeast. Dam is 

on the south, or upstream side 

of the landslide and is approxi-

mately 3-4 m in height. Note the 

old stream channel to the left of 

the dam (northeast side of chan-

nel) that has been uplifted by 

the landslide. B. Close-up view 

of landslide dam looking toward 

the northeast. Dam is composed 

primarily of large boulders of 

basalt. Some erosion of the dam 

has occurred due to stream 

downcutting. Photo taken Sep-

tember 24, 2003.

B

Figure 10. Composite of photo-

graphs taken March 2001 show-

ing toe of landslide encroaching 

into Salmon Falls Creek down-

stream of the landslide dam. View is from west to east. Photo 

composite courtesy of Karen 

Shilling, BLM. 

10
Figure 11. Topographic map of section of Salmon Falls Creek canyon 

showing old landslide dams approximately 4 km and 7 km upstream of 

Sinking Canyon and the Bluegill 

landslide. Ages of these older dams 

are uncertain. The topographic 

map (Balanced Rock 7.5-minute 

Quadrangle, USGS) was compiled 

in 1987. The sizes of the impounded 

lakes are similar to the size of the 

lake now formed behind the Bluegill 

landslide.

11
0.85 and 8.5 m3 /sec (30 and 

300 cfs) (fig. 12). The seasonal 

streamflows have remained 

fairly constant over the 32-year 

period of record, and only rarely 

has the maximum daily-mean 

streamflow exceeded 11 m3/sec 

(400 cfs). The maximum meandaily streamflow during the 32-

year recording period was about 

100 m3 /sec (3500 cfs) in early 

1984. Because of the regulating 

effect of Salmon Falls Dam, 

there is a low probability of an 

abnormally high stream-flow 

event impacting the Bluegill 

landslide dam. 

A simplified dam-break 

simulation was conducted by 

the BLM (K. Gebhardt, BLM, 

Figure 12. Daily mean stream flow of Salmon Falls Creek between 1970 and 2003 at written commun., 2001) using 

USGS gaging station approximately 10 km downstream from the Bluegill landslide. (Data the FLDWAV model developed 

plot from USGS web site http://nwis.waterdata.usgs.gov/id/nwis/discharge). by the National Weather Service 

(http://weather.gov/oh/hrl/

wide, the breadth of such a dam would be large compared 

to its height. The dam would also be composed of the same 

erosion-resistant material as the current dam, such that rapid 

erosion and down cutting would therefore not be expected.

Some indication of the future stability of the Bluegill 

landslide dam can be inferred from other landslide dams 

in this area of the canyon. Examination of the USGS 

Balanced Rock quadrangle topographic map (dated July 1, 

1987) indicates two apparent landslide dams within about 

7 km upstream of the Bluegill landslide (fig. 11). These 

topographic landforms are typical of the landslide features 

in Sinking Canyon. While the ages of these landslide dams 

are not known, they at least occurred prior to July1987, 

and are probably much older. The impoundments behind 

these older dams are similar in size to the Bluegill landslide 

impoundment, and their presence indicates that landslide 

dams formed in this geologic environment and topographic 

terrain may remain stable for many years. The ages of these 

older landslide dams could perhaps be better constrained by 

researching old maps and county records.

The flow of Salmon Falls Creek is regulated to some degree 

by Salmon Falls Dam, located about 50 km upstream of 

the landslide. The dam was built in 1911 (C. Skinner, 

Idaho Department of Water Resources, Southern Region, 

oral commun., 2003). USGS streamflow records since 

1970 for Salmon Falls Creek near Hagerman, about 10 

km downstream of the landslide, indicate that seasonal 

variations in daily-mean streamflow generally vary between 

rvmech/fld_avail.htm). The 

simulation was done for dam 

heights of 4.5, 6.0, and 7.6 m (15, 20, and 25 ft) using two 

different times to reach maximum breach (12 and 30 min.). 

A width of breach of 15 m (50 ft) was used in the simulation. 

The actual height of the Bluegill landslide dam on its 

downstream side is about 3.6 m (12 ft). The model predicted 

discharge at specified distances downstream from the 

landslide dam. For a dam height of 4.5 m (15 ft) and a time 

to maximum breach of 12 minutes, the simulation predicted 

peak flows less than 85 m3/sec (3000 cfs) at a distance 10 

km (13 km measured along the stream channel) downstream 

from the dam. Thus, based on the FLDWAV model 

simulation, peak flow from a rapid breach of the current dam 

would produce a flow rate at the USGS gaging station that is 

less than the 30-yr maximum flow measured in 1984.

According to information provided by BLM (K. Shilling, 

written commun., 2003), a 1984 newspaper account indicates 

that high water from Salmon Falls Creek did spread out over 

the flood plain below the mouth of the canyon. Apparently 

no homes were present on the flood plain at that time, but 

10-15 homes are now located on or near the flood plain. 

It is therefore possible that sudden failure of the Bluegill 

landslide dam could cause flooding of these structures. Also, 

if high water similar to what occurred in 1984 were to cause 

sudden failure of the dam, then flows could be about double 

those measured in 1984. Such flows would have the potential 

to produce downstream flooding. 

The current rate of movement of the Bluegill landslide, the 

apparent breadth and composition of the landslide dam, the 

12
long history of relatively stable flow rates of Salmon Falls 

Creek, and the presence of older landslide dams in the same 

vicinity of the canyon all provide strong indications that a 

sudden and massive failure of the Bluegill landslide dam is 

unlikely. The dam-break simulation, although a simplified 

analysis, also suggests that a rapid breach of the dam would 

not produce flows greater than have occurred in the past 30 

years at downstream locations where people or structures are 

located.

Other Landslide Hazards

Recent movement of the Bluegill landslide has created large, 

linear-trending open fissures on the 4.5 ha area of downdropped canyon rim. This area probably also contains open 

fractures that are bridged by soil and are thus not visible at the 

ground surface. Some of these fractures likely reach depths of 

several tens of meters. 

The 50-80 m thick basalt flow that forms the upper canyon 

rim also contains numerous open fractures associated with 

jointing planes behind the exposed cliff face. These fractures 

have been widening with progressive landslide movement, 

making the cliff face highly unstable. Areas below the cliff, 

including the trail to Bluegill Lake, are therefore exposed to 

significant risk from rock fall, as are areas of the cliff used 

by recreational rock climbers. Such hazards in the 4.5 ha area 

above the canyon rim affected by the landslide and the areas 

along and below the canyon rim pose a significant safety 

threat to hikers and climbers, as well as wild or domestic 

animals.

Future Considerations

Movement of the landslide has slowed dramatically in 

recent months, but the possibility of a future acceleration 

of movement cannot be ruled out. Any significant increase 

in movement rates would result in an increase in the size 

and depth of the existing landslide-formed lake, either by 

continued uplift of the existing dam and/or closing off of the 

stream channel below the dam. As discussed above, such 

a scenario would not necessarily increase the likelihood of 

a dam failure. However, the potential consequences of a 

failure of a much larger landslide dam would be more severe. 

As such, movement of the landslide should be carefully 

monitored until it is apparent that the landslide has stabilized. 

Thereafter, periodic visual inspections and annual surveys of 

prism locations should be done to confirm that the landslide 

remains stable, especially during the spring and early summer 

months when ground-water levels may be at seasonal highs 

and irrigation of nearby cropland is in progress.

Mitigation Options

It is highly unlikely that actions taken to prevent the 

movement or possible future movement of the landslide would 

be financially feasible. However, mitigation options focused 

on eliminating or reducing the impacts of hazardous events 

that could potentially result from movement of the landslide 

are less expensive and much more likely to be cost effective 

than efforts to prevent landslide movement. The most serious 

hazard associated with the landslide is downstream flooding, 

and/or debris flows, that might occur in the event of a sudden 

failure of the current or any future landslide dam. The only 

viable option for elimination of this potential hazard is to 

drain the impounded water from behind the dam.

Draining of the landslide-formed lake could be achieved 

relatively inexpensively by lowering the height of the 

landslide dam. Heavy equipment, such as a track-mounted 

excavator or bulldozer, could be used to gradually excavate 

the dam to a level near the original stream elevation. Dam 

material would need to be removed in small stages to prevent 

rapid flow and incision of the dam by the escaping water. Any 

significant future movement of the landslide would likely 

require additional excavation. An existing road from the west 

side of the canyon currently provides access into the canyon 

near the dam, and a short access road to the dam could easily 

be cut by bulldozer if necessary. Costs for conducting such an 

excavation of the dam would probably be less than $10,000. 

The only other feasible option to drain the lake would be 

to construct a large-diameter siphon tube to carry water 

around the area of the canyon affected by the landslide. Such 

an action, however, would be extremely costly and would 

require that the tube pass along the west side of the canyon to 

avoid damage from future landslide movement. This option 

is considered impractical considering the high cost and the 

current low probability of rapid dam failure.

Another option, in lieu of draining the lake, would be to 

control the lake level by constructing an irrigation pumping 

station upstream of the dam. A pumping station currently 

exists a short distance downstream of the landslide that 

diverts water into an irrigation canal on the west canyon rim. 

This pumping station could be threatened by future landslide 

movement, or lost in the unlikely event of a landslide dam 

failure. Relocation of the pumping station upstream of 

the dam would eliminate that risk and provide a means of 

controlling the amount of water behind the landslide dam. It is 

our understanding that the land on the west side of the canyon 

is under BLM management. The possibility and feasibility of 

a BLM/private sector cooperative funding project to relocate 

the pumping station upstream of the landslide dam should 

be explored. Although more costly than draining the lake, 

such an arrangement could benefit the BLM in mitigating the 

potential hazard by routing some of the Salmon Falls Creek 

flow around the landslide dam. Depending on the pumping 

capacity, such a pump station could be used to regulate the 

13
level of water behind the dam, especially during periods of 

high flow in Salmon Falls Creek. At the same time, private 

sector irrigation concerns could also benefit by having 

an alternate source of irrigation water not threatened by 

the Bluegill landslide. A canal to carry the pumped water 

currently exists on the west rim of the canyon, and if 

necessary the diverted water could be returned to Salmon 

Falls Creek downstream from the landslide.

Because a rapid failure of the landslide dam is considered 

unlikely, the authors do not believe that elaborate or 

expensive mitigation actions are warranted. As longer 

term options are investigated, current mitigation efforts 

should focus on continued monitoring of landslide 

movement and periodic inspections of the landslide dam. 

An expansion of the current monitoring effort is warranted, 

and investigations should be undertaken to better define the 

boundaries of the currently active slide. Public access to the 

active areas of the landslide should be restricted, or at least 

warnings of the potential dangers of entering the area should 

be clearly posted. The following options for continued 

monitoring and investigation of the landslide are suggested:

1. Continue cooperative effort with researchers at 

Idaho State University to monitor the landslide using 

remote-sensing techniques. Specifically, continue 

the efforts to utilize InSAR and LiDAR data to 

develop DEM models of the slide and to delineate 

slide boundaries. Establishment of a baseline DEM 

will allow for changes in landslide topography due 

to slide movement to be quantified from future 

LiDAR and InSAR surveys. 

2. Continue the in-place GPS monitoring currently 

being conducted by Idaho State University. At a 

minimum, try to maintain two GPS stations along 

the toe of the slide, one station in the middle area 

of the slide, and one station on the top of the upper 

block of the slide. If additional GPS receivers can 

be acquired, deploy one to the north and one to the 

south of the observed displacement to help delineate 

the active part of the landslide. This monitoring 

should be continued for an additional 1-2 years 

unless displacement ceases. If movement continues 

beyond 2 years, then the need for continuing the 

GPS monitoring should be reassessed. 

3. If the additional GPS stations discussed above are 

acquired and deployed, then the periodic movement 

surveys of prism locations on the landslide could 

be discontinued. Otherwise, it is suggested that 

additional survey monuments be installed to define 

the boundaries of active landslide movement, 

and that the surveys of the established and new 

monuments be continued on 2-month intervals. The 

movement surveys should be conducted on a more 

frequent basis in the event of an increase in the rate 

of landslide movement.

4. The geology of the landslide area should be mapped 

in detail. Of particular importance would be the 

determination, if possible, of the thickness of the 

underlying sedimentary rocks. It would also be 

helpful to determine if the volcanic rocks that 

outcrop in the canyon just upstream of the landslide 

are in-place, or are derived from the upper canyon 

rim by landsliding.

5. Continue near real-time monitoring of movement 

across the headscarp of the landslide using a cellphone equipped datalogger in order to provide early 

notification in the event of accelerated landslide 

movement. The GPS monitoring provides better 

overall indication of slide movement, but because the 

data is collected and processed every 2-3 weeks, it 

does not provide an early-alert capability.

6. Although rapid failure of the landslide dam is 

considered unlikely, the phone numbers and contact 

information compiled by BLM for the few residences 

and businesses located within the floodplane 

downstream of the landslide should be kept current 

and made available to local emergency officials. 

If not already in existence, a plan should also be 

developed to notify emergency officials to evacuate 

potentially affected downstream parties should a dam 

failure occur or appear imminent.

The following actions are also considered desirable should 

funding support be feasible. While these actions would 

greatly enhance the landslide monitoring effort and result 

in an improved understanding of the causes and potential 

behavior of the landslide, they are not considered essential 

with respect to the current level of hazard. 

1. Contract for real-time GPS monitoring of the 

landslide movements. Real-time GPS monitoring and 

alarm services, including installation of the remote 

GPS stations, are commercially available. Such 

services tend to be costly, but offer the advantage of 

real-time data acquisition and alarming capability 

via the Internet. Depending on the number of GPS 

receivers, the cost of real-time GPS monitoring of 

the Bluegill landslide are estimated to be between 

$50,000 to $75,000 for capital equipment purchases 

and $20,000-$25,000 per year for a services contract. 

Such real-time monitoring would eliminate the need 

for other remote monitoring and periodic prism 

surveys, and as such, some of these costs would 

be recovered by BLM salary and equipment cost 

savings from no longer having to conduct labor 

intensive and time consuming periodic movement 

surveys.

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2. Expand the existing cell-phone based monitoring 

system to include deformation monitoring at the toe 

of the slide and instrumentation to detect changes 

in water level or flow below the dam. Hardware 

for expanding and upgrading the current system 

to provide a network of additional monitoring 

instruments would cost an estimated $15,000-$20,000. 

Monthly cell phone charges to collect the data and 

provide a pager alert in the event of significant 

changes are estimated at less than $35 per month. 

3. Drill a 150-200 m deep exploratory boring just east 

of the landslide headscarp to determine the geologic 

stratigraphy in this part of the canyon, and to monitor 

long-term ground-water pressure variations that likely 

play a critical role in triggering and maintaining 

landslide movement. Data from such a boring would 

be essential to establish a better understanding of the 

landslide and thus provide a basis for forecasting its 

long-term behavior. Construction of an exploratory 

boring and monitoring well would likely cost $50,000-

$75,000. Data collection could be integrated with the 

existing cell-phone based monitoring system at the 

landslide site.

Summary

The most significant hazard posed by the Bluegill landslide 

is a rapid failure of the landslide dam that could produce 

catastrophic flooding, and possibly debris flow from 

entrainment of channel debris in the flood waters. Some 

residences and structures located in the Salmon Falls Creek 

drainage beginning at a distance of about 10 km downstream 

from the landslide could be at risk should flooding occur. 

However, based on the available information and a visual 

inspection of the landslide, the authors do not believe the 

current landslide dam poses an imminent risk.

The landslide dam is a relatively wide feature caused by 

uplift of the canyon floor and streambed. The breadth (stream 

parallel width) of the dam imparts stability that makes a 

sudden failure improbable. The dam is composed primarily 

of large boulders of volcanic rock, and thus does not appear 

vulnerable to rapid erosion. The dam has not increased in 

size significantly since the landslide movement dramatically 

slowed in late 2001. Some erosion of the dam by Salmon Falls 

Creek is visible, and the level of water behind the dam has 

apparently decreased somewhat over the past several months 

(K.F. Shilling, BLM, oral commun., 2003). The presence of 

two impoundments of similar size from apparent landslide 

dams upstream of the Bluegill landslide indicate that such 

features are not uncommon in this part of the canyon. Because 

these landslide dams have been present for at least 16 years 

(fig. 11), and perhaps much longer, suggests that they are 

stable for the flows that typically occur in Salmon Falls Creek, 

and we believe that the Bluegill landslide dam is probably 

equally stable.

Results of a simple dam-burst simulation indicates that a 

rapid failure of the existing dam would produce flows 10 

km downstream that are equivalent to or less than the 30-

year maximum flow recorded at that location. Although this 

simulation can only be considered a rough approximation of 

expected flows, it does suggest that flows from a rapid dam 

failure would probably not exceed the highest seasonal flows 

measured over the past 30 years, unless the dam were to fail 

during a rare period of equivalently high seasonal flows.

Continued or sudden renewed movement of the landslide may 

increase the height of the dam, but would also likely increase 

or maintain the breadth of the dam. Renewed landslide 

movement could also choke off the stream channel below the 

existing dam and create a much broader dam that would be 

composed of similar erosion-resistant materials as the existing 

dam. Impounded water would eventually overtop the dam 

and flow over the landslide, ultimately creating a new stream 

channel. However, rapid erosion of such a potential dam 

would also be highly unlikely. 

Other hazards associated with the landslide derive mainly 

from the presence of wide, deep fissures on the 4.5 ha upper 

block, and from rockfall from the cliff that forms the canyon 

rim. Closing the area to public access would effectively 

mitigate these hazards, but if closure of the area is not an 

option, then warnings of the hazards should be clearly posted 

at key locations around the site.

References

Chadwick, J., Thackery, G., Dorsch, S., Glenn, N., and 

Sklar, J., 2003, Use of GPS and interferometric SAR 

for monitoring a large landslide in south-central Idaho: 

Proceedings of the International Symposium on Remote 

Sensing of Environment, 30th, November 10-14, 2003, 

Honolulu, Hawaii.

Chleborad, A.F., and Schuster, R.L., 1995, A protocol for 

monitoring movement and seepage conditions at the 

Hagerman Fossil Beds National Monument, south-central 

Idaho: U.S. Geological Survey Admin. Report, Denver, 

CO, 20 p.

Cruden, D.M., and Varnes, D.J., 1996, Landslide types and 

processes, in Turner, A.K. and Schuster, R.L., eds., 

Landslides—investigation and mitigation: National Res. 

Council, Transp. Res. Board Special Report 247, p. 36-

75.

Lee, C.A., 1938, Recent landslides in Idaho: Journal of 

Geology, 46(4), p. 660-665.

Malde, H.E., Powers, H.A., and Marshall, C.H., 1963, 

Reconnaissance map of west-central Snake River Plain, 

Idaho: U.S. Geological Survey Miscellaneous Geologic 

Investigations Map I-373, scale 1:250,000.

15
Malde, H.E., 1991, Quaternary geology and structural 

history of the Snake River Plain, Idaho and Oregon, in 

Morrison, R.B., ed., Quaternary nonglacial geology; 

Conterminous U.S.: Boulder, Colorado, Geological 

Society of America, the Geology of North America, v. 

K-2, p. 251-281.

Schuster, R.L., Chleborad, A.F., and Hays, W.H., 1989, 

The White Bluffs landslides, in Galster, R.W., ed., 

Engineering Geology in Washington, Wash. Div. 

Geology and Earth Resources Bull. 78, v. 2, p. 911-920.

Varnes, D.J., 1978, Slope movement types and processes, 

in Schuster, R.L. and Krizek, R.J., eds., Landslides—

analysis and control: National Res. Council, Transp. 

Res. Board Special Report 176, p. 11-33.

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