PROJ00072

• Contract/PO #: C153592
• Contractor Name: West Divide Water Conservancy District
• Contract Type: Loan
• Water District: 0
• County: Garfield
• Bill Number: SB 89-85
• Loan Projects - Doc Type: Feasibility Study

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5.3 Dam Site Geology:

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Alsbury Dam site is located on East Divide Creek at an elevation of about

9,042 feet (stream bed elevation) where a natural lake occurs in a small

mountain valley known as Reservoir Park. The creek enters a steep, narrow

canyon immediately downstream from the dam. The gen~al area is very

mountainous with nearby ridges and peaks rising to more than 10,000 feet in

elevation. A small dam about 10 feet in height was constructed to enlarge the

natural lake reportedly to supply water for natural gas drilling on the adjacent

Divide Creek Anticline. This gas field was discovered in 1956 (J. Dan Powell in

Grand Junction Geological Society Guidebook - 1982 Field Trip, page 57). For

unknown reasons, the embankment was breached some time in the 1970s; and at this

time only the original, natural lake exists (at a water surface elevation of

9,042 feet and an area of 4.5 acres).

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Bedrock of a fairly competent sandstone member of the Wasatch Formation is

well exposed on the right abutment of the dam site and for approximately 800

feet upstream along the east shoreline (see Geology and Borrow Area Map, Figure

5.1). Much of the remainder of the dam and reservoir area; however, is covered

by slopewash, alluvial fan, and landslide materials. The sandstone on the right

abutment also is exposed for a short distance downstream from the dam and is

thin bedded and fairly well cemented. The beds strike N750W and dip 5 to 100

downstream. A set of vertical joints in the sandstone strikes approximately

east/west and the joints are spaced about one to 20 feet apart. The strike of

the jointing at a right angle to the stream is a favorable orientation to

minimize seepage loss from the reservoir.

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The left abutment of the dam site is created by a huge landslide mass that

is about one mile wide and 2~ miles long. Reservoir Park and the Alsbury Dam

site are located at the upstream or south edge of this landslide. The present

surface of the slide area is hummocky with numerous closed depressions which is

typical of landslide topography. It is possible that the movement occurred in

several phases rather than in one sudden movement. The large-scale movement was

caused by the downslope dip of the bedding (to the northeast) into the canyon

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