PROJ00345

• Contract/PO #: C153330
• Contractor Name: Upper Yampa Water Conservancy District
• Contract Type: Loan
• Water District: 0
• County: Routt
• Bill Number: SB 77-35
• Loan Projects - Doc Type: Feasibility Study

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Mr. Joseph L, Jordan
August 29, 1978
Page 2

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3.

"Yamcolo reservoir project, feasibility report")
by Western Engineers, Inc., November, 1975.

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"Yamcolo reservoir geotechnical data", by Western
Engineers, Inc., November, 1977,

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5.

I1Yamcolo damsite geological exploration report",
by Western Engineers, Inc., November, 1977.

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The geology of the reservoir and damsite is typical of
northeastern Garfield County. The bedrock consists of marine
shale and some siltstone and sandstone of the Mancos Formation.
Overlying the bedrock are surficial deposits that include debris
flows, glacial till, alluvial and glacial lake deposits. The
materials that will affect the construction of the dam are the
surficial deposits, The right abutment is chiefly glacial till
which consists ot unstratified and unsorted boulders, gravel,
sand, silt and clay, The logs of test holes and pits, and
percolation tests, show that the fill has a high density and
generally a low permeability. ,The left abutment is an old debris
flow with thin glacial till on top, The debris flow consists
chiefly of sandstone boulders, sand, silt and clay derived from
the Mancos Formation, Incorporated locally in the debris flow
are boulders derived from the Brown's Park Formation and the lava
flows of the Flat Top Mountains. The debris flow, which consists
of several individual flows, has been glaciated and formed before
or during the last period that glacial ice occupied the valley.
The test holes and pits show that the material of the debris flow
is almost of maximum moisture content and density, and has a very
low permeability.

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The proposed dam site is almost at the same location as a
natural dam that was formed by the retreat of the glaciers. A
small recessional moraine was formed by the ice and dammed the
Bear River. A lake formed behind the natural dam and extended
more than a mile upstream. The lake must have existed for a
considerable period of time before the natural dam was breached;
a test pit showed more than six feet of lake sediment in the
bottom of the valley upstream from the natural dam.

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This review of the geology of the Yamcolo reservoir and dam
site, and the results of the testing, have shown that the
materials at the site are in equilibrium with their present envir-
onment and would not be adversely affected by the construction of

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