PROJ00370

• Contract/PO #: FS0031X
• Contractor Name: Yampa Project DNR 1972
• Contract Type: Miscellaneous
• Water District: 0
• County: Moffat
• Loan Projects - Doc Type: Feasibility Study

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On June 24, 1972, the undersIgned investigated the new reservoir

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embankment condition from both soi'l ant! geological standpoInt. The maIn

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purpose of the inspection was to ~etermlne if the new high water level

wil I pose a problem for reservoir leakage. Our geologist walked along

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the high water 1 ine of the proposed reservoir and recorded all outcrops

of sandstone and locations where terrace deposits of gravel and boulders

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were found. The geological features are noted on the attached figure.

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RESERVOIR EMBANKMENT

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from the information obtained from Geological Survey Bulletin 1027-0,

the reservoir basin consists essentially of alluvial and terrace deposits.

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On the east bank of the Elkhead River, the lance Formation predominates.

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On the west bank of the Elkhead River, the Fort Union Formation is

found. The main soil and geological features are described as follows:

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Alluvial and Terrace Deposits: The entire Elkhead River basin is covered

by a layer of silty clays. The thickness of this overburden material extends

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to approximately 10 feet. layers and lenses of silty sands and clean sands

were found In the silty clay deposit. Generally, the material is a fine

grained soil with grain size smaller than #4 sieve. The material has low

plasticity and is highly susceptible to erosion. The erosive character-

istics of the overburden soil should be entered in the design of the dam.

Possibil ity of silting of the reservoir should be taken into consideration.

Terrace deposits consisting. of boulders and sand and gravel underlie

.the fine grained alluvial soi Is and are exposed on the surface at several

locations. The west side of the bridge crossing an existing borrow pit
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reveals that the thickness of this deposit is 10 to 15 feet. Terrace