PROJ00259

• Contract/PO #: C153424
• Contractor Name: Colorado River Water Conservation District
• Water District: 0
• Bill Number: XB 99-999
• Loan Projects - Doc Type: Feasibility Study

The most abundant joints are parallel to the bedding. Cross joints at
right angles to the bedding are approximately parallel to the strike
of the beds or at right angles to the strike of the beds.

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The bedrock is for the most part covered by surficial
deposits. The surficial deposits include colluvium, including
landslide deposits, and alluvium. Colluvium is derived from the
weathering of the underlying bedrock and consists of cobbles and gra-
vel of sandstone in a sandy and sUty clay matrix. The colluvium on
the slopes becomes saturated with water under conditions of high pre-
cipitation and fails to form landslide deposits. Such deposits form
the right abutment of the proposed dam. In general, the deposits have
a maximum thickness of a few tens of feet. In the stream valley are
alluvial deposits. These deposits are chiefly derived from erosion of
the colluvial deposits and consist of gravel of sandstone in a sandy
and silty matrix. Erratics, from c:obble to gravel size, composed of
basalt, derived from pre':'existing terrace deposits, occcur in the
alluvium. (See Figure IV-4)

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The bedrock is better exposed in the left abutment of the
West Divide Creek site, but there is thick colluvium on the right
abutment. A large landslide has beeen mapped (Ref. 33) above the
right abutment. The dip of the bedrock units ranges from 2. to 7. to
the southwest, and jointing in the ,~ompetent units is similar to that
at Willow Creek.

IV-l6

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Material for dam construction can probably be derived
locally. Some test pitting was done in the alluvium at the Willow
Creek site. The alluvium and colluvium (including landslides) could
be used for an earthfill dam. The alluvium could probably be pro.
cessed for rollercrete, aggregate for concrete and sand for aggregate
or filler material would probably have to come from pits along the
Colorado River to the north. Riprap might be derived from processing
the basalt boulders in the alluvium and colluvium. There is not ade.
quate rock of hard composition in the area for a rockfill dam.

The geologic and geotechnic:al investigations of the proposed
damsites on Willow and West Divide Creeks are not adequate for preli-
minary design. For preliminary design, at either location, a
topographic and geologic map will need to be prepared, holes drilled
for core along the dam axis, the formation tested for permeabili ty,
and possible sources of materials for dam construction investigated.