Lower South Platte River Study

• Contractor Name: Colorado Water Conservation Board
• Loan Projects - Doc Type: Feasibility Study

ILower South Platte River Study
Colorado Water Conservation Board
IDecember 2001
Gerk
IThe Gerk Reservoir site is located northeast of Julesburg along the Peterson Ditch.
Water stored at this site could be released back to the South Platte River at a similar
Ilocation to that of the Troelstrup Reservoir site, about 3.5 miles upstream from the
state line. A ring-dike with maximum height of 55 feet would be required to create a
I 5,000 acre-foot reservoir at the Gerk site. The embankment length would be 13,800
feet. The reservoir basin area includes wetlands based on USFWS mapping. Water
would need to be pumped from the Peterson Ditch or from a large capacity well or
I wells. Pumping from the river during months when water is available also would be
possible. The cost of storage is estimated to be $18,720,000 or approximately
$3,740 per acre-foot.
INew Sites on the South Side of the River
In addition to the LSP sites discussed previously, two other sites were identified as
part of the current study—Johnson Lake Enlargement and Johnson Lake West.
I These sites are located southeast of Ovid and approximately 10 miles(Johnson Lake
Enlargement) and 11 miles(Johnson Lake West)upstream from the state line. The
local geologic setting in this reach of the South Platte River Valley consists of nearly i
Iflat river terraces abutting erosional scarps known as the Gravel Breaks(Sedgwick,
County Soil Survey, 1969). Intermittent tributaries forming broad gullies and draws
I dissect the scarps along the south side of the valley. The ridges dividing the draws
are capped with a gravelly loam (clay, silt and very fine sand) overlying a clayey
sand and moderately sandy soils. These layers generally lie in contact with the
I underlying weathered bedrock (Brule Formation)but exact thicknesses are unknown.
The draws are filled with a sandy loam and stratified sandy alluvium presumably
overlying the Brule Formation.
I The reservoir sites in this group would include the ridges within the"breaks" as
�' P g
potential abutments. The soils capping the ridges have lab-tested permeabilities
greater than 6.3 inches/hour(4.4E-03 centimeters/second) (Sedgwick, County Soil
Survey). The depths to bedrock are unknown at the proposed dam sites but, if
I shallow enough, the Brule Formation would have a lower permeability as
exemplified by the existence of Johnson Lake.
IMaterials suitable for use in an embankment dam are potentially available within a
few miles of each site either as a known commercial source or as a mapping
I geologic unit. On-site materials containing low permeability clay(embankment
core), free draining sands (filter blankets and drains) and silty, clayey sand (soil
cement) may be available. Drilling or trenching would be necessary to confirm the
Ioccurrence and volume of materials.
IGEI Consultants, Inc. 4-18 01002 Dec 2001 Lower South Platte River Study