C150104 Feasibility Study

• Contract/PO #: C150104
• Contractor Name: Groundwater Appropriators of the South Platte
• Contract Type: Grant
• Water District: 0
• Loan Projects - Doc Type: Feasibility Study

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GASP Sedgwick Reservoir ,Feasibility Study

I. Introduction

The Groundwater Appropriators of the South Platte River Basin (GASP) have sponsored this study to
address the feasibility of constructing an off stream augmentation reservoir near the Colorado-
Nebraska border. The objective of the study was to identify constraints associated with construction
of the reservoir and to analyze the effectiveness of the reslervoir in achieving the primary function of
re-regulating the flows in the South Platte River at the Nebraska State line. Re-regulation of flows in
the South Platte River will benefit GASP members by increasing flows at the state line during the
irrigation season.

The lower section of the South Platte River is defined as the reach of the river from the west
boundary of Washington County to the Colorado-Nebraska State line. This reach of the river is
commonly known as Water District 64. The South Platte River Compact requires the State of
Colorado to administer water rights with adjudicated priority dates junior to June 14, 1897 in such a
way that such diversions do not diminish the flow of the riiver at the Interstate Station on any day
below 120 cubic feet per second (cfs) during the period from April I-October 15.

Although the South Platte River has seen an overall increase in flows due to urbanization,
agricultural practices, transmountain diversions, and groundwater development, the flow at the State
line still drops below 120 cfs compact threshold. Many times the natural flow in the river will not
meet compact entitlements even with a curtailment of diversions in District 64. But the possibility
for Colorado surface water users in Water District 64 that ,are junior to June 14, 1897 to have to cease
diversions is still a concern of water users.

The South Platte River in Water District 64 is a gaining reach ofriver. This means that return flows
to the river tend to increase progressively as it moves towards Nebraska. Approximately 300,000
acre-feet per year in excess of the compact entitlement flow into Nebraska every year. However, the
majority of these excess flows occur in the winter months due to the lag time for return flows from
irrigated agriculture to enter the river. The proposed reservoir's primary function will be to
supplement the flows of the South Platte River at the State line on days when the flow of the river at
the State line is below 120 cfs during the period of April I.. October 15 on as many days as possible
within the constraints of limited storage. By supplementing flows of the South Platte River at the
State line on days when the flow of the river at the State line is below 120 cfs, the potential that
GASP members would be forced to cease diversions during critical periods would be reduced.

GASP has previously commissioned a feasibility study for reservoir sites on the south side of the
river in the same approximate geographic area. This study was completed in March 1997 by The
Sear-Brown Group. The results of this study indicated that construction of an off-channel reservoir
in the uplands sites chosen for study were not conclusively a good choice for a re-regulation project.

This work is an extension of evaluating reservoir sites on the north side of the river to determine if
there are better alternates.

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