WSP05705

• File Number: 8407
• Description: Platte River Basin - River Basin General Publications
• State: CO
• Basin: Statewide
• Date: 3/11/1964
• Author: CWCB
• Title: Engineering and Geologic Investigation and Analysis of Alternate Proposed Dam Sites on the South Platte River - Morgan and Weld Counties-Colorado
• Water Supply Pro - Doc Type: Report/Study

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site, the basic differences are these:

1.

The overburden ou the left abutment is much less at

Narrows.

2.

The foundation under the Narrows is a part of a shallow

"bypass' channel of the river aud does not contain the

deeper inner channel present at the Weld County site.

As a result, it does not have the subsurface width and

depth of the Weld County site. However, although the

overall subsurface channel at Weld County is wide, it

contains all of the subsurface flow of the river under

the dam site proper and offers the possibility of complete

river cutoff and seepage control if such is desired.

The l"ight abutment of the Weld County site shows bedrock

rising continuously to offer impervious confinement of

the reservoir, whereas the right abutment of Narrows is

composed of hi~hly pervious sand and gravel. The bedrock

dips strongly away from the reservoir toward the old

deep channel of the South Platte which bypasses the dam

site.

4. Impervious borrow materials can be obtained nearer to

the Narrows site than to the Weld County site.

EFFECTS ON WATEJl WELL IRRIGATION PUMPING

It is evident that coustruction of a dam aud reservoir at

either one of the subject sites will change the ground water con-

ditions adjacent to and downstream from the dam site. At the

Weld County site a complete cutoff will shut off the underflow of

the river channel and flood plain area inuuediately below the dam.

This effect will gradually diminish as return flow from irrigation

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