Moffat Collection System Project - Mitigation Plan 6-9-11 - Colorado ...

Proposed Mitigation for the Moffat Collection System Project 06/09/2011
Table 7 - EAST SLOPE - South Platte River
Proposed
Mitigation
EIS Impacts
Surface Water
Antero Reservoir surface water elevation fluctuation :
The maximum increase and decrease in reservoir elevation (averaged over the month) for any month over the
45-year study period between Full Use of Existing System and the Project is 9 feet to 7 feet, respectively.
Antero is utilized for multi-year droughts, therefore is not affected by the project.
None ---
Eleven Mile Canyon Reservoir surface water elevation fluctuation :
The maximum increase and decrease in reservoir elevation (averaged over the month) for any month over the
45-year study period between Full Use of Existing System and the Project is 4 feet to 3 feet, respectively.
Eleven Mile is utilized for multi-year droughts, therefore is not affected by the project.
Cheesman Reservoir surface water elevation fluctuation :
The maximum increase and decrease in reservoir elevation (averaged over the month) for any month over the
45-year study period between Full Use of Existing System and the Project is 61 feet to 6 feet, respectively.
Strontia Springs Reservoir surface water elevation fluctuation :
Reservoir evaporation, contents, and elevation changes to Strontia Springs Reservoir would be negligible. No
recreation is allowed on Strontia Springs Reservoir.
Mitigation
Agency
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Chatfield Reservoir surface water elevation fluctuation :
Reservoir evaporation, contents, and elevation changes to Chatfield Reservoir would be negligible. The project None --has
no impact on storage or operations of Chatfield Reservoir.
Surface water flows :
Flow changes along the South Platte would be relatively minor and vary depending on the location. South
Platte flows at the Waterton Canyon gage would decrease on average in summer months (max. decrease of
5% in June) due to Denver Water's additional direct diversions and exchanges to Strontia Springs Reservoir and
Conduit 20. There would be little change in flows at the Waterton gage in most winter months (1% in Dec and
0% in Jan-March). Flows at the Henderson gage would increase on average during winter months from Oct-
Nov (2-9%) and little change in flow May-Sept.
Stream Morphology and Sedimentation :
Given the minor flow changes to the South Platte, impacts to channel morphology under the project are likely
to be negligible.
Groundwater
Below Chatfield Reservoir and at the Denver gage, flows increase in the winter and decrease in late summer in
an average year. The maximum expected increases and decreases in flow would have minor effects on
groundwater and would be limited to only the areas near the river and would be well within the normal
seasonal fluctuations typical for aquifers along streams in mountainous terrain.
Riparian/Wetland
The changes in stream flow associated with the Moffat Project would have no measurable effects to wetlands,
other waters, and riparian area along the South Platte.
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