Analysis of Colorado-Big Thompson Project Operation With Respect to East Slope Diversions and Non-Charge Deliveries

• Description: For the River Colorado Water Conservancy District
• State: CO
• Basin: South Platte
• Basin: Colorado Mainstem
• Water Division: 1
• Date: 10/1/2000
• Author: Helton & Williamsen, P.C.
• Title: Analysis of Colorado-Big Thompson Project Operation with Respect to East Slope Diversions and Non-charge Deliveries
• Water Supply Pro - Doc Type: Report/Study

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Maryanne C. Bach
Brian Person
October 6, 2000
Page 6
Program as a reasonable and prudent alternative for Upper Colorado River Basin
depletions.
UTILIZATION OF EAST SLOPE WATER RIGHTS
CBT operations have also been altered from that contemplated in Senate
Document 80 by the failure to utilize the CBT Project's East Slope water rights to
fill Horsetooth Reservoir and Carter Lake. The CBT Project is being operated in a
manner that sends West Slope water to the East Slope through the Adams Tunnel to
fill Horsetooth and Carter at the end of March or early April. Because these
reservoirs are almost completely filled early in the year with Colorado River Basin
water, there is insufficient space in the reservoirs when the CBT Project East Slope
decrees would allow diversion of native water for storage.
As described above, Senate Document 80 authorized and required a manner of
operation of the CBT Project that would "reserve capacity" in the East Slope
reservoirs until the "latter part of June." Utilization of forecasts based on snowfall
was intended to allow the CBT Project operators to determine the amount of
capacity in the East Slope reservoirs to be reserved in order to fully make use of the
available East Slope native water. A corresponding amount of water would be "held
back" in Granby Reservoir:
Senate Document 80 estimated that approximately 16,000 acre feet per year
would be derived from these East Slope sources. A point flow study by Helton &
Williamsen has calculated the amount of Big Thompson River water available for
storage by the CBT Project at a level remarkably similar to the original estimates.
The Helton & Williamsen point flow study showed that an average of close to
22,000 acre feet of native Big Thompson water could have been diverted at the
Olympus and Dille diversions and integrated into the Project water supply (based on
a 1983 -1998 study period). Of this total amount, approximately 18,000 acre feet
was available during May and June. The failure to utilize this in -basin available
water results in unauthorized and unnecessary diversions of West Slope water by the
CBT Project,
The Helton & Williamsen report includes an operational scenario that shows
full use of the CBT Project East Slope water rights can be accomplished while
meeting all historic Project water demands. Helton & Williamsen also conclude that
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