WSP05022

• File Number: 8200.200
• Description: Colorado River - Basin Hydrology
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 11/13/1985
• Author: Unknown
• Title: Inflow-Discharge Data for Blue River Basin - Snake River-Dillon Reservoir - Blue River Memorandum Flows Below Dillon Reservoir
• Water Supply Pro - Doc Type: Report/Study

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BLUE uvu. MEMOllAJlDlDI FLOWS BELOW DILLOII RBSERWIIl

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Concern has been expressed by the Colorado Division of Wildlife. the Town

of Silverthorne and others in regard to potential flows of less than 50

cfs in tbe Blue River below Dillon Reservoir, Dillon Reservoir is'

operated under an agreement to bypass tbe smaller of tbe inflow or 50

cfs, If depletions upstream of Dillon Reservoir were sufficient to

reduce tbe inflow to Dillon Reservoir significantly below 50 cfs. the

outflow could be correspondingly reduced.

As part of the Green Mountain Reservoir Water Sales EIS. the Bureau of
Reclamation made a model study of the Blue River hydrology using a 21
year period of historic flows (1962-1982) and operating Dillon Reservoir

so as to show the greatest
With simulated full water

potential impact
sales. the model

on the Blue River flows.
,

showed Dillon Reservoir

.

re leases of significant ly less than 50 cfs in only two months. 45 cfs in
December 1964 and 46 cfs in December 1967. Five other months in the 21
year period showed releases of about 49 cfs, The model study was done on
sn averalle monthly basis using upstream depletions from the hydrologic
datahas shown in the EIS.

.

In the analysis of the inflow to Dillon Reservoir. it was noted that the
inflow used in the BOR model was developed from change in reservoir
storage and measured outflows. This is the actual method used in
operating Dillon Reservoir and is the method used in measuring inflow to
all large storage reservoirs as far as I know. However. if an attempt to
measure inflow is made by sUSling the gaged flows of the Snake River.
Keystone Gulch. Blue River and Tenmile Creek and adding an increment (17
percent) for ungaged inflow. this method generally shows a smaller inflow
in the winter time and a greater inflow during the spring runoff, T'his
apparant discrepancy is believed to result from bank storage effects
whereby some inflow is lost to the ground water system as the reservoir
level rises during filling in the spring. and additional inflow is gained
as the reservoir level falls due to releases from storage in the winter,
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