BOARD00933

• Board Meeting Date: 1/25/2005
• Description: ISF Section - Instream Flow Appropriations - West Fork Dry Creek
• Board Meetings - Doc Type: Memo

Biologic Flow Recommendation
The summer flow recommendation, which meets 3 of 3 criteria and is within the accuracy range
of the R2CROSS model is 3.4 cfs (See Table I). This was derived by averaging the results of
the three data sets. The winter flow recommendation, which meets 2 of 3 criteria and is within
the accuracy range of the R2CROSS model is 1.0 cfs (See Table I). It is our belief that
recommendations that fall outside of the accuracy range of the model, over 250% of the
measured discharge or under 40% of the measured discharge may not give an accurate estimate
of the necessary instream flow required.

Hydrologic Data

After receiving the cooperating agency's biologic recommendation, the CWCB and BLM staff,
conducted an evaluation of the stream hydrology to determine if water was physically available
for an instream flow appropriation. Although there is a substantial amount of streamflow gage
data available for the Uncompahgre Plateau and Glade Park, most of this data is severely
impacted by diversions and irrigation use. This situation makes it difficult to estimate the natural
flow regime for the watersheds on the Plateau. Without specific gage data to evaluate, the next
best approach is a regional equation that estimates annual flow characteristics. The USGS has
developed regional equations (Estimation of Natural Streamflow Characteristics in Western
Colorado, Water Resources Investigations Report 85-4086, 1985) that apply to the Uncompahgre
Plateau and Glade Park based on basin drainage area, mean annual precipitation, mean basin
elevation and mean basin slope. The report "Uncompahgre Plateau and Glade Park Annual
Hydrograph Estimation" in Appendix C, explains staff's water availability analysis in more
detail. Table 2 below displays the estimated stream flow of West Fork Dry Creek.

Table 2 shows that for this reach, the summer flow recommendation of 3.4 cfs is available only
from April I to June 30. The summer flow recommendation was reduced to 0.85 cfs for the time
period of July I through July 31 based on limited water availability. The winter flow
recommendation of 1.0 cfs is not available at least 50% of the time. The winter flow
recommendation was reduced to 0.3 cfs for the period of August I to February 28, and to 0.85
cfs from March I to March 3 I based on limited water availability.

Table 2: Estimated Stream Flow on West Fork Dry Creek:

(cfs)

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