ISFAPPC01446 ‎(3)‎

• Case Number: 04CW0085
• Stream Name: La Plata Gulch
• Water Division: 2
• Water District: 11
• County: Chaffee
• Instream Flow App - Doc Type: Supplemental Data

Biologic Flow Recommendation
The summer flow recommendation, which meets 3 of 3 criteria and is within the accuracy range
of the R2CROSS model is 3.5 cfs (See Table 1). The winter flow recommendation, which meets
2 of 3 criteria and is within the accuracy range of the R2CROSS model is 2.8 cfs (See Table 1).
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 staff conducted
an evaluation of the stream hydrology to determine if water was physically available for an
instream flow appropriation. The hydrograph below was derived from data collected by the
USGS stream gage for Halfmoon Creek near Malta, CO (ill #07083000), which has a drainage
area of 23.6 square miles (See Gage Summary in Appendix C). The total drainage area of the La
Plata Gulch Drainage is approximately 5.1 square miles. The period of record for this gage was
1946-1998, the period of record used by staff in their analysis was 1946-1998, or 52 years of
record. Table 2 below displays the estimated stream flow of La Plata Gulch, in terms of a
percentage of exceedence.

Table 2: Estimated flow La Plata Gulch.

Exceedences
1%
5%
10%
20%
50%
80%
90%
95%
99%

Januarv Februarv March ADril Mav June Julv Auaust SeDtember October November December
1.94 1.73 2.38 5.62 34.55 63.32 63.97 28.96 10.83 5.62 4.11 2.16
1.49 1.34 1.51 3.46 25.50 51.86 42.87 17.72 7.35 4.32 2.81 1.90
1.21 1.12 1.30 2.81 19.67 45.38 35.01 13.33 6.27 3.89 2.38 1.64
1.08 0.97 0.97 1.86 15.13 38.68 25.50 9.94 5.19 3.03 1.99 1.38
0.82 0.76 0.76 1.21 8.00 25.50 15.13 6.48 3.46 2.16 1.49 1.04
0.65 0.58 0.56 0.76 3.03 17.07 9.08 4.32 2.59 1.62 1.17 0.84
0.54 0.52 0.50 0.58 1.99 13.83 6.92 3.67 2.16 1.45 1.06 0.76
0.45 0.43 0.32 0.52 1.49 11.67 5.62 3.24 1.90 1.32 0.95 0.69
0.32 0.37 0.26 0.37 1.06 7.48 4.13 2.59 1.51 1.10 0.73 0.58

La Plata Gulch Estimated Stream Flow

5

1

4

3
~
o 2

o

JAN

Fm

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

oov

DEe

Month

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