BOARD02010

• Board Meeting Date: 9/24/2001
• Description: Snowmass Water and Sanitation District 404 Permit
• Board Meetings - Doc Type: Memo

only documents the use of the side channels by trout fry, but demonstrates that they survived an a
extreme flow event of 1995 after emerging in the spring of 1995, ..

Chadwick also prepared a report for the SWSD on the effects of a 4 cfs minimum flow on
the resident trout populations of Snowmass Creek (Chadwick 1998), In this report, he assessed
the impacts of a 4 cfs flow for 1, 3, 5 and 30 days.; Again, he does not supply time series analysis
tables to display impacts and his an.alysis concem~d the change in habitat from 7 cfs to 4 cfs,
nnlike our comparison of existing flows to 4 cfs. He reports only a 20% loss of habitat for
juvenile brown trout as the highest impact. This nl.unber appears accurate using the Miller data
when you lower the stream from 7 cfs to 4 cfs (21 'Yo). The problem with this analysis is that
impacts have already occurred when the stream is lowered from existing conditions, 10 to 12 cfs,
down to 7 cfs, From 10 cfs down to 7 cfs juvenile brown trout habitat decreases 11 'Yo,

Spring high flows obviously impact trout populations regardless of available refuge
habitat. Nehring (1985,1993) reported decreasedjyoung-of-the-year (yay) age classes in
streams following low habitat levels modeled by IFIM/PHABSIM at high flows, Chadwick
(1998) states that the current becomes too fast at high flows and fish, especially YOY, are
essentially flushed downstream and out of the system. This information actually supports the
fact that a 4 cfs minimum flow would affect the trout populations drastically. Although we
believe that the refuge habitat allows YOY to escape high flows, many are lost during spring
runoff. The problem with a 4 cfs flow is that it wj\l drastically reduce spawning habitat (Miller
1992 reports 63% loss of spawning habitat), thus there will be less of the YOY age class, thereby
compounding the impact of high flows. This is v,(hy the CDOW did not recommend a 4 cfs .-
minimum flow for the CWCB in-stream flow right. Chadwick (1998) reports the use of the ..
R2CROSS, the hydraulic method used by the CDOW, would reveal that the 4 cfs does not meet
the minimum criteria of the CDOW for selection of a winter low flow.

iv) We reviewed the Nehring reports referenced by Chadwick for his assumption that
spring high flows limit trout populations. Nehririg did not simply conclude that spring high
flows create the only habitat bottleneck. He stat~s that two bottlenecks occur and that protecting
incubation flows over the winter is important. N~hring (1993) clearly states (page 13) "
However, these data actually indicate that flow stability from egg deposition (in October-
November) through hatching (April) is critical toibrown trout year class recruitment", In fact,
Nehring (1987 and 1993) did not find the strongest correlations between Mean Monthly Flow
(discharge) and fry populations but between Weighted Useable Area (WUA) and fry
populations. He explains that the relationship between discharge and fry abundance as bimodal.
When the flows are low, WUA is high, As the fl0WS increase to fill the channel (bankfull) the
WUA decreases for fry but when the flows exce~d the channel, the WUA increases again
representing the available side-channel and wetl4nd habitat discussed for Snowmass Creek
above. The overbank flows create the shallow s~ow moving water that provides fry habitat.
Nehring took cross-sections all the way across the channel into the floodplain whereas the IFIM
studies on Snowmass Creek only measured the active channel. This is why discharge does not
correlate as well with fry abundance (a negative ,correlation) as well as with WUA because, as
the discharge exceeds bankfull, fry abundance av-d habitat (WUA) actually increased.

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