BOARD02010

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

e

Walsh & Walsh then conducted an analysis of variance (ANOV A) statistical test on the
data collected to determine if statistically significant differences existed for the variables over
changes in stream discharge. The ANOV A test allows for comparisons of variance between
groups and tests the null hypothesis that there is no mean difference between redd depths and
velocities with changes of discharge. The results section of the report is extensive but we will
only report the relevant results. Redd pit and tailspin depths were significantly reduced relative
to December ice conditions. Redu<;tions in stream discharge significantly decreased redd pit
velocities from 0.71 fils (November 16.9 cfs) to 0.3 ftls (February 8.9 cfs), Reductions in stream
flow significantly reduced redd tai1spill velocities from .93 ft/s (November 16.9 cfs) to 0.47 ft/s
(February 8,9 cfs), Walsh reports that these velocity reductions were highly significant
(p=O.OOOO). The percentage of fines increased significantly over the winter period.

Next, Walsh & Walsh compared their measured velocities at the redds with redd
velocities from Habitat Suitability Curves developed by Bovee (1978) and Raleigh et. aI. (1986)
for both brook and brown trout spawning sites. These curves provide known optimal velocities
at redds for successful reproduction, Walsh used the lowest end ofthe optimal range to remain
conservative. The February low flow data of8.9 cfs create "profound reduction of velocity
optima," where upstream velocities were reduced by 52% (brook trout) and 92% (brown trout)
and mean tailspill velocity was reduced 29% for brook trout and 60% for brown trout.

e

Walsh & Walsh (1995) also assessed the substrate composition of the redds comparing
November to February after several months of decreasing flows. They found that fines increased
significantly from 11.1 % in November to 28.9% in February, They cited other studies that found
that the percentage of fines in a redd becanle critical at the 20 to 30% level. Again, fines fill the
spaces between the gravels preventing oxygen from reaching the incubating eggs and later
preventing the sac-fry (aelvins) from leaving the gravel.

This study documents the physical changes that occur at trout redds on Snowmass Creek
as flows naturally decrease from fall spawning flows through the winter. The decreases are not
entirely natural as the SWSD diverted small amounts of water during the study, The report
further evaluates how these physical attribute changes compared to other known studies on the
physical requirements for successful trout redds, Obviously, as in any other river system, the
trout populations at Snowmass Creek have adapted to the flow regimes and a certain percentage
of the redds for the winter of 1994-1995 were not successful. The trout population adjusts to
these natural variations. A higher percentage of redds will be successful in higher flow years.
The trout population in Snowmass Creek is almost entirely dependent on spawning as the State
of Colorado does not stock and whatever private stocking occurred in the past will be even more
limited now with the strict state regulations on private stocking aimed at controlling the spread of
whirling disease

The State of Colorado has developed a winter minimum flow for Snowmass Creek that
incorporates a stair-step or graduated flow based on the hydrologic cycle and the flow year
present in the stream. This in-stream flow is a water right held by the State's Colorado Water
Conservation Board (CWCB) and it only limits snowmaking diversions for the Snowmass Ski
tit Area. The municipal diversions are a water right senior to the CWCB' s in-stream flow right and

11