WSP04463

• File Number: 8460.500
• Description: Platte River Recovery Plan
• Basin: South Platte
• Date: 6/1/1997
• Author: Colo Div of Wildlife
• Title: Inventory and Status of South Platte River Native Fishes in Colorado
• Water Supply Pro - Doc Type: Publication

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ST VRAIN RIVER DRAINAGE - HYDROUNIT 10190005
The St. Vrain River drainage originates in Rocky Mountain National Park and in the
Indian Peaks Wilderness Area. The entire drainage covers roughly 433 square miles. The
transition and prairie zones composed approximately 40 percent of the total area. Much ofthe
land in this drainage (where sampling occurred) is irrigated cropland, but a large portion of the
area is heavily populated and supports extensive light-industry. Small ranches, and rural acreages
are very abundant throughout the sampling area. This drainage has been separated into two
sections for analysis: the mainstem river downstream from Hygiene to the South Platte River
confluence, and the tributaries from the foothills transition zone to the South Platte River
confluence.

The mainstem St. Vrain River was sampled in 16 locations from Hygiene to the South
Platte (Table 17). Earlier efforts and sampling sites further upstream in and near Lyons in 1994
show the St. Vrain is dominated by trout at these points; therefore, these sites were not included
in this analysis. The mainstem fish community changes rather abruptly between Lyons and
Hygiene (a distance of 4 miles) to a native minnow complex with only a few trout appearing in
downstream collections through Longmont. This abrupt change is probably triggered by
extensive water diversions which lead to water temperatures in the dewatered sections which
exceed critical thermal maximums for trout.

Overall sampling efficiency for the mainstem St. Vrain was very good. Lefthand Creek,
Dry Creek and Boulder Creek all enter the St. Vrain River between the city ofLongmont and its
intersection with 1-25. It was very difficult to sample the river downstream of these creeks
effectively all summer long because of high flows and turbid conditions. The crew finally
experienced fair sampling conditions in late October, but the river never reached the very low,
clear water conditions which were present during sampling at sites further upstream.
Consequently, the collection of rare or sensitive species collections in the downstream sites was
also infrequent. Darters, dace and topminnows are probably underrepresented due to the
difficulty in sampling. Stonecat distribution may be greater than was found because they are
underrepresented in daytime sampling.

The St. Vrain River from Hygiene to 1-25 is unique in northern Colorado with respect to
species diversity, presence of rare species, and the fact that over 25 percent of the total numbers
of fish in the entire river are either environmentally sensitive or uncommon species (Table 18).
This unique character persists in spite of three major pollution/toxic spill events in this section of
river in the past 20 years. The fish in this area are also subject to increasing pressures from
human-related water quality degradation (Woodling 1995) and decreasing water quantity (City of
Longmont, pers. comm.). The central stoneroller was followed in abundance by sand shiner,
creek chub, white sucker, and longnose dace, which comprised anywhere from 11 to 16 percent
of total numbers.

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