PUB00108

• Year: 1986
• Title: St. Vrain Basin Reconnaissance Study
• CWCB Section: Administration
• Author: Colorado Water Resources and Power Development Authority
• Description: Extensive study for future study and development toward meeting the water-related needs of residents with the St. Vrain Basin
• Publications - Doc Type: Brochure

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11-5

Two aquatic species of special interest occur in the area. A popu-
lation of greenback cutthroat trout, a Federally endangered species, is known
to occur in Como Creek, a tributary to North Boulder Creek. The johnny
darter, which occurs in the foothi lis zone of St. Vrain Creek, is currently
I isted as a threatened species in Colorado by the Colorado Division of Wi Id-
life.

3. Water Quality

Water quality in the St. Vrain Creek Basin is highly variable. The
headwaters and upstream reaches have good to excellent water quality while
there is significant deterioration in several water qual ity parameters down-
stream from Lyons. The upper reach, above the town of Lyons, is characterized
by generally good water quality with low water temperature, high dissolved
oxygen and rocky and gravel bottoms. This upper segment of the Basin general-
ly maintains sustained flows.

A 1983 study of St. Vrain Creek indicated that the water quality of
the stream at Lyons is good and meets the state stream standards of a Class I
cold water fishery. Water qual ity below Lyons to Hygiene Road also appears to
be capable of supporting a Class I cold water fishery; however, periods of low
stream flow during the winter months appear to be the limiting factor in sus-
taining a fishery (DRCOG, 1983c).

The St. Vrain streambed through the Lyons and Longmont reaches is
si Ited as a result of increased development along stream banks and the resul-
tant increases in storm runoff. Agricultural withdrawals below Lyons have
reduced flows considerably from those of the past. Water qual ity downstream
of Longmont is generally poor due to agricultural return flows.

Non-point sources deliver significant quantities of pollutants to
the St. Vrain Basin waterways; however, few data are avai lable for quantifying
the relative contribution of non-point to point source pollution. The data
indicate that loadin9 from non-point sources for total dissolved sol ids, fecal
coliforms, fecal streptococci, and nitrates outweigh loadings from point
sources. Ammonia and phosphate contributions from point sources exceed those
from non-point sources. Significant loadings of organic materials (BOD) occur
from both point and non-point sources. It is believed that agricultural irri-
gation return flows are the primary cause of the high total dissolved solids
contributions from non-point sources. The high levels of microorganisms
del ivered to St. Vrain streams are bel ieved to be a resul t of stormwater
runoff (DRCOG, 1981).

Water qual ity data from the EPA STORET retrieval system were analy-
zed for five sampling stations in the upper and lower basins. Data summaries
are shown in Table 11-1. Measured pH levels increase as the St. Vrain Creek
flows from the high mountain sources through the foothi lis down through the
plains to Weld County. The increase in pH provides evidence of dissolved salt
loading which buffers the water at higher pH levels. This is supported by
data which show significant increases in conductivity from 28 to 1,300 mg/l as