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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111-3

annual precipitation decreases from approximately 16 to 30 inches in the moun-
tains to 12 to 16 inches in the plains.

Creek.
tat ions
and are

There are four USGS stream gag i ng s ta t ions a long lower St. V ra i n
All gages in this reach are affected by upstream diversions and impor-
for irrigation. Gaged flows are substantially regulated by reservoirs
affected by trans-Basin imports.

Gaging Station 7240, St. Vrain Creek at Lyons, is located just
downstream of the confluence of North and South St. Vrain creeks where the
drainage area is 212 sq mi. There are several ditch diversions above the
gage, but the St. Vrain Supply Canal, which imports Colorado-Big Thompson
Project water into the St. Vrain Basin, is located just downstream. There-
fore, this gage is a useful reference point for inflows to the plains portion
of the Basin. Based on an 1895 to 1981 continuous period of record, the aver-
age annual gaged discharge at Station 7240 is 92,470 ac-ft. Calculation of
virgin flows for the period 1951 to 1980 results in an estimated average
annual discharge of 116,000 ac-ft. Note that the 1951 to 1980 period contains
the 3 years of least runoff during the enti re period of record. These are
1956, 1966, and 1977, in order of decreasing gaged flow.

Gaging Station 7251, St. Vrain Creek near Longmont, with 370 sq mi
of drainage area, was in place from 1965 through 1968, while Station 7254.5,
St. Vrain Creek below Longmont, reflecting a drainage area of 424 sq mi, has
data from 1977 to 1981.

Gaging Station 7310, St. Vrain Creek at mouth (confluence with the
South Platte River), drains 976 sq mi, including Boulder Creek. During the
1928 to 1981 period of record, the minimum and maximum recorded flows are
12 cfs and 11,300 cfs, respectively. The average annual discharge, which is
greatly affected by diversions and storage, as well as trans-Basin imports, is
approximately 150,000 ac-ft.

C. Groundwater Hydrology
1. Geology and Hydrology of the Study Area

The St. Vrain Basin is geologically diverse, but may be classi fied
into three distinct hydrogeologic terrains, which are generally coincident
with the three topographic provinces described in Chapter II: (1) the Pre-
cambrian igneous and metamorphic mountains west of the hogback ridges
(mountains), (2) the upturned sedimentary strata exposed as hogback ridges,
immediately east of the mountains (foothi lis), and (3) unconsol idated allu-
vial, eol ian and glacial deposits which overl ie sedimentary rocks in most of
the area east of the hogback and limited areas in the mountains (plains).

The
permeabi I i ty,
water suppl ies

mountainous portion of the St. Vrain Basin is underlain by low
igneous and metamorphic rocks, which yield limited domestic
at best. The plains portion of the Basin is underlain by 8,000