WSP12086

• File Number: 8449.800.A
• Description: Metro Study SB 96-74
• State: CO
• Basin: South Platte
• Date: 4/1/1998
• Author: CWCB
• Title: Denver Basin and South Platte River Basin Technical Study, Technical Addendum 1 and 2, Senate Bill 96-074
• Water Supply Pro - Doc Type: Publication

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Chapter 1: South Platte Basin Inventory

March 21,1998
Page 1-7

Left Hand Creek Near Boulder natural flow +
St. Vrain Creek at Lyons natural flow +
Big Thompson River at Mouth of Canyon. Near Drake natural flow +
Cache La Poudre River at Mouth of Canyon, Near Ft. Collins natural flow +
(estimated runoff for remaining mountain drainages) +
(estimated runoff for remaining plains drainages).

Natural flow estimates for Boulder Creek at 75th Street, which covered a 1950-
1985 period of hydrologic record, were obtained from previous studies by Hydrosphere
for the City of Boulder (Hydrosphere. 1993).

Natural flow estimates for Left Hand Creek and St. Vrain Creek. which covered a
1970-1980 period of hydrologic record, were obtained from previous studies by Leonard
Rice Consulting Water Engineers for the Colorado Water Resources and Power
Development Authority (LRCWE, 1984). These estimates were extended to cover the
1950-1969 period of record by correlation with natural flows for Boulder Creek at 75'"
Street.

Natural flow estimates for the Big Thompson River, which covered a 1950-1994
period of hydrologic record, were obtained from previous studies by the Northern
Colorado Water Conservancy District (NCWCD, 1997).

Natural flow estimates for the Cache La Poudre River at mouth of , which
covered a 1950-1980 period of hydrologic record, were obtained from previous studies
by RCI for the City of Fort Collins (RCI, 1985).

The remaining mountain drainages tributary to Kersey include the Coal Creek,
Little Thompson, Buckhorn Creek and Cottonwood Creek drainages. While each of
these drainages have been gaged for some period of time, none of the records for
these drainages extend for more than 10 years. Natural flow estimates for these
remaining mountain drainages were developed by correlation of gaged flows for these
drainages with natural flow estimates for the Cache La Poudre at Mouth gage location
based upon drainage area and runoff per square mile. The resulting flow component
for ungaged mountain tributaries is approximately 31,000 acre-feet per year, or
approximately 2% of the estimated natural flow at the Kersey gage.

Natural flow estimates for remaining plains drainages tributary to the Kersey
gage were developed by measuring the remaining plains drainage area, applying an
average annual unit runoff factor of 1.08 inches per year (USGS 1970), and distributing
the estimated annual runoff into monthly fractions based on Longmont precipitation
data. The resulting flow component for the remaining plains drainages tributaries to
Kersey is approximately 155,000 acre-feet per year, or approximately 11% of the
estimated natural flow at the Kersey gage.

Hydrosphere Resource Consuhants, 1002 Walnut Suite 200, Boulder, CO 80302