FLOOD10184

• Designation Number: 430
• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: FIS - Fort Collins, CO
• Date: 3/1/1996
• Designation Date: 7/1/1996
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Associations have moderate infiltration rates, while the soils of the Fluvaquents-FluvenlS
Association have moderate to low infiltration rates (Reference 10).

The Cooper Slough study reach is approximately 1.5 miles long. The tributary area for
the Cooper Slough basin is approximately 10.2 square miles (Reference 2). The basin
is approximately 6 miles long and I. 75 miles wide, and is fairly flat, with poorly defined
drainage patterns. Cooper Slough flows in a southerly direction and joins Boxelder Creek
in the vicinity of Prospect Street and 1-25, east of the City of Fort Collins.

The Dry Creek study reach is approximalely 1. 9 miles long. The stream is a tributary
of the Cache La Poudre River, which flows from the northwest in a southeasterly
direction to ilS confluence with the Cache La Poudre River on the eastern corporace limits
of the City of Fort Collins. Over 500 structures are located within the base floodplain
in the City of Fort Collins. The total number of structures throughout the Dry Creek
floodplain for both County and City aleas was estimated as over 800 structures in 1995
(Reference 11). The channel is approximately 3 to 4 feet deep and 10 to 15 feet wide
from the downstream limit to Andersonville, after which the channel is basically
nonexistent to the upstream limit of the study. The stream channel gradient within the
study reach is approximately 18 feet per mile. The soils within the Dry Creek basin
floodplain are of the Fluvaquents-Fluvents and Otero-Nelson Associations (Reference 10).

Boxelder Creek flows in a southerly direction through Larimer County and the City of
Fort Collins. The Boxelder Creek watershed, approximately 32 miles long with an
average width of 8 miles, flows from an altitude of 7,720 feet at its headwaters in
Wyoming to 4,860 feet at its confluence with the Cache La Poudre River. Geological
formations exposed within the watershed consist of rocks and sediment ranging from
Precambrian to Quaternary in age, and the soils in the basin are of the Fluvaquents-
Fluvents and ATguistolls-Haplustolls Associations (Reference 12).

Principal Flood Problems

Floods in the City of Fort Collins alea usually result from general rain and cloudburst
storms from May through September. Floods resulting from heavy rainfall over the
watershed are characterized by high peak flow of moderate volume and duration. The
cloudburst storm is a high-intensity, short-duration rainstorm that produces a flood with
a high peak flow, short duration, and relatively small volume of runoff. Peak runoff will
usually occur within a few hours after a single storm. Flooding can also occur as a result
of rapid spring snowmelt. Snowmelt-caused floods are characteristically oflong duration.

Notable floods occurred on the Cache La Poudre River in 1844, 1864, 1884, 1891, 1904,
1923, and 1930 (Reference 13). The three largest floods occurred in 1864, 1891, and
1904, with flood peaks of approximately 21. 000 cubic feet per second (cfs). The 1904
flood probably did the greatest financial damage. During this flood, approximately 150
houses were swept away and all bridges, with the exception of one, were destroyed. The
snowmelt runofffrom 1983 produced a peak of 7,000 cfs. This was the highest peak in
53 years. Extensive channel damage occurred because of prolonged duration of the
runoff (Reference 14).

Floods occurred on Spring Creek in 1902, 1904, 1938, 1949, and 1951, prior to the
completion of the Horsetooth Reservoir, which cuts off the upper portion of the Spring

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