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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Creek Drainage basin. The Horselooth Reservoir was completed in the early 1950s and
has helped reduce flooding problems by reducing drainage area. Floods occurred in 1975
and 1977, causing flooding in several basements, but there are no recorded discharges or
damage estimates available (Reference 15). A flood occurred on June 25, 1983, that
produced 1.9 inches of rainfall over 2 hours. The stonn sewer that carries Spring Creek
under the mobile home park surcharged and water ponded to a depth of several feet.
Property damage was estimated in the $5,000 to $10,000 range (Refelence 16).

The Cooper Slough floodplain is predominantly flat. Channel capacity is limited in
places, promoting overbank flows and divided-flow conditions. Channel flow is restricted
by relatively small culverts at Vine Drive, the Colorado and Southern Railroad (C&SRR),
and SH-14. Due to an undersized culvert at SH-14, a ponded area will fonn north of the
highway, and eventually overtop the highway during stonn events. In places, the width
of the 100-year floodplain averages over 1,000 feet, although the depth of flooding is
generally less than 3 feet, except in areas where ponding occurs. Hydrologic data from
stream gages were not available, thus no discharge and recurrence intervals for major
floods along Cooper Slough were detennined.

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Floods have been recorded in the Boxelder Creek watershed on 13 occasions since 1900.
On August I, 1961, a stonn with a frequency of 50 to 100 years caused an estimated
$76,150 in damages in the Wellington vicinity. In May and June 1976, two overtopping
25-year stonns caused an estimated $46,100 in damages and took four lives in the
Wellington vicinity (Reference 17).

Limited information is available regarding past flooding from Dry Creek. In 1904, a
flood occurred that resulted in the drowning death of a child when floodwaters overtopped
the Eaton Ditch (which intercepts Dry Creek near Willox Road) (Reference 18).
Flooding also occurred in 1924 with depths of flows several feet deep. However, it is
unclear as to whether the flooding was due to overflow from the Cache La Poudre River
(Reference 19).

Numerous irrigation canals cross the Dry Creek channel and directly intercept drainage
flows. In the past, much of the excess drainage in the lower Dry Creek basin (below
Eaton Ditch) was intercepted by irrigation canals. However, the impact of development
has increased the magnitude and frequency of drainage flow, and many of the canals no
longer have the capacity to intercept the increased drainage flows. Several of the canals,
including the Larimer and Weld Canal (Eaton Ditch), Larimer County Canal, Terry Inlet,
Poudre Valley Canal, and North Poudre Ditch, have large enough flow capacities to
impact flood magnitudes on Dry Creek (Reference 19).

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Flood Protection Measures

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There are no designated flood-control structures on the Cache La Poudre River in the
study area. Two irrigation water-supply storage reservoirs are located on the North Fork
of the Cache La Poudre River; however, the effects of the dams on the study reach were
considered negligible (Reference 6).

The Horsetooth and Dixon Reservoirs are two major reservoirs within the Spring Creek
basin. The Dixon Reservoir is located on the western edge of the basin immediately
below the hogback that impounds !he Horsetooth Reservoir. The Dixon Reservoir

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