FLOOD09485

• Designation Number: 206
• County: Larimer
• Community: Fort Collins
• Stream Name: Fossil Creek Drainage Basin
• Basin: South Platte
• Title: Master Drainageway Planning - Fossil Creek Drainage Basin, Fort Collins, Larimer County, Colorado
• Date: 8/1/1982
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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V. SEDIMENTATION AND EROSION ANALYSIS

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The sedimentation and erosion analysis of Fossil Creek and its tribu-

taries consisted of two levels 1 an initial qualitative assessment of the

system, and a long-term equilibrium slope analysis. The qualitative

assessment was based on evaluation of aerial photographs, sediment yield, and

stream bank stability inventory. The qualitative assessment agreed closely

with the results of the equilibrium slope analysis. In general, the system is

relatively stable with the exception of problem areas discussed in the next

section. The developed condition experienced more degradation and less aggra-

dation throughout the system than the existing condition. The increase in

discharges and decrease in sediment supply associated with developed con-

ditions increases the sediment transport capacity while decreasing the sedi-

ment supply of most reaches, causing an increase in degradation over the

existing condition.

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5. 1 Aerial Photographs

A review of aerial photographs dated 1937, 1950, 1956, 1969, and 1978 of

the Fossil Creek watershed was made. Scales ranged from approximately one

inch equals 1300 feet to 1500 feet. The photographs showed most irrigation

ditches, diversions, channel crossings, and reservoirs to be in place by 1937.

Major changes were related to new channel crossings, improved channel

crossings, and ponds located along the channel system. Observable changes are

listed below.

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Fossil Creek between Fossil Creek Reservoir and the Union Pacific
Railroad had been realigned prior to 1937. The existing channel
alignment is the same today as it was in 19371 in the 1937 pho-
tographs, old channel meanders are apparent.

1.

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2.

Upstream of Trilby Road
sloughing has occurred.
increase from the early

(County Road 34) along Fossil Creek, bank
The amplitude of the meanders appears to
photos as compared to later photos.

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3.

Upstream of County Road 13 (Lemay Avenue) the meanders are shown to
have shifted and are more closely spaced in the 1978 photographs
than in the 1937 photograph. Some bank sloughing has occurred to
cause a more irregular channel.

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4.

The channel downstream of Highway 287 has been changed due to resi-
dential development prior to 1969.

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10.

5.

Upstream of the sediment pond at Fossil Creek Drive the meanders are
shifting and by 1969 the amplitude of the meander increased as com-
pared to earlier photographs.

6.

The reaches of Fossil Creek and Burns Tributary upstream of Shields
Street appear to have followed the same meander pattern since 1937.
The only difference is that since the road embankment for Taft Hill
Road has been in place some aggradation has occurred upstream of the
channel crossing, where it appeared less defined in 1978 compared to
1937.

7.

stanton Creek is not covered by the photographs in significant
detail to allow comparison.

8.

Lang Gulch appears to follow the same meanders in 1978 as it did in
1937 downstream of Trilby Road (County Road 34).

9.

The channel for Smith Creek is less defined in 1978 than it was in
1937 at the Shields Street crossings.

Burns Creek at Shields Street is less defined upstream of the road-
way embankment in 1978 than it was in 1937. The channel upstream of
Shields Street appears to be more vegetated in 1978 than it was in
1937 and the meanders appear to have less bank sloughing indicating
stability or lack of high flows.

5.2 Sediment Yield

Two formal site visits of the study area took place besides numerous

general inspections. General observations of the watershed indicated:

1.

Bank sloughing with excessive bank heights is common along Fossil
Creek and. Burns Tributary downstream of Shields Street and along
stanton Creek below Lemay Avenue (County Road 13).

2.

Slight sedimentation behind embankments with culvert crossings has
occurred along with scour at the downstream culvert exits.

3.

The overland slopes are fairly well vegetated.

4.

Grazing and rural development are common land uses.

5.

A rock outcrop has stablized the banks and channel bed along Lang
Gulch upstream of the first Colorado & Southern Railroad Crossing
#1.

6.

A few depositional bars of fine gravel were visible along the chan-
nels. Generally, the material was very fine.

7.

Side channels which bring irrigation water into the system are
deeply eroded and have unstable banks.