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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The Pacific Southwest Interagency Committee (PSIAC, ref. 11) has deve-

loped a method of rating sediment yield from a watershed for use as an aid for

broad planning purposes. The method consists of numerically rating nine fac-

tors which influence sediment production in the watershed, and then summing

the nine ratings. This final rating corresponds to a range of sediment yields

(Sy) in acre-feet per square mile, Table 20. The ranges of estimated Sy

values indicate that precision was not the intended result of the PSIAC

method.

Table 20. sediment Yield Classification (ref. 11)

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The PSIAC method has been tested against actual Sy values measured in

ponds and dams in the southwest. The comparisons were done on watersheds of

less than about 20 square miles, and PSIAC results agreed with or were

slightly lower than actual measurements.

Estimation of Sy based on the PSIAC method is used as a guideline to

annual yields. However, due to the erratic nature of precipitation and

floodwater flow in the study area, a more useful approach is to predict Sy

on an event basis. More accurate prediction of Sy is possible only by

detailed analysis using physical process models of runoff and sediment produc-

tion on an event basis. Predicted values of long- and short-term sediment

production are dicussed in a subsequent section.

The nine factors that are rated are; surficial geology, soil, climate,

runoff, topography, ground cover, land use, upland erosion, and channel ero-

sion and sediment transport. These factors were rated for the Fossil Creek

watershed based on data from aerial photography, soil surveys and maps, as

well as site visits. The following paragraphs present a brief discussion of

each of these factors in the Fossil Creek watershed.

Annual
Sediment Yield
Rating Classification ac-ft/mi2
> 100 1 > 3.0
75 to 100 2 1.0 to 3.0
50 to 75 3 0.5 to 1.0
2S to 50 4 0.2 to 0.5
0 to 25 5 < 0.2

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5.2.1 Geology

In most areas of the Fossil Creek watershed, geologic formations are

covered with alluvium, colluvium, or residual materials. Bedrock consists of

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sandstone which outcrops along a reach of Lang Gulch. Also, in the upland

hills of the watershed, rocks and rock exposures are common. There is no

active faulting in the area.

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