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I I I I II I I ,I I ,I ,I I I II !I ,I ,I I I STUDY AREA DESCRIPTION DRAINAGE BASIN CHARACTERISTICS The Cache La Poudre River, with a total drainage area of 1,890 square miles, is a major tributary of the South Platte River. Beginning at its headwater in Rocky Mountain National Park, the Cache La Poudre River drains into portions of Larimer and Weld Counties, flowing first in a north and then easterly direction and joining the North Fork Cache La Poudre where the river leaves the mountains. At this point the Cache La Poudre River enters the high plains where it flows in a southeasterly direction through Fort Collins and Greeley to its confluence with the South Platte River. The Cache La Poudre River flows 121 miles from the headwaters at an elevation of over 12,000 feet mean sea level (m.s.l.), to its mouth, at an elevation ofapproxirnateIy 4,600 feet m.s.l. No further investigation was performed on the areas of the river outside of the study reach area other than the previous flood investigation studies sited earlier in the report. STIllY REACH DESCRIPTION The portion of the Cache La Poudre River studied in this report extends from the bridge located at Weld County Road 27 to the bridge located at Weld County Road 29. The floodplain is relatively flat and averages a Y:z mile in width. The channel is moderately sinuous, the 10,500 feet length of river within the study reach covers only 1-1/4 miles. Channel dimensions vary with nominal depths ranging from 3 to 8 feet and widths of 16 to 100 feet in width. The stream bed is gravelly and has an average slope of I I feet per mile through the study reach. The banks of the stream are vegetated with weeds and occasional clumps of trees. The floodplain is used predominantly for agriculture and gravel mining. Buildings associated with the aforementioned uses occur at intervals along the floodplain. A moderate degree of urbanization occurs within the 1-1/4 mile strip of the floodplain. The urbanization consists of mostly residential acreage tracts. There are two roadway bridges which cross the Poudre River within the limits of the study. The upstream bridge occurs at the Weld County Road 27 crossing, which is also the upper most limit of the study reach. The downstream bridge occurs at the Weld County Road 29 crossing, which j.s also the lower most limit of the study reach. The decks of both bridges are considered to be above the 100 year flood elevation. Portions of the approach roadways may be overtopped at both bridges during the 100 year event. Elevated roadways that cross the floodplain impede flood flows and raise the water surface to a degree dependent on the height of the roadway and the size of the waterway opening at the bridge. Likewise, irrigation diversion structures may impede flood flows. The diversion dams or weirs impede channel flows, while the embankments adjacent to the diversion ditches impede overbank flows. 3