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I I I I I STUDY AREA DESCRIPTION Study Reach Description lefthand Creek was studied in detail from its mouth near Longmont, Colorado to a point approximately 0.5 miles upstream of the Foothills Drainage Basin Characteristics Lefthand Creek is located in Boulder County approximately 30 miles northwest of Denver. Its he<ldwaters originate on the eastern slope of the Continental Ohide at elevations in excess of 13,000 feet. The creek Highway. Tne length of study was about 13.7 stream miles. The then flows in an easterly direction draining into St. Vraln Creek near contributing drainage <lrea varies from 72 square miles at the mouth to 56.4 squaremi1es below Geer Canyon which is imediately IJpstreamof the limits of this study. Stream bed elevations vary from 4916.3 feet mean Longmant. Table I sea level (m.s.1.) at the mouth to 5614.8 feet m.s.1- at the upstream limit of study. Stream bed slopes range from 8.5 ft/mile near the mouth to 135.7 ft/mile near the upstream limit of the study. Development along Lefthand Creek consists mainly of single-family Approximate drainage areas are given in Table 1. , DrainaqeAreas houses and their related structures in both rural and urban settings. Location Area (sq.mi.) 72.0 56.4 Development in the floodplain occurs throughout the study reach, but is concentrated in an area about 1.5 miles upstream from the mouth. Obstructions to flood flows within the study reach include natural obstructions such as large rocks, brush, and other vegetation growing along the streambank, and man-made obstructions such as bridges, Lefthand Creek at mouth Lefthand Creek at u/s study limits Normal annual precip1tation over the study area is,approximately 16 inches. Temperatures range from a high of abnut 95 degrees Fahrenheit to buildings, and culverts. During floods, these obstructions cause a low of -20 degrees (Reference 6). The topography of the Lefthand Creek baCkwater conditions that may increase the flood depths or they may cause and cottonwood. constrfctions that increase the flow velocities. 8rush washed out during floods is carried downstream and may collect at bridges and in culverts, thus creating a darrming effect and overbank fJow. As flows irn;rease, masses of debris may break Joose and cause a wall of water and debris to basin var1es from rugged and heavily forested canyon in the western part of the basin, to slightly rolling prairie land east of the canyon mouth where pasture and crop Jands border stream t>anks lined with willow brush surge downstream until another obstruction is encountered. In SO!ll€ instances, debris may collect to the pOint where structural capability is exceeded and a hrid'le is destroy..d or abutments and approaches eroded and rOddbeds damaged. 3