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<br />The topography of St. Vrain Creek, Lefthand Creek, and Dry Creek <br />No. 1 Basins ranges from rugged and heavily forested Rocky Mountain <br />canyons in the west, to slightly rolling prairie land near Longmont, <br />where pasture land and cropland border streambanks lined with brush, <br />willow, and cottonwood trees. Spring Gulch has a basin that ex- <br />tends to Terry Lake, but it has a channel only from 15th Avenue to <br />St. Vrain Creek. Loomiller Basin lies entirely within the Longmont. <br /> <br />. <br /> <br />Dry Creek No.1, Spring Gulch, and Lefthand Creek originate outside <br />the PMA in the Rocky Mountain foothills and follow generally east- <br />erly courses through the city. Spring Gulch consists of an improved <br />channel that starts south of East 15th Avenue and continues as a <br />concrete-lined channel through Spring Gulch Linear Park to 3rd <br />Avenue, then down to St. Vrain Creek. Loomiller Basin is a depres- <br />sion oriented in a northwest-southeast direction through central <br />Longmont. Floodwater accumulates in the depression and subsequent- <br />ly drains into Spring Gulch Basin near 3rd Avenue and Atwood Street. <br />A large portion of the area within the PMA limits and adjoining <br />the streams is fully developed. <br /> <br />Commercial and residential development exist in the flood plains <br />of all the streams studied. On St. Vrain Creek, from Airport Road <br />to the confluence with Dry Creek No.1, industrial and commercial <br />buildings, farms, and mobile home parks share the flood plain with <br />gravel-mining sites. The Dry Creek No. 1 flood plain is occupied <br />by both commercial building and single-family dwellings from Sun- <br />set Street to the confluence with St. Vrain Creek. Development <br />along Spring Gulch is also represented by pastureland and a linear <br />park bounded by single-family dwellings from 15th Avenue to 3rd <br />Avenue. Commercial and industrial buildings occupy the Spring <br />Gulch flood plain from Third Avenue to the confluence with St. <br />Vrain Creek. Loomiller Basin is totally developed with residential <br />and commercial buildings. <br /> <br />2.3 Principal Flood problems <br /> <br />St. Vrain Creek flows through Longmont in a broad channel bed of <br />shifting sandbars bounded by banks 10 to 15 feet high. Lefthand <br />Creek, Dry Creek No.1, and Spring Gulch flow in relatively steep, <br />narrow channels. The over banks of all the streams studied are <br />relatively flat, with the exception of the north over banks of St. <br />vrain Creek and Lefthand Creek, which rise more steeply. <br /> <br />. <br /> <br />As is characteristic of the region, flooding on these streams gen- <br />erally occurs between May and September. peak annual flows usu- <br />ally occur in May and June, however, resulting from a combination <br />of snowmelt runoff and spring rains. Floods having the most dam- <br />aging effect in the Longmont area, such as the flood of June 2-7, <br />1921, occur when a long-duration rainstorm forms over the St. Vrain <br />Creek basin with the heaviest rainfall accumulation downstream of <br />the Lyons gaging station. Other floods, such as that of June 22, <br /> <br />5 <br />