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I I I I I 'I II II 'I t ~ '. I '. " I '. I I INTRODUCTION Northwest of Denver, in Jefferson and Adams Counties, Little Dry Creek rises at the eastern edge of the foothills and flows southeastward to its confluence with Clear Creek. Topography of the Little Dry Creek basin is dominated by the bordering hills and ridges. The upstream region in Jefferson County and Arvada has a narrow floodplain. The downstream lowlands in Adams County and Westminster are fairly level and wide. While no serious flood damage has been recorded on this stream, the Cities of Westminster and Arvada have experienced rapid growth in recent years and the trend is toward fully developed conditions within a few years causing increased runoff potential in all reaches of Little Dry Creek. Industrial and residential developments already exist along the lower reaches of the stream and without regulation, anticipated future growth could spread urbanization to agricultural areas located in probable flood areas. Data in this report cover 15.4 miles of the main channel and tributaries of Little Dry Creek and identifies flood hazard areas in these reaches. STUDY AREA DESCRIPTION Drainage Basin Characteristics Little Dry Creek, located in Adams and Jefferson Counties, is a left bank tributary to Clear Creek. From its origin in the hills about a mile and a half southwest of Standley Lake, the Little Dry Creek basin slopes southeastward to Clear Creek. The confluence is immediately downstream of the Colorado and Southern Railroad bridge over Clear Creek. The Little Dry Creek drainage basin is oblong in shape, approximately 9 miles long and 2.7 miles at the widest point. It drains 13.3 square miles. It rises at about elevation 5,650 feet above mean sea level and falls about 480 feet to elevation 5,170 feet in its 8 mile course. -3- Just downstream from Sheridan Boulevard, Little Dry Creek is joined by the Shaw Heights Branch, a left bank tributary draining approximately 1.5 square miles. Two other tributaries draining a total of 2.5 square miles flow into Lake Arbor, an irrigation lake which acts also as a detention pond, before discharging into Little Dry Creek just upstream from Sheridan Boulevard. In the upper reaches, the Farmer's Highline Canal and Croke Canal intersect Little Dry Creek as they meander across the drainage basin. The Croke Canal carries storm runoff from nearby residential developments and a spill structure has been constructed at Little Dry Creek to separate storm from irrigation waters. The Farmer's Highline Canal presently has no flow separation structure at Little Dry Creek. Further upstream, minor dams for stock watering or irrigation have been constructed across the channel. The development status of the entire Little Dry Creek basin has been classified as "B" by the Denver Regional Council of Governments (DRCOG) in their 1971 Project REUSE, meaning the basin is not yet fully developed, but will become so in the immediate future. East of Sheridan Boulevard in Westminster and Adams County, the basin is now nearly fully developed with medium and high density residential areas on the hillsides and residential, commercial and industrial areas in the valley. West of Sheridan, residential development has occurred throughout Arvada and is rapidly spreading into the unincor- porated areas of Jefferson County at the westernmost limits of the basin. With this development comes a significant increase in storm runoff. Study Reach Description Typical of South Platte River tributaries around Denver, the Little Dry Creek channel can contain only minor flows. Development in the floodplains east of Sheridan Boulevard has severely encroached on the channel primarily because most reaches of the channel are on private property. The channel has been further constricted by numerous culverts placed beneath roads, parking lots, and the Colorado and Southern Railroad tracks.