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The Frenchman Creek drainage basin is located in the northeastern corner of Colorado in Logan, Sedgwick, and Phillips Counties. Its headwaters are within Logan County about 20 miles west of Holyoke. The creek flows in an easterly direction, emptying into the Republican River near Culbertson, Nebraska. The basin divides into two subbasins at the west edge of Holyoke. North Fork Frenchman Creek has a drainage area of 164 square miles. South Fork Frenchman Creek drains an area of 63 square miles. The combined contributing drainage area for Frenchman Creek is 227 square miles at U.s. Highway 385. The topography of Frenchman Creek basin consists of low hills dividing the waterways. The larger waterways have well-defined channels, whereas the smaller ones lie in swales between drainage divides. The basin is elongatedr with its major drainage flow patterns in a west to east direction. Ground elevations vary from a maximum of 4,500 feet above National Geodetic Vertical Datum (NGVD) in the headwaters to 3,700 feet NGVD at Holyoke. The climate of northeast summers and cold winters. are the primary source of thunderstorms account for Colorado is semiarid, with hot, dry Air currents from the Gulf of Mexico moisture. Localized and short-duration much of the total annual precipitation. 2.3 Principal Flood Problems Floodflows in this area of Colorado generally result from high- intensity, short-duration, cloudburst-type thunderstorms. Frenchman Creek has experienced few of these thunderstorm, whereas adjacent basins have experienced extremely intense thunderstorms. Most of these thunderstorms occur from late spring to late summer. Because of these highly localized storms, runoff is characterized by a quick buildup to a peak and then a tapering off. Obstructions to floodflows within the study reach include natural obstructions, such as brush and other vegetation growing along the streambanks, and nonnatural obstructions, such as bridges, road embankments, buildings, and culverts. During floods, these obstructions impede floodflows and cause backwater conditions that may increase the flood heights upstream and increase velocities downstream. Three bridge obstructions exist in the flood plain of Holyoke. The first two are for Interocean Avenue (U.S. Highway 385) and Burlington Northern. The third is for Denver Street (U.S. Highway 6) at the east edge of Holyoke. During high floodflows, these bridges can cause a backwater effect upstream of the structure. The Denver Street bridge will cause a divided flow area along the north side of the roadway because of the backwater effects of the structure. Brush and other debris washed downstream may also collect at these bridges, thus creating a more serious damming 4