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The South Platte River in Arapahoe County is a contin- uous flowing stream, whereas the tributaries are inter- mittent flowing streams. The South Platte has two major flooding characteristics--snow melt and summer weatherfronts or thunderstorms. The tributary basins are narrow, hydraulically steep, and composed of highly erodible, clay loam soils. In the undeveloped portions of the basins, the ground cover is predominantly a short grass called buffalo grass, willows, and cotton- wood trees. ~ i Development has occurred up to the channels on the tributaries. The flood plain on the South Platte River in the past was mostly agricultural, but today commer- cial, industrial, and residential development has encroached into the flood plain. In various reaches of the flood plains, development pressures continue to exist. The county government is working to retain the open space of the flood plain areas. 2.3 Principal Flood Problems The South Platte River flows through the western edge of Arapahoe County in shifting channels in a broad, shallow bed with low, flat overbanks. The streams tributary to the South Platte River are ephemeral and flow in steep, narrow channels; whereas those in the eastern two-thirds of the county flow in wide, flat channels similar to the South Platte River. All the streams studied have seen various structural improvements but the intense and infrequent thunder- storms characteristic of the area can generate floods much in excess of existing structural capacities. The flood threat throughout the community has not been adequately defined and urbanization has occurred in certain areas without regard for the hazard. Major floods have occurred on the South Platte and its tributaries throughout the history of Arapahoe County since 1844. During that period, eleven devastating floods have occurred on the South Platte River, seven- teen on Cherry Creek, and three each on Box Elder, Comanche, and Bijou Creeks. !! ~ 6 .........