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Almost all record floods on the South Platte River have been generated near the river's headwaters on the slopes of Monument Divide, a high ridge located between Castle Rock and Colorado Springs, extending from the Rocky Mountains down to the plains near Limon, Colorado. Past floods have resulted from snowmelt and intensive rain storms over the mountain tributaries, rainstorms over the eastern tributaries, and combinations of these conditions. In 1965, a unique combination of orographic effects and meteorological conditions in the South Platte River Basin caused the worst flooding in the region's recorded history. Severe thunderstorms commenced over the headwaters of Plum Creek and Cherry Creek on June 16, and moved northeasterly down the creeks following and augmenting peak flows. More than 14 inches of rain were recorded at Palmer Lake in 4 hours. Overnight, westerly winds moved the storm front to a position over the Kiowa and Bijou Creek Basins where it met with thunderstorms forming just south of Agate. Here, 5.25 inches fell in 45 minutes. The net resul t s of these conditions were six persons drowned, two other deaths caused by flood-related activities, and estimated damages of $500 million in the South Platte River Basin, of which $300 million occurred in the Denver area. Major floods affecting the City of Commerce City area have occurred on the South Platte River and Sand Creek since 1844. During that period, 11 floods occurred on the South Platte and 10 notable floods occurred on Sand Creek. The major cause of floods on the South Platte River and Sand Creek are cloudbursts of intensive rainstorms which normally occur during the period of May through August. The South Platte River flooding is also aggravated by snowmelt runoff on the tributary streams during the rainstorm period. There are two areas of shallow sheetflow within Commerce City, both of which are along Sand Creek. The upstream area is on the southwest side of Sand Creek between the corporate limits and east 49th Drive. In this area, the ground slopes away from the channel and the flow cannot return. The second area is under Interstate Highway 270, at the two railroad underpasses to the northwest. During a large flood event, the flood waters will pass under the interstate bridge and flow along the low ground away from the channel, returning to the South Platte River north of East 64th Avenue. Severe flood runoff is transported through the City of Federal Heights as both overland shallow flow and as channel flow. The steep slope of the land, the close proximity of mobile homes to Tributary M of Niver Creek, and the presence of several culverts that are inadequate to convey major storm runoff combine to create flooding problems. 10