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<br /> <br /> <br />FIGURE 9. - South Platte River just west of Brighton, l'day 6, 1973. Silt and mud damaged hundreds of acres of <br />cropland and pasture. Aerial view is northeast toward East 160th Avenue in middle distance. Denver Post <br />photograph by Barry Staver. <br /> <br />ment, and deposition occurred where and when <br />flow velocity was slowed. Sedimentation was <br />subordinate to other flood effects in Greater <br />Denver in terms of damage to property, but it <br />caused local hardship to many people. Overbank <br />flooding was the chief cause of sedimentation <br />along most major streams, even while they were <br />still scouring their channels. Scores of acres of <br />flooded ground along the South Platte, includ- <br />ing much cropland, received sediment from <br />overbank flooding. North of Commerce City the <br />South Platte River was out of its banks in a <br />strip of land one-half to nearly 1 mile across <br />(fig. 9), At Chatfield Dam, 60 acres was inun- <br />dated. These areas were coated with mud when <br />the water disappeared. Downstream, at Platte- <br />ville, about 30 miles north of Denver, the mud <br />reportedly was 3 feet deep and caused severe <br />damage to pastures and cropland. <br />Sheet flooding resulted when temporary rivu- <br />lets spread out fanlike at the mouths of small <br /> <br />" <br />1: <br />l~~:;', <br /> <br />ravines or gullies, Much sediment was deposited <br />on roads and streets. Some of these deposits <br />graded into mudflows. <br />Many tributaries without well-defined flood <br />plains deposited large quantities of sediment <br />when they were ponded by obstructed bridges <br />or culverts, Obviously, these problems would <br />not have arisen had the channels remained clear <br />(fig. 10), <br /> <br />MASS WASTAGE <br />"Mass wastage" is a general term for the <br />processes by which rock or earth material is <br />transported en masse downslope. It includes <br />landsliding (fig. 11) but excludes sediment <br />transport by streams, Gravity,is the chief trans- <br />porting agent. <br />Many small landslides, perhaps hundreds of <br />them, were triggered in the western hilly part <br />of the Greater Denver area by the storm of <br />May 5-6. The moderate but steady precipitation <br />saturated slide-prone areas, led to renewed <br /> <br />11 <br /> <br />