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FIGURE 3. - Flooding along Kennys Gulch at 21st and Ford Streets in Golden. Urban runoff and overflow of Welch ditch exceeded capacity of storm drains. Photograph by John R. Keith, May 6, 1973, at height of storm. overwhelmed by the backup of water from over- flowing ditches and storm sewers. Outside the urban area, according to Don Svedman, State Deputy Commissioner of Agri- culture, farmers and ranchers sustained heavy flood losses, partly the result of overfilled washed-out irrigation ditches (The Denver Post, May 8, 1973). GEOLOGIC EFFECTS OF THE STORM The processes of erosion, transport, and re- deposition of materials by the floodwaters, and the infiltration of moisture into the ground- either directly from precipitation or indirectly by lateral soaking from flooded stream courses - altered the landscape in many ways, Stream- banks and structures along the banks were at- tacked by scour, and, in many places, \vere destroyed, Thousands of acres of flooded lands were coated with mud and sand, Saturated hill- sides and stream banks gave way under the added weight of the moisture. Locally, rocks bounded down hillsides onto roads and high- ways. For discussion purposes, the geologic effects of the storm are here grouped by process under the three main headings "Scour," "Sedimenta- tion," and "Mass Wastage." Scour predomi- nated over other processes in its effects on manmade structures and the resultant damage costs, Mass wastage-mainly landsliding-was widespread in hilly areas, but its economic con- sequences were much less than those of scour. Sedimentation was the least destructive of all in the Greater Denver area, although it did much damage to croplands downstream along the South Platte River. SCO(;R The sediment-carrying capacity of a stream increases enormously in flood. As the volume of water rises, so does the velocity, and with in- creased velocity, the carrying capacity increases geometrically - the enlarged stream, flowing faster, scours its channel and quickly acquires a load of sediment. Conversely, the sediment carried in flood is quickly dropped when stream velocity is reduced. Nearly all perennial streams in the area, and countless intermittent ones, scoured segments of their banks and beds, Scour was a maj or cause of damage along the South Platte itself, as well as along its tributaries. Scour was most noticeable in cut banks at the outsides of bends, where the force of the current has the most 6 " " m . I '" f! FI ki es co b~ to Pl be th th .., gr af ni ~ in S( ag be ac it, (~ er, ph