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I I I I ~ j ) i 1 ! ~ I I I I I I I I 7 3. The riverbed is alroost completely covered bank-to-bank with water during low-flow. The~ are hardly any exposed gravel and sand bars. 4. Both banks are stabilized with rubble, riprap, concrete walls, timber or steel pilings to prevent the river from moving laterally. 5. There 1s an absence of trash as compared to the suburban areas. Efforts are made to keep the river clean and to beautify its surroundings. 6. There 1s hardly any riparian vegetation in many places and little evidence of pristine vegetation. The old cottonwoods are gone; only a few willows volunteer to come back. 7. Works of men abound. The floodplain is completely covered with roads, houses, buildings and a few parks. There are numerous bridges and sewer outfalls, and more diversfon dams and drop structures. Facilities have been constructed in and next to the rfver for people to enjoy the rfverine envfronment. The urban river fs well illustrated by reports of the City of Denver (l966) and photographs fn the book by Shoemaker with Stevens (1981). Geoloqical Settinq The geological setting for the South Platte River in the Metro- politan Denver area has been described by Hunt (1954). Unless otherwise noted, the infonnation given below is from his paper. The South Platte River flows in a valley cut into the landscape during Pleistocene or Recent times. The valley floor is protohistoric and historicalluvium,(T(lstlyreworkedgraveloverl;J;inbyafewfeetofdark humus--rich sandy silt. The v;J;lley floor is from 112 to 2 miles wide in the Denver area. The average width is approximately 3/4 mile. Upstream from Sixth Avenue W. in Denver, there are a few outcrops of Wisconsin Stage gravel fills in the valley floor. One is at the Overland Park Municipal Golf Course. These gravels are chiefly granitic pebbles mostly less than Tinch fn diameter, well-bedded and well sorted. The Denver Formation underlies the protohistoric and historic alluvium and granitic gravels. This formation is deeply weathered but compact conglomerate and siltstone and some clay shale of Upper Cretaceous and Paleocene age. Theformat1onhaslentfcularbedding. Theconglomerate commonly forms channel deposits in fjner textured beds. The topography of the upper surface of the Denver formation is highly irregular. The ASCE COl11llittee on Denver So11s (1954) reports that ". . . it was actually found that at some [buildinaJ sites, the surface of the rock would be 20 feet deeper at one point in the site than at another." . I 6 r f I I I I I I I I I I I I I I I I Their observations are that the Denver and underlying Arapahoe Formations consist of nearly flat-lying shale and sandstone. The Arapahoe Formation crops out in some low areas near the South Platte River. These two forma- tions are very similar fn lithology and are sometimes not differentiated. The Committee (1954) reported that the Denver Formation is composed mainly of silty shale wfth many layers and channels of soft silty sand- stone. Thetoprrostl1thologicunitfsabrown,highlyfractured,smooth textured, weathered claystone which f5 easily excavated. Immediately underneath are sandy claystones and claystones, brown, with some fractures. They are gritty, weakly cemented and fairly hard when exposed. Thereafter, thereal'e a blue soft claystone and blue and hard siltstones and claystones, the hardest material in the formation. All samples of rocks in the Denver Formatfon disintegrate in water after being allowed to dry in air. The floodplain and terraces of the South Platte River are from 5 to 30 feet thick with a maximum of 50 feet in a few places. The Denver Formation outcrops in the r1verbed at quite a fe....places through the study reach. The locations where these outcrops could be identified from the river bank are shown on the river profile in Appendix 1. The Broadway Terrace is the most prominent of the terraces in the Metropolitan Denver area. It forms the rigntvalleywdll of th" South Platte River throughout the study re;J;ch. It also exists along the left side of the valley floor downstream from the confluence with Clear CreeL The terrace varies in height from 50 feet above the river near the confluence with Cherry Creek to 20 feet near Brighton. The South Platte River cuts into the Broadway Terrace on the left side of the valley floor near Eighty-eighth Avenue east of Thornton in Adams County. The Broadway Terrace is sand and gravel fill of the Wisconsin Stage. The upper phase is clean coarse sand composed of granite, quartz and gneiss fragments, approximately 15 feet In thickness. Underneath,are coarse gravel and cobbles, some exceeding 6 inches 1n diameter composed of the same materials as the sand above. At other places along the valley floor through Denver and along the left valley wall downstream from Clear Creek, there are terraces of Piney Creek Alluvium. This is mostly highly calcareous, well stratified clay, silt and sand contafning thin lenses of gravel. Its thickness is generally IT'ON! than 8 feet. TneSoutn Platte River does not cut into the Piney Creek alluvium. The Denver Formation constitutes the left or west valley wall of the South Platte River from its confluence with Cherry Creek to its confluence with Clear Creek. To the east side of the valley, the landscape surface is eolian sand and silt blowout from the river valley and areas to the west. To the west