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Bank Materfals The Corps of Engineers collected samples of the river bank materials tn the Metropolitan Denver area at the same time as bed samples were taken. A summary 1s given in Table B. The bank materials are very non.homogeneous ranging from silt to mediu!!l gravel. The percentage of fines (silt and thy) in the bank samples varies from 0.3 to 60 percent. Most samples have less than 20 percent silt and clay. Bank materials with not much silt and clay are easily erodible. Cherry Creek Clear Creek 26 Suspended Sediment Observations Catchment Period of NU'TTlber of Location Area, mi2 Record Observations Littleton 30S0 ", lS4l-Jun1952 1287 Henderson 4713 ", 1941-Jun1952 785 Melvin 3" ", 1941-Oct195l 1083 Derby 600 Apr 1944.0ct lS5l 39 I I I I I I I I I I I I I I I I I I I 1 ] ~ I B I n g 9 I I I I I a I I I I 25 mouth of Bear Creek. The bed samples from Cherry Creek are very coarse sand and very flne gravel. Two of the samples were taken nine years after the completfon of the Cherry C:eek Dam in 1953. The other was collected in 1977. The material now belng deposited in the bed of Cherry Creek as it enters the reservoir is also very coarse sand and ftne gravel. Stream South Platte The materials on the bed of Sand Cherry Creek. Clear Creek is different. very coarse gravel. Creek are the same as on the bed of Here, the bed material sample is Distrfct. The measurements were taken 2 to 3 times weekly ....ith additional samp~es during hf9h stage. A measurement consfsted of a sinsle sample obta1ned in the stream at or near the surface of the stream. The sample was taken with the bottle, can, bucket or any unimproved container used to dip samples. Once a month and during peaks, 3 to 5 verticals ....ere sampled across the river. Suspended Sediment Sources of Data. There are no measurements of the bed load in the South Platte River and lts major tributaries in the Metropolitan Denver area. However, numerous suspended sediment measurements were taken by the Denver Oistrfct of the Corps of Engineers in the period between 1941 and 1952. Thereafter the Denver Oistrict....as disbanded and the measuring pro - gram ceased. Only a few records of the suspended sediment measurements have been found. These are in the Colorado Division offices of the U.S. Geological Survey in Denver. References to the South Platte measurements are made in the Soil Conservation Service's (1949, 1952) inventory of published and unpublished sediment-load data in the United States. Unfortunately, the SouthPlattedataareunpublfshed. The data l1stedby the Soil Conservation Service include the following (see next page). _ Harris (1952) u~dated the Soil Conservation Inventory for the period 1900 to 1960. Observations of suspended sediment in the South Platte River at Littleton and Henderson were continued until June 1952. Sampling in Cherry Cree~ and Clear Creek was dfscontinued in October 1951. A minimum of one sample per week was taken at each station. During changing stage supplemental observations were made. Henderson. The suspended sediment data collected in the South Platte River at Henderson for the period from 1941 through 1945 are plotted in Figure 11. These data were obtained from Jim Kircher of the COlorado . The larg:st concentration measured was 16,350 milligrams per liter. Thl~ load of sedlment occurred on 5 August 1943 when the discharge was 602 CUblC feet per second. All the other concentrations are in the range from 100 to 10,000 milligrams per liter. There are no records available to indicate the sizes of the sediment being transported in suspension in the river. The average suspended sediment load in the South Platte River at Henderson has been calCulated using the rating curve for the period 1941 to 1946, shown in Figure 11 and the daily flow duration curve for the period 1927 to 1979. During this interim, the average annual streamflow was 354 cubic feet per second. The calculated annual suspended sediment load 1s 611,000 tons or 130 tons per square mile, a very modest amount. Others. The Corps of Engineers (1978) has computed the suspended sediment load at other gaging stations in the South Platte drainage. These estimates are based on the analysis of samples obtained at the sites (see following page). .~e~di~nt Yield Estimates of the sediment yield in the South Platte River Basin have been made by various government agenc1es. Sediment yield is greater than the suspended sediment load by an amount equal to the bed load transport. South Platte River. Estfmates of sediment yield were made by the COrps of Engineers (1973) when they designed the Chatfield Darn. An allow- ance was made to store 20,000 acre.feet of sediment fn the reservoir in a