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I ~ a D I I I I I ~ ~ g ~ I I I I I I 47 6. Fluvial morphological studies indicate that parts of the South Platte River in its Urban and Suburban subreactles lire unstable. ~ere the river is moving laterally at II rapid pace. Unstable reaches are chariIC;. terfzed by sharp, short.radiu5 bends. 7. Overall, the expected changes due to past, present and future developments are towards a narrower, deeper, straighter river with stabil- ized banks and II degraded bed. Degradation will be arrested 1n many areas as the river encounters cobbles under the present-day bed. 8. There is no indication that vegetation is but a very minor,fac- tor in controlling the morphology of the river. Vegetation does not 519- nificantly affect the flood carrying capacity of the river channel at the present time. 48 IX. RECOMMENDATIONS General The processes whereby an alluvial gravelbed river changes from one form to another in response to changing supplies of water and sediment are intricate and not well described by the one-dimensional mathematical equa- tions available at this time. If a river were truly one-dimensional, it would be straight with rectangular cross-sections and a plain bed. The presence of meanders and bars and the wide variations in bed material sizes and channel width attest to the fact that the flow phenomena are complex. Moreover, very little 1s known about the spatial and size distributions of the sediment under the riverbed. The degradation process which has occurred or which is anticipated to occur in the future is very similar to that experienced downstream from Glen Canyon Dam. The field studies conducted in this reach of the Colorado River and reported by Pemberton in 1976 are surrrnarized below. Construction of the Glen Canyon Dam on the ColoradO River began in 1956. Closure was officially made in 1963. The da~ traps the entire incoming annual sediment load of 104,000 acre.feet. The l5-mile reach of river downstream from the dam was studied over a 2Q-year period to deter- mine the actual degradation rate. In the detailed sediment sampling program in 1956. it was observed that a gravel bar existed approximately 3.6 miles downstream from the dam. At the mouth of the Paria River 16 miles downstream, there was another bar of gravel and cobble-size material. The particles were rounded hard rocks varying in size from 4 millimeters to over 8 inches in diameter. Where exposed, they were Shingled and densely compacted. The average size of al' samples taken on the surface of the riverbed in the study reach was 0.21 millimeters (fine sand). During the initial construction period, much of the sediment load was trapped benin~ the coffer dam so degradation was i~diate. floods in 1957, 1958 and 1959 caused 6 feet of degradation ilTmediately downstream from the dam. Inspection of the riverbed indicated that the floods had left a series of controls in tne riverbed which were well-armored with gravel and cobbles. Another flood in 1965 created some deep holes and degraded the lower part of the reach. 8y then, 3 major and 6 minor controls had developed. Ihese were bars armored with a l-diameter thick 1aJer of cobbles with a median diameter of 4 inches. Underneath, the material was 20 percent sand, 75 percent gravel and 5 percent cobbles up to 8 inChes in diameter. From 1965 to 1975, there were no floods of ~ignificance. 8y 1975, anotner control had developed making 7 in the 8 miles upstream and 3 in the 7 miles downstream. The sediment on the riverbed between controls was pre. dominantly gravel with very little sand. I I I I I I I I I I I I I I I I I I I