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J ~ I ~ D I I I I I I I I I . I I I I I 33 " narrower and smaller than in other reaches. The lower Suburban and Rural reaches had 8 and 7 bars per mile respectively. None of these were entirely vegetated. The spring floods of 1983 changed the shape, size and number of bars in sectiOnS of the Suburban and Rural reaches. For example, 1nrnedlately upstream from the Eighty-eighth Avenue Bridge, the channel between the side bar on the left bank and the adjacent middle bar was completely filled with sediment during the flood. Bars are prominent inrnedfately downstream from Sand Creek ref1ectin9 the fact that this tributary carries a large load of sediment. These bars are barren. VI. EXPECTED OiANGES General In its pristine state, the South Platte River adjusted its width, depth, and sinuosi~ in response to the amount of water and the amount and type of sediment supplied to it by its tributaries. Prior to the arrival of settlers, major influence on the river's form was the occurrence of prolonged drought and wet cycles in the climatic regime. Stable river banks are often treed whiTe erosion of the unprotected bends has removed the vegetation between the river and the cleared land on the floodplain. In the Rural reacheswherethebanksareerodlngand there are trees, the channel bed is littered with trunks and roots of trees removed by bank caving. The vegetation shown on the photographs used to prepare the bank- line maps in the Henderson Road reach shown in Figure 14 was studied to determine if the vegetation has been changing in recent times. In 1937, vegetation was generally sparse. There were some large trees along the bank here and there, on bars downstream from the road, and on areas recently abandoll~d by tfl~ river. Upstream there was hardly any veg!.'tation. In 1953, there s!.'ems to be even less vegetation than in 1937. ~~st of the trees were alon9 the banks, former abandoned banks or on recent deposits. In 1963, there was a little more vegetation in the same places. By 1983, there was hardly any vegetation left. Some trees may have been removed when levees were constructed; others were stripped out in the gravel mining operations. The important factor is that there has been no recent proliferation of vegetation in the South Platte River between Chatfield Dam and Baseline Road near Brighton, which would change its flood carrying capacity or its morphology. In the irrmediate past, the vegetation has been sparse and continues to be so. In the nineteenth century, the natural regime of river was inter- ruptedpermanently. Climaticcyclesst111occurredandtheriverresponded. However, it is man's influence on the supply of water and sediment and his desire to control those natural forces which cause him anguish and grief that are the major factors creating the new and changing form of the South Platte River. Agriculture began in the South Platte Basin following the first gold rush to Colorado in 1858. Major diversions from the river began in the Tatter part of the century. Nadler and Schumm (1981) have studied the effect of early developments in the basin on the geomorphology of the South Platte River between Greely and Fort Morgan. Their conclusions were that the river underwent a metamorphosis (a complete change in morphology) from a multiple-thalweg channel to a single-thalweg channel. The process of ,change was complex. Between 1885 and 1897, return flows from crop irriga- tion began to change the hydrologic character of the stream. The return flows raised the water table above the riverbed in late summer changing the stream flow from intermittent to perennial. Droughts in the late lSlDs and early 1930s and again in the 1950s temporarily decreased the streamflow but the perennial nature of the flow remained. When the stream flow was 1ntermittent, woody vegetation was sparse along the South Platte River. On the floodplain there were cottonwood groves alternating with prairie. Shrubs were not plentiful. The channel width was in the range from 1500 to 2500 feet. The channel contained transient grave' bars. The bank heights were in the neighborhood of 4 feet to 5 feet. The sinuosity was 1.14 and the average gradient was approxi- mately 7 feet per mile. The raised water table and continuous streamflow resulting from the irrigation practices resulted in an increase in riverbank and floodplain vegetation which in turn caused the river metamorphosis, characterized by stream shrinkage and the complex vertical accretion of sediment. The droughts after 1900 triggered the great change in width. It is Nadler and Schumm's (1981) conclusions that the thalweg did not aggrade, but the floodplain land was formed adjacent to the thalweg by island construction and channel filling. When a drought occurred, vegeta- tion quickly colonized areas below the mean high water level, thus stabilizing the newly.fonned bars. With additional sedimentation the veg- tated bars grew in height to become islands. The channels flowing around