Physical History of the Platte River in Nebraska

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2.4.6 Pre - Development Sediment Transport An estimate of pre- development sediment transport was made by Simons & Associates (1987a). This estimate is for the quantity of sediment delivered to the Platte River from both the North and South Platte Rivers. It was based on regression equations relating sediment transport to flow using available bedload and suspended load data. The estimate used the regression equation with an estimate of the pre - development annual flow. The result of the estimate was 1.25 million tons for bedload and 2.28 million tons for suspended load on an annual basis. To this number from the regression equations, an estimate of sediment trapped in reservoirs of 4.25 million tons per year was added obtaining a total of 7.8 million tons per year. This estimate is admittedly crude and based on a number of assumptions that may not be valid. Some attempt at verification should be made before relying on this estimate, however, it does provide an initial guess at pre - development sediment transport. 2.5 Channel Geometry The channel geometry of the Platte River prior to development can be characterized as wide and shallow, and classified as braided in form. Maps of the vicinity of the surveys show a generally wide and open channel or one that had some islands with multiple braided channels (Figures 2.20 and 2.21). The earliest aerial photographs from the late 1930's confirm the generally braided character of the Platte River system (Figure 2.22). 2.5.1 Total Channel Width One of the most notable features of the Platte River is its width. As discussed by Eschner et al. (1983), "Most travelers, comparing the Platte River to rivers in the Eastern United States, found the width of the river remarkable; comments on channel width were recorded in most journals. " Two early measurements of width were provided by Eschner et al. (1983). They gave a bank -to -bank width measurement by Captain Bonneville in 1832 (Irving, 1837) of 6600 feet at a location 25 miles downstream of the head of Grand Island. The other measurement was done by Fremont (1845) just downstream of the confluence of the North and South Platte of 5280 feet (one mile). They further summarize other width estimates stating that such estimates during the period 1800 -1860 ranged from 3960 feet (three - fourths of a mile) to 15,840 feet (3 miles) with most measurements ranging from 5280 (1 mile) to 10,560 feet (2 miles). Some channel surveys were conducted in the 1920's associated with the construction of bridges. Channel widths on the Platte River ranged from 1240 feet to 5500 feet (0.23 - 1.04 miles), averaging 0.7 miles. These widths appear to be generally less than those cited above made on the order of 60 to 75 years earlier. This decrease in width could reflect the effect of water resources development that occurred prior to the 1920's, or it could reflect the fact that these were bridge sites which might have been somewhat narrower than the width of the river in general. 2.5.2 Active (unveaetated) Channel Width The active channel width is defined as the portion of the width of the river channel within its banks that is not vegetated by perennial or woody vegetation. The active channel generally consists of the barren sandy bed above water combined with the portion of the hi 22 1