Physical History of the Platte River in Nebraska

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In order to place the Platte River into perspective, several basic geomorphic techniques are applied. The "idealized fluvial system" (Schumm, 1977) subdivides a river system into three basic components: the sediment supply zone, the transfer zone, and the sediment deposition zone. These three components of the idealized fluvial system are shown in Figure 2.3. While the upper portion of the Platte Basin (i.e., the portion of tributaries that are principally found in the mountains and foothills where steep channels flow with significant turbulence and swiftness) lies in the sediment supply zone, the Central Platte River lies in the transfer zone. Most of the sediment supplied to this zone from the sediment supply zone is transported through the transfer zone to the deposition zone farther downstream, hence the designation as the transfer zone. Thus, the Central Platte River is an alluvial river in the intermediate zone, designated as the transfer zone, flowing over an alluvial plain. Figure 2.4 shows a profile of the Platte River as it extends to the continental divide. The slope of the river in the mountain region is clearly steeper than the river in the plains region. Figure 2.5 focuses on the portion of the river in Nebraska. Rivers come in many different types including such categories as straight, braided, and meandering. Lane. (1957) presented a graphical method of evaluating stream type based on flow and slope. The Platte River, in Lane's diagram, plots as a braided river (Figure 2.6). It should be noted, however, that the point on the diagram representing the Platte (in pre - development conditions) plots close to the line separating braided and transitional or intermediate streams. This implies that even relatively small changes in characteristics governing the geomorphology of the Platte could cause a shift or change in channel type. Descriptions, maps, and historic photographs confirm that the Platte River was a wide, shallow, braided river. Since it is an alluvial river with sandy bed and bank material, and it is subject to relatively large.flow events, the river was quite dynamic. In other words, the Platte River was subject to shifting of the river bed and other features such as bars or islands, as the continuously changing flow regime and upstream sediment supply flowed through this reach of the river. One of the key issues regarding riverine habitat along the Platte River deals with riparian vegetation and changes that have occurred over time with woody vegetation along and within the river. Woody, riparian vegetation (typically cottonwoods, willows, and some other species) that grows to heights greater than about 3 feet obstructs the view of the bird species of concern. The lack of a sufficient distance of clear view, or what may be called unobstructed view, reduces habitat quality since the birds cannot see potential predators. The species of interest prefer areas of habitat with sufficient unobstructed view so that they have sufficient time and space to deal with predators. Since the Platte River channel consists primarily of sand, the river provides suitable substrate for establishment and growth of riparian vegetation. Williams (1978), as well as numerous other researchers, has documented the fact that woody,. riparian vegetation has expanded onto formerly active channel areas of the North Platte, South Platte, and Platte Rivers over the past century. The causes for the expansion of woody, riparian vegetation and potential approaches to control vegetation are of significant- concern for the maintenance or enhancement of habitat. 4