Progressive abandonment and planform changes of the central Platte River in Nebraska, central USA, over historical timeframes

摘要

This paper presents data compiled from aerial photography, General Land Office maps (GLOs), and discharge data from the central Platte River in Nebraska that quantify and provide a new understanding of channel evolution and abandonment in a braided stream. Analyses of aerial photographs and GLOs in ArcMap™ show that channel area decreased by an average of 46% from 1938 to 2006 and that channel widths in 1858 were an average of 539% greater than in 2006. Annual discharge during 1942–2006 does not show a clear long-term relationship with the decrease in channel width and area. Rather, stabilization through vegetation of bars and channel banks is cited as the major contributing factor to channel shrinkage and the development of an anabranching system within the boundaries of the 1858 channel. The stabilization of anabranches and the encroachment of vegetation onto banks, abandoned channels, and islands have significantly impacted the riparian ecosystem as mobile sandy bars gradually disappeared. Annual aerial photographs of the Platte River from 1984 to 2001 show that the river responds to year-to-year changes in mean discharge through the formation and disappearance of in-channel features such as medium-size transverse bars, large transverse bars, open channels, exposed channel surfaces, laterally accreted surfaces, and vegetated channel islands. Vegetated channel islands and stable anabranches also increased during 1984–2009. Significant change has also occurred since the beginning of the twentieth century on the Wood River, a local tributary to the Platte River. The Wood River experienced a long-term increase in sinuosity after it occupied the abandoned North Channel of the Platte River, which last experienced major sediment transport 350 ± 50 years ago, according to an optically stimulated luminescence date. The modern Wood River is also superimposed on a prehistoric meandering pattern with significantly greater meander amplitude.We present the first channel abandonment model to account for the gradual abandonment of the Platte River braided stream system and its evolution from a braided to an anabranching system: in stage 1, the channel prominently features macroform bars comprising downstream-migrating foresets; in stage 2, downstream-migrating dunes are deposited over channel bed features; and in stage 3, there is a shift from a braided stream to a stream dominated by anabranches and only channel bed features (e.g., three-dimensional dunes) are present. Our new model characterizes the features expected to be found in abandoned braided channels, as well as more temporary features that form and are destroyed through successive stages of abandonment.