ANNUAL DISCHARGE CONTROLS THE NATURE OF BRAIDED CHANNEL FEATURES IN THE CENTRAL PLATTE RIVER NEAR GRAND ISLAND NEBRASKA

An annual time series (1984-2001) of aerial photographs of the Platte River shows that channel area and width vary positively with annual discharge through the formation and destruction 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. During the study period, the Platte River evolved from a braided channel to an anastomosing channel as long-lived vegetated in-channel islands formed and persisted. Such islands became especially important as exposed channel surfaces developed into channel islands following the high-flow year of 1993, and as semi-permanent secondary channels developed. Channel features were also reworked or remobilized over a period of several years in the aftermath of that high-flow year. Our observations demonstrate that mesoforms and macroforms in the study area are primarily affected by shifts in overall yearly discharge, rather than isolated peak high and low discharge events lasting less than one week. Therefore, channel features are shown to be stable only within certain annual average discharge parameters. For example, large transverse bars occurred when annual average discharge rates ranged from 79-151 m³/s, medium-size transverse bars were stable at average annual discharges of 33-85 m³/s, and exposed channel surfaces dominated during years having average discharges of <32 m³/s Vegetated islands are not strongly impacted by variations in discharge, but rather they remain stable and are increasingly under the influence of invasive vegetation. Observations and correlations from this study can predict the step-by step formation and destruction of braided channel features given specific annual discharge values. Inversely, the recognition of certain channel features in ancient and modern abandoned braided systems can be interpreted in terms of discharge variations using a general predictive model.