HISTORICAL METAMORPHOSIS OF THE ARKANSAS RIVER ON THE KANSAS HIGH PLAINS

Abstract

River metamorphosis is a well-documented global phenomenon, particularly for the historic period. The Arkansas River in western Kansas is an example of a river channel that has undergone a major historical metamorphosis: the pre-1900’s channel was wide, shallow and braided, but subsequently transformed into a narrow, sinuous meandering system. This study establishes the relationship between the hydrology and dynamics of channel morphology, determined the period in which the metamorphosis occurred, and quantified the channel change. In order to document channel change along a reach of the Arkansas River within the Kansas High Plains this study used ArcGIS to evaluate aerial photography for seven discrete years within the past 75 years, USGS stream gages to document the historical decrease in discharge, Public Land Survey records to characterize early settlement channel widths, and lesser sources such as historical ground-based images and bridge construction plans to further document historical changes in channel morphology. Channel width and sinuosity were measured and recorded for each year of aerial photography to quantitatively determine the magnitude of change and to characterize progression of the historical metamorphosis. The river channel has narrowed by about 145 meters and increased in sinuosity from 1.22 to 1.46 since the acquisition of the first aerial photography (1939). Historical changes in the channel morphology have occurred because of many anthropogenic modifications including a dam, irrigation diversion canals, and groundwater pumping for center pivot irrigation systems. These anthropogenic influences have directly altered the hydrology of the river by decreasing mean annual discharge, reducing peak annual flows, and lowering the water table. The upstream part of the study reach, near the Colorado-Kansas border experiences sporadic flows and has a narrow sinuous channel, where the discharge is actively building and stabilizing the floodplain and channel banks. The downstream reach, below irrigation diversions, channel width increases and sinuosity decreases, where the surface flow is extremely rare, resulting in little or no channel change. Upstream reaches, near Syracuse, have high sinuosity values from riparian vegetation stabilizing point bars and cutbanks, whereas downstream reaches, near Garden City, have low sinuosity values due to minimal riparian vegetation. The character of the Arkansas River channel within the Kansas High Plains may continue its present trajectory as long as the present-day hydrologic regime is maintained and the prevailing climate is unchanged.