A DYING RIVER—HISTORICAL TRANSFORMATION OF THE ARKANSAS RIVER, WESTERN KANSAS

When European settlers first migrated to the Arkansas River valley of western Kansas in the mid to late 1800s, the river had an archetypal braided habit, being 1-2 m deep and up to 600 m wide. Spring snowmelt in the Rocky Mountains produced annual freshets that filled the channel and frequently produced flooding, which often swept away early bridge works. At present, the river channel is much narrower and more sinuous, a response to upstream dam and reservoir installation and to groundwater pumping, which dropped the water table, and diversion canals for irrigation. Consequently, the channel is completely dry throughout the year in most of western Kansas. To chronicle the change in the Arkansas River channel of western Kansas, a classic geomorphic approach was used to assess the degree and timing of the channel transformation, with data sources including original federal land survey records that indicated the early settlement (1871) channel widths, historical ground-based photography, successive bridge construction surveys, seven discrete years of aerial photography (B&W photography and NAIP imagery) covering 75 years, USGS stream gage data, and LiDAR coverage. By 1939, the first aerial photography, the average channel width was 175 m, and as of 2014 the width was only 30 m. Sinuosity increased from 1.2 to as much as 1.8 during the same period. As the discharge waned during the course of the 1900s, the channel entrenched 2-3 m, producing a floodplain consisting of the pre-settlement channel bed and a terrace form coincident with the braided channel floodplain; this degradation is attributed to the trapping of sediment in the upstream dam. These anthropogenic activities have directly altered the hydrology of the Arkansas River by decreasing mean annual discharge, reducing peak annual flows, and lowering the water table. The character of the Arkansas River channel within the Kansas High Plains will continue its present trajectory as long as the present-day hydrologic regime is maintained and prevailing climate remains relatively unchanged. The Arkansas River in western Kansas is an example of another Great Plains river that has undergone a major historical metamorphosis in response to anthropogenic forcing (e.g., Platte River of Nebraska, Cimarron River of New Mexico, Kansas and Oklahoma).