Metamorphosis of South Platte and Arkansas Rivers, Eastern Colorado

The Arkansas and South Platte Rivers in eastern Colorado have changed dramatically during the past 150 years. Earlier, they were relatively straight, wide, braided, intermittent streams. However, percolation of irrigation water into and through valley alluvium has raised water tables, and flow regulation has produced more uniform flow into the rivers. As a result new floodplains formed, and bank and floodplain vegetation became denser, as if a climatic change to more humid conditions had occurred. The rivers became narrower and more sinuous due to perennial stream flow, abstraction of sediment with irrigation water, and a decrease in discharge during drought. Three reaches of these rivers responded differently to the changes. The South Platte River and a reach of the Arkansas River narrowed and developed a single thalweg. In contrast, a reach of the Arkansas River at Bent's Old Fort began to meander as a result of an increase of suspended sediment load. All of the reaches are near a pattern threshold.