VANISHING RIVERSCAPES: HISTORICAL CHANNEL CHANGE ON THE WESTERN GREAT PLAINS

The shortgrass steppe of the United States primarily occupies the region from the 100th meridian west to the base of the Rocky Mountains and north from the US-Mexico to the US-Canada border. This one of the most altered and endangered ecosystems in the U.S. The extent and intensity of historical changes in the steppe associated with land-use patterns are reflected by the changes that have occurred in the region's rivers. Numerous studies document the shift of larger perennial rivers from shallow, braided, largely unvegetated systems during the 19th century to narrow, sinuous, densely vegetated river corridors today. Less well known are the changes in smaller, intermittent streams that are fed by seeps and springs. As regional water tables have dropped, these streams have become progressively drier, eliminating riparian and aquatic habitat critical to many species that are now listed as being of concern. Changes during the past century in the South Platte River basin of Colorado, as exemplified by small streams in the Pawnee National Grasslands, provide an example of changes occurring throughout the shortgrass steppe. Maps, air and ground-based photographs, other historical documents, and soil characteristics are used to estimate the location of seeps and springs and the extent of intermittent stream flow at selected sites during the past century and at present. Results indicate that seasonal wetlands in the shortgrass steppe, which are often associated with stream channels, are vanishing rapidly.