HISTORIC CHANGES IN STREAM CHANNEL MIGRATION RATES IN LONG PINE CREEK, NEBRASKA

Historic changes in channel form, discharge, migration rates, and sinuosity are commonly identified in modern streams. Many of these changes can be related directly to human activity, while others can be attributed to changing climatic conditions and/or other factors. Our study aimed to determine if stream erosion rates had changed appreciably in the 20^th century along a 25 km stretch of the Long Pine Creek valley, which is a tributary of the Niobrara River in northeastern Nebraska. In our study reach the stream is contained within a narrow bedrock valley that is 30 m deep and approximately 800 m wide at its widest point. We mapped the position of the Long Pine Creek channel from aerial photographs that were taken between 1937 and the present, and the results show that both stream length and channel migration rates were significantly reduced in the 20^th century. We cannot attribute these reductions in stream length or erosion rates to either climatic factors or human activity. We instead suggest that the dramatic reduction in migration rates result from an increase in woody vegetation on the Long Pine Creek floodplain. Woody vegetation encroachment, which is evident from historic aerial photographs in our study reach, has been an ongoing problem throughout the Great Plains and has been attributed to the suppression of wildfires following European settlement. This study shows that woody vegetation encroachment has stabilized stream banks significantly impacting the nature of both the Long Pine Creek channel and its floodplain.