VARIABLE CHANNEL MIGRATION RATES OVER THE PAST TWO CENTURIES, POWDER RIVER, MT

Channel migration is the primary mechanism of floodplain turnover in meandering rivers and is essential to the maintenance of floodplain ecosystems. Channel migration is dictated by river flows, and even modest perturbations to the flow regime may decrease migration rates. Ongoing research on Montana’s Powder River began in 1975 and has contributed to a diverse array of fluvial geomorphology literature. Although this past research thoroughly describes recent processes occurring along the Powder River, it is unknown how representative documented conditions are compared to those that occurred before agricultural expansion, incremental water development, and climate change. We calculated channel migration rates from topographic cross-sections collected between 1975-2014 (Moody and Meade 2014 and unpub. data). We then extended the spatiotemporal perspective of channel migration up to two centuries by delineating the river channel in air photos (1939-2013) and by aging transects of cottonwood trees (1829-2014). Channel migration calculated from the recent cross-sections occurred at 0.63 m/yr, compared to rates of 1.68 m/yr for the medium-length air photo record and 2.78 m/yr for the long cottonwood record. Examining different periods from within the air photo record alone supported these findings; the post-1978 photos showed a similar migration rate to that calculated from cross-sections surveyed in the same period (0.81 vs. 0.63 m/yr), which is half the rate found over the entire 74 year air photo period (1.68 m/yr). All lines of evidence suggest that channel migration and floodplain turnover have decreased in recent decades, and the recent intensively studied period is not representative of past fluvial geomorphic processes. Corresponding to the decreased channel migration rates are a decrease in channel width (111 vs. 52 m for 1939 vs. 2013), increase in sinuosity (1.55 vs. 2.01 for 1939 vs. 2013), decrease in flood peaks, and an exotic shrub invasion. We conclude that even the modest degree of land development and flow management in the watershed has caused channel migration and floodplain turnover to decrease, threatening the native floodplain ecosystem that depends on dynamic fluvial processes.