Long-tailed Sediment Storage Time Distribution from the Meandering Powder River, Montana

As sediment is carried downstream through river corridors, it may be deposited into alluvial storage, remaining immobile until it is re-entrained. Because the time spent in storage greatly exceeds the time spent in transport, storage timescales largely control sediment delivery. We quantify the storage time distribution of Powder River for 1998-2013 by determining the age distribution of eroded sediment. Our approach integrates surveyed cross-sections, analysis of historical aerial imagery, research-grade aerial LiDAR obtained in 2016, geomorphic mapping of lateral accretion elements, and age control provided by optical stimulated luminescence (OSL) and dendrochronology. OSL samples from banks indicate sediment ages ranging from 0 to 1800 years at annually surveyed cross sections, which provide erosion volumes. Erosion by lateral migration within a 70- km reach is also quantified by historical aerial imagery and analysis of LiDAR data. Laterally eroded sediment is placed into relative age categories that are classified into broad age groups corresponding to regionally defined alluvial terraces (the oldest of which has been provisionally interpreted by radiocarbon dating to be around 4000 years old and the youngest around 600 years old), and into decadal age groups based on dendrochronology of cottonwood trees for the last 100-150 years. Combining these age estimates yields a distribution of sediment storage times ranging from 0 to 4000 years. The first quartile of eroded sediment is less than 50 years old, the median is approximately 150 years, while the third quantile is around 1000 years. The distribution is long-tailed, indicating that Powder River erodes younger deposits in preference to older deposits. These results demonstrate that downstream sediment transport along meandering rivers will involve millennial timescales, and that alluvial deposits will not behave as well-mixed reservoirs with a single characteristic timescale.