Floodplain Sediment Storage Timescales of the Laterally Confined Meandering Power River, USA

As sediment is transported through river corridors, it typically spends more time in storage than transport, and as a result, sediment delivery timescales are controlled by the duration of storage. Present understanding of storage timescales is largely derived from models or from field studies covering relatively short (<= 10(2) year) time spans. Here we quantify the storage time distribution for a 17 km length of Powder River in Montana, USA by determining the age distribution of eroded sediment. Our approach integrates surveyed cross-sections, analysis of historical aerial imagery, aerial LiDAR, geomorphic mapping, and age control provided by optically stimulated luminescence (OSL) and dendrochronology. Sediment eroded by Powder River from 1998 to 2013 ranges from a few years to similar to 5,000 years in age; ages are exponentially distributed (r(2) = 0.78; Anderson-Darling p value 0.003). Eroded sediment is derived from Powder River's meander belt (similar to 900 m wide), which is only 1.25 times its meander wavelength, a value reflecting valley confinement rather than free meandering. The mean storage time, 824 years (95% C.I. 610-1030 years), is similar to the time required to rework deposits of Powder River's meander belt based on an average meander migration rate of similar to 1 m/yr, implying that storage time distributions of confined meandering rivers can be quantified from remotely sensed estimates of meander belt width and channel migration rates. Heavy-tailed storage time distributions, frequently cited from physical and numerical modeling studies, may be restricted to unconfined meandering rivers. Plain Language Summary As sediment moves downstream through a watershed it is intermittently stored in a river's deposits before being eroded and transported farther downstream. Storage times vary from less than a decade to millennia. Storage time greatly exceeds the time sediment is being transported by the river. Consequently, the time required for sediment to reach a point downstream is largely controlled by the time spent in storage. This can influence how the movement of contaminants are monitored and restoration strategies are developed. Sediment particles spend different amounts of time in storage, which can be represented as a probability distribution. Here we date sediment eroded by Powder River in southeastern Montana from 1998 to 2013 and find that the storage time distribution is exponential. Furthermore, the mean storage time of 824 years (which fully characterizes the exponential distribution) can be determined from the meander belt width and the channel migration rate, both of which can be measured using aerial imagery, providing a simple method for assessing storage times in laterally confined rivers.

Automated summary

The distribution of sediment storage time in river corridors affects the transport and movement of contaminants. By studying Powder River in Montana, USA, it was found that the storage time distribution follows an exponential distribution and can be determined by measuring the meander belt width and channel migration rate.