Floodplain development in the Powder River, southeastern Montana, USA

Floodplains are highly dynamic landscapes that are formed by a range of processes that are active at multiple temporal and spatial scales that result from the interaction of hydrology, sediment, and vegetation. Unfortunately, our understanding of the processes that form floodplains and moderate sedimentation within the floodplain are not well understood, largely as a result of limited data from modern floodplains. The Powder River of southeastern Montana offers a unique opportunity to observe how floodplains develop and are modified within a highly mobile, meandering river in a system with limited accommodation. The Powder River can be thought of as representative of alluvial rivers in relatively steep and confined valleys close to the sediment source. Here, we use sedimentologic and geochemical data in conjunction with remotely sensed maps and statistical models to investigate the evolution of the Powder River floodplain and how sediment is exchanged between the channel and the floodplain. We find that there is a characteristic sequence of floodplain growth driven by the episodic migration of the channel due to the growth and translation of meander bends during flood events. The probability of deposition within the floodplain depends on the antecedent topography formed by past locations of the channel and the distance and elevation relative to the current channel position, as floods preferentially move sediment in the "swales" left behind between abandoned and active point bars. On the other hand, the probability of erosion more strongly depends on the age of the floodplain and the distance to active cutbanks. Similarly, the grain size deposited within the floodplain is controlled by the antecedent topography and the history of flood events that occurred while the floodplain was close enough to the active channel to receive sediment. Our results suggest that even in a low accommodation, constrained floodplain sediment can be stored within the floodplain for geologically long timescales (~thousands of years at minimum) and can superimpose sediment of very different ages without leaving a clear stratigraphic signature.