Ontogeny of point bars on a river in a cold semi-arid climate

During the extreme flood of May 1978 in Powder River, Montana, USA, three new point bars were created: two by meander cutoffs, and one by 65 m of lateral channel migration. By using annual cross-sectional surveys and by sampling sediment in dug trenches, we documented the creation of point-bar platforms upon which sediments were later deposited to create point-bar features that evolved with time. Our observations of point-bar growth and evolution on Powder River are compared with similar observations of other point bars in humid subtropical and subarctic climates. Powder River is a meandering alluvial river in a cold semi-arid climate, characterized by a diverse flow regime that includes occasional ice-jam floods in late winter, annual snowmelt floods in late spring, episodic flash floods in the summer, and infrequent floods in the fall. The building of the point-bar platform took place during a relatively short time span, by an apparently random process of deposition and erosion of unit bars during floods that gradually changed a concave channel surface to a convex surface upon which the point-bar features evolved. Point-bar features on Powder River were built by a superposition of multiple unit bars associated with five flood types over a period of years rather than during a single year. Erosion was found to be a significant process in the shaping of the point bars. Erosion varied spatially between point bars and temporally at multiple time scales over the 33 yr time span. Erosion at the decadal time scale indicated that on average 19%-41% of the initial deposits (older than 10 yr) were eroded. This quantification of erosion has implications for interpreting the geological record.