Can we distinguish flood frequency and magnitude in the sedimentological record of rivers?

Consideration of the origin of alluvial deposits and their paleoenvironmental interpretation has traditionally involved two schools of thought: that they are either the result of processes that, on average, have acted uniformly through time, or that they are related to exceptional events that occur infrequently. Despite the long-running debate of gradualism versus catastrophism within the Earth Sciences, there are surprisingly few quantitative data to assess the magnitude of events that produce alluvial sedimentary successions. This paper reports on a unique ‘natural experiment’ where surface (digital elevation model, DEM) and subsurface (ground penetrating radar, GPR) data were taken immediately prior to, and after, a large (1-in-40 yr) flood event on the sandy, braided, South Saskatchewan River, Canada. Results show that although this high-magnitude flood reworked the entire braidplain, the scale of scour and style of deposition was similar to that associated with lower-magnitude, annual, floods. The absence of a distinct imprint of this large flood within the deposits is related to the fact that as river discharge rises, and begins to flow overbank, channel width increases at a much faster rate than flow depth, and thus the rate of increase in channel bed shear stress declines. Hence, rather than being a product of either frequent or rare events, alluvial deposits will likely be created by a range of different magnitude floods, but discriminating between these different scale events in the rock record may be extremely difficult.