Consideration of the origin of alluvial deposits and their
paleoenvironmental interpretation has traditionally involved two
schools of thought: that alluvial deposits 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.
We report on a unique natural experiment where surface (digital
elevation model) and subsurface (ground penetrating radar) 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 were similar to those
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 over the
bank, 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 are likely created by a range of different magnitude
floods; however, discriminating between these different scale events in
the rock record may be extremely difficult.