A STRATIGRAPHICALLY INFORMED THEORY OF ALLUVIAL SEDIMENT RESERVOIRS AND ITS USE IN ROUTING FINE SEDIMENT FROM SOURCE TO SINK

Sediment storage often controls the timing of fine grained sediment delivery from source to sink, but storage processes are often neglected in sediment routing models. Reservoir theory and sediment budget data can account for the influence of storage on sediment delivery, but these methods typically fail as sediment pulses create time-varying, downstream-migrating waves of transport and storage. To replace the constant sediment residence times typically assumed by reservoir theory, it is hypothesized that stored sediment is exposed to erosion as a characteristic function of age (time spent in storage), and that this “erosion exposure function” (denoted EP(A)) is a time-independent function characteristic of each fluvial geomorphic system. The rate of erosion of sediment from storage in an age category “A”, is then specified as the product of M(A)rEP(A), where M(A) is the stored mass in an age category A, and r is a time-varying erosion rate constant with units of 1/time. The erosion exposure function thus defines the fraction of M(A) exposed to erosional processes at the rate “r”, and therefore reflects the stratigraphic architecture of stored sediment in an alluvial reservoir. To account for sedimentation into storage, deposition is modeled as a function of sediment supply and unfilled accommodation space. A 3-D chronostratigraphic model and observations of erosion patterns of the Little Missouri River suggest an erosion exposure function of ~ 1 (independent of deposit age), with r = 0.0127/yr. When combined with expressions for downstream suspended sediment flux, numerically tractable predictions of suspended sediment routing are feasible for continental-scale sediment routing systems. The age and transit time distributions of stored sediment are predicted, not assumed, allowing some tests of model predictions using stratigraphic data. Straightforward extensions allow coupling between bedload and suspended load, thus including episodes of channel aggradation and degradation and the formation of alluvial terraces.