LONG-TERM STORAGE AND REMOBILIZATION OF LEGACY SEDIMENT: CONCEPTUAL MODELS OF ANTHROPOGENIC SEDIMENT REDISTRIBUTION

Anticipating effects of global climate and land-use change on fluvial sediment calls for accurate conceptual models of anthropogenic sediment behavior at watershed scales. Episodic production of legacy sediment often resulted in large alluvial archives, which can—in variable proportions—be remobilized over intermediate (decadal to centennial) time scales. Anthropogenic sediment production and transport rates may be orders of magnitude greater than background rates. Storage and entrainment may involve processes unique to the Anthropocene with non-linear dynamics such as lag times, thresholds, and dis-equilibria. Conceptual models of anthropogenic processes are needed for accurate sediment forecasting and landscape evolution models with realistic uncertainties.

Concepts for long-term storage and remobilization of legacy sediment include sediment delivery ratios (SDR), cascades, sediment waves, and lagged yield (fast-in/slow-out). SDRs—simple linear, black-box models—can’t recognize differences within geomorphic systems that govern local storage and entrainment. The cascade model—developed to describe early anthropogenic sediment in central Europe—describes filling of topographic hollows near headwater sources before transport extends to floodplains. Thus, colluvial records may predate floodplain alluvial archives. Legacy sediment can be transported in symmetrical waves where connectivity is great and storage is minimal, but skewed waves—with arrival faster than removal—is more likely. Pulses in receding limbs are common as remobilization is spatially and temporally heterogeneous and idiosyncratic to geomorphology and climate. Aggradation-degradation episodes (ADE) may involve long-term storage with sporadic time lags. The degradation phase of a floodplain ADE may be described by the channel evolution model with rapid channel incision followed by a long period of widening controlled by connectivity between high flows and stored sediment. Cut-offs are extreme examples of delayed remobilization in an ADE where large volumes of sediment can be stored for millennia. As global-scale sediment models are down-scaled to watershed scales, applicability of competing concepts of idiosyncratic sediment behavior at the geomorphic boundary layer should be considered.