(Non)Equilibrium Sediment Flux in Coastal Plain Rivers

Steady-state equilibrium sediment flux, whereby sediment yield roughly balances sediment production within a watershed, is rare and transient in coastal plain rivers. Five southeast Texas rivers show evidence of nonequilibrium in contemporary and historic sediment fluxes, and with respect to evidence of Quaternary sediment dynamics. Further, even though channel boundaries are readily adjusted, few longitudinal profiles show the strongly concave shape often associated with equilibrium. Yet, some forms of equilibrium do exist in these rivers. Geomorphological equilibrium concepts occupy a continuum from weaker to stronger forms in terms of the assumptions and criteria involved. The weakest form, relaxation time equilibrium, requires only that the response to a disturbance has an opportunity to be completed (or slow to a negligible rate) and therefore progress toward this form of equilibrium is inevitable in many situations, such as downstream adjustment to sediment trapping in reservoirs. Also, a nonlinear dynamical systems perspective shows that equilibrium and nonequilibrium are emergent properties. Thus there is always some scale at which equilibrium pertains. The problem with equilibrium concepts in fluvial sediment dynamics is not that equilibrium does not exist, or is not relevant, but rather with the ontological and definitional baggage associated with the term. While steady-state equilibrium sediment flux is a valid reference condition, it is not necessarily a normative condition. Further, the association of equilibrium with more general “balance of nature” concepts and with conceptual frameworks which postulate inevitable progress toward a single normative end-state creates the impression that there is a (single) correct or “natural” state for a fluvial system. The actual geographical and temporal variations in sediment flux (and other aspects of fluvial geomorphology), reflecting equilibrium and nonequilibrium dynamics at various times, places, and scales, is far richer than any single normative condition.