BACKWATER FLOW DYNAMICS IN LOWLAND RIVERS, INFLUENCE ON CHANNEL AVULSIONS, AND THE DEVELOPMENT OF FLUVIAL-DELTAIC STRATIGRAPHIC ARCHITECTURE

Where rivers enter the coastal zone, gradually varied non-uniform flow conditions develop due to ocean influence. The section of the river affected by non-uniform flow is commonly referred to as the backwater segment. For large lowland rivers, this segment can extend many tens to hundreds of kilometers upstream of the outlet. Recent studies have shown that the backwater segment is characterized by temporal and spatial changes in boundary shear stress which in turn influence the timing and magnitude of sediment transport. Channel morphology, alluvial sediment cover and grain-size character, and channel kinematics all respond to dynamic conditions of the sediment-transport field. For example, the Mississippi River channel bed tends to aggrade due to deposition of coarse-grained bedload sediment at the backwater transition which coincides with the location of four major avulsions during the past 5000 years, as well as with enhanced rates of lateral mobility. Farther downstream, backwater flow significantly diminishes sand transport during low and moderate water discharge. However, during high water discharge, transport capacity increases significantly and sand movement is dominated by suspension transport. These conditions limit bar formation and alluvial cover of the channel bed near the outlet, resulting in channel incision and diminished lateral mobility. These morphodynamic feedbacks, recognized for a modern lowland river, have influences on the spatial patterns of sediment deposition, and influence the development of stratigraphy when integrated over longer time scales. Recent studies in the Campanian Lower Castlegate Sandstone (Utah) have shown that backwater-influenced non-uniform sediment transport exerted a major control over regional facies distributions, and consequently, the backwater length scale provides a quantitative means of facies prediction within fluvial strata. Importantly, regions of enhanced lateral mobility and channel avulsions at the paleo-backwater transition coincide with amalgamated, coarse-grained channel bodies which become finer and more isolated down the backwater segment. We demonstrate that backwater flow leads to important morphodynamic feedbacks that enhance sediment deposition and the occurrence of distributary-channel-forming avulsions.