Downstream trends in discharge, slope, and stream power in a lower coastal plain river

SummaryConceptual models of river-estuary interaction are typically based on a notion of systematic downstream change in the intensity of fluvial processes. Low slopes, backwater effects, and effects of antecedent topography and landforms may complicate downstream tends in water and sediment flux in coastal plain rivers. An analysis of the lower Trinity River, Texas shows no consistent downstream pattern of increases or decreases in the discharge, stream power, or water surface slope. Flows may decrease downstream due to coastal backwater effects in the lowermost reaches, and due to diversion of flow into valley-bottom depressions during high flows in both the fluvial and fluvial-estuarine transition reaches. In general, however, stream power and slope decrease in the lower reaches, consistent with earlier findings of limited fluvial sediment delivery to the coastal zone. Some tributaries may become distributaries at high but sub-bankfull flows, as backwater effects reverse flows into depressions associated with paleomeanders. The paleomeanders, and possibly the locations of these “reversible” channels, are related to antecedent topography associated with aggradation/degradation cycles over the past 100 Ka. Low-gradient coastal plain rivers may not function as simple conduits from land to sea. Further, the transition from fluvial to coastal dominance may be variable along the river, with the variability controlled not just by the relative magnitude of river and tidal or backwater forcing, but also by valley topography controlled in part by antecedent landforms.