Net ebb sediment transport in a rock-bound, mesotidal estuary during spring-freshet conditions: Kennebec River estuary, Maine

Abstract

Hydrographic data—obtained concurrently along the 25 km longitudinal axis of the Kennebec River estuary during a 13 h semidiurnal tidal cycle of a spring freshet superimposed on near-perigean spring tides—revealed a strong ebb-current dominance along the length of the estuary. Ebb- current dominance is produced by riverine flow that supplants a substantial part of the flood-tidal prism. In addition, side-scan sonograms showed a suite of large bed forms (1–12 m height) with nearly ubiquitous ebb orientations veneering the estuary bottom. Embayment geometry, salinity, water temperature, discharge, current velocity, and bed-form data all suggest that ebb- velocity asymmetry, set up by seasonal changes in freshwater discharge superimposed on ebb-directed tides, is the most important control on net bed-load sediment transport within this high-latitude, rock- bound estuary. These data augment a model that shows that a freshwater discharge threshold exists for net seaward bed-load sediment transport. The results from this study can be used to refine existing conceptual sedimentologic and morphologic classifications of estuaries.