The Influence of Tides on Coastal Plain Channel Geomorphology: Altamaha River, Georgia, USA

Dr. Raymond Torres, Professor
University South Carolina – School of Earth, Ocean and Environment

Wednesday, March 17th, 2021

Rivers are ubiquitous features along the terrestrial-marine interface of many continents and their lower reaches are influenced by both terrestrial and marine flow processes.  However, few studies provide details on how the interactions of fluvial and tidal processes translate to channel geomorphology. Here we quantify channel properties along the fluvial-tidal transition reach of a coastal plain river and provide greater insight on the origin of those properties. The study site is a 47 km long tidal, freshwater section of a river at 29-76 river kilometers inland of the estuary mouth, upstream of the distributary system, and with average riverbed slope of 10-5-10-4. Results show that a tidal wave approaching the study reach loses 15-17% of its incident energy (per horizontal area) per kilometer of along-channel distance, and at 51 km upstream of the mouth the incident energy is reduced to <1%. Also, at or near 51 km we observed breaks in along-channel trends of channel cross section geometry, bed particle size, sinuosity, and channel bed and water surface slopes. Overall, the fluvial-tidal transition channel is 15 km long, from 36-51 km inland of the estuary mouth, upstream of the delta/distributary system, with most discontinuities in geomorphic trends occurring at or near 51 km.  We propose that fluvial-tidal flow processes and corresponding geomorphic feedbacks are apparent as discontinuities in channel properties highlighting the influence of tides, and these discontinuities may be endemic to fluvial-tidal transition zones in general.  How these transition reaches should self-adjust in response to sea level rise remains largely unexplored but are likely to become important geomorphic hotspots.