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

Rivers that traverse the terrestrial-marine interface may have lower reaches that are influenced by both terrestrial and marine processes. However, only a handful of studies have focused on how the interactions of fluvial and tidal processes translate to channel geomorphology, and those are largely from delta/distributary systems. Here we quantify channel properties along the fluvial-tidal transition reach of a coastal plain river and provide insight into their origins. The study site is a 47 km long tidal, single-thread freshwater section of a river at 29 to 76 river kilometers inland of the estuary mouth, upstream of the delta/distributary system, and with average riverbed slope of 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 channel, 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 grain size, sinuosity, channel bed and water surface slopes. We propose that fluvial-tidal flow processes and corresponding geomorphic feedbacks are apparent as abrupt changes in channel properties that highlight the influence of tides, and these discontinuities may be endemic to fluvial-tidal transition zones in general. How these transition reaches self-adjust in response to climate change remains largely unexplored but these reaches are likely to become important geomorphic hotspots.