Propagation of subtidal sea level oscillations in the river channel: A case study of the St. Johns River, Florida, USA.

Analysis of water level and river discharge time series collected at three locations in the St. Johns River, FL reveals that subtidal variability with periods of several days is associated with long waves propagating from the ocean into the river channel. These dynamics are similar to tidal wave propagation: both tidal and subtidal frequency bands have the same ratio of free surface-to-discharge standard deviations, which is not the case on oceanic shelves. However, important differences also emerge: as waves pass through the river mouth, tidal oscillations exhibit much stronger attenuation, while subtidal oscillations propagate at a lower speed. Further upstream, where the channel cross-sectional area contracts (between Palatka and Buffalo Bluff), the waves in two frequency bands adjust differently: tidal waves are amplified and continue upstream, while a significant fraction of subtidal energy is reflected. The amplification of tidal waves occurs mostly through the generation of overtides. Also, tidal wave attenuation in the river relative to the mouth is nearly constant over the observation period, while the attenuation of subtidal waves exhibits strong changes. Variations in subtidal attenuation are linked to the influence of the river discharge: higher discharge (relative to the subtidal water level variability) causes stronger attenuation of subtidal waves.