Tides affect water levels and current speeds in rivers as they approach the ocean. These effects can reach much farther inland—sometimes hundreds of kilometers—than the brackish estuary, where salty seawater mixes with fresh river water. The section of a river that is affected by tides but too far upstream to contain salt water is called the “tidal river.”
Tidal river dynamics affect wetland restoration efforts, river and coastal sedimentology, changes in delta structure, and saltwater intrusion into fresh aquifers, which increases as sea level rises. However, relatively little research has addressed tidal rivers. In a new study, Hoitink and Jay review existing knowledge of tidal river dynamics and challenges for future research.
At the local level, observations of tidal rivers reveal features like the tidal bore—a visible wave of water at the edge of the oncoming tide that travels against the river current. Although studies have analyzed the characteristics and effects of tidal bores, scientists still face challenges in predicting them.
On a larger scale, ocean tides can be thought of as very long waves with peaks separated by hundreds of kilometers. The authors highlight the physics of how these long waves travel up rivers and the resulting effects, noting that interest in these effects greatly increased in 1953 after a storm surge flooded much of the coast of the southern North Sea. Since then, increasingly sophisticated equations and models have been developed to describe long-wave dynamics. These models take into account the seaward flow of the river, as well as friction caused by water flow over coastal wetlands and other factors.
Today, some models calibrated with gauge station data can predict tidal river surface level on the basis of river discharge and conditions at sea. Interactions between river discharge and tides can also be used to subtract the influence of tides when estimating river flow into the ocean.
Branching river deltas also pose unique challenges for scientists. Networks of multiple water channels introduce much more complexity than is seen in a single channel. Nonetheless, scientists are continually improving models of tidal river effects on sediment transport, flooding, ecology, and other factors in delta settings.
Although scientists agree on the importance of tidal rivers, they disagree on their definition. It is often useful to have a distinct name for the freshwater portion of a river that is affected by tides, but a definition based on salinity makes less sense for rivers with large seasonal differences in the upstream extent of saltwater. Instead, the authors propose a definition based purely on the extent of specific tidal influences.
Regardless of the precise definition, the authors say, scientists will need to overcome many challenges in order to predict how rising sea level and sinking deltas will threaten the fate of tidal rivers and nearby wetlands. (Reviews of Geophysics, doi:10.1002/2015RG000507, 2016)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2016), When rivers and tides collide, Eos, 97, doi:10.1029/2016EO051221. Published on 28 April 2016.