Where a river slows: investigating the oscillic freshwater zone

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As rivers approach the sea there is surprising, complex, and poorly understood "oscillic freshwater zone" where the river flow may slow, stop, or even reverse direction with the tide, but not an ounce of seawater is seen. This zone can behave like a fast-flowing river or a slow-flowing lake or a mixture of the two, at different times. Although these behaviors are poorly understood, any variations will affect how fast the river transports freshwater and pollutants to the sea. Pollutant runoff from rivers is an (inter)national problem that significantly impacts the health of river and ocean ecosystems and fisheries. Oscillic zones likely already remove some pollutants from rivers before they harm the coast, by slowing their flow and enabling more natural chemical mitigation along the way. The degree to which this is occurring, however, and the specific processes involved are unknown. To ensure that these free, natural benefits are not lost due to changes in river management, rainfall, or sea level, we need to better understand how oscillic zones work. This project is the first to define and investigate the oscillic zone as an important section of a river that does not fit previous fast-or-slow river classifications. This research will answer three questions: 1) What alternation of fast and slow flows defines an oscillic zone? 2) What effect does this zone have on nitrogen pollutants reaching the coast? and 3) How sensitive is this zone to changes in sea level, rainfall, and river management? The project includes detailed field study of two Texas river' oscillic zones. These real-world insights will be combined with computer models to better understand dynamic physics-chemistry links and to predict how human and natural influences affect freshwater and pollutant delivery to the coast. This research will be globally shared on a public web portal hosting animated simulations of oscillic zone physics/chemistry. The portal will illustrate the oscillic zones of all the US western Gulf Coast rivers and their connections to the Gulf in real-time. The portal will also host a more general tool enabling anyone to explore and test oscillic zone dynamics for different types of rivers. To further connect to education and management, the project will work with the local water quality committee relevant to the field sites to seek to directly inform real river management policy and will enhance elementary school-and-up educational materials on river-sea connections, in partnership with a National Estuarine Research Reserve.