Practice question

An estuary is a partly enclosed coastal water body where river water mixes with seawater and tidal action influences water level and circulation. Mixing produces brackish water, whose salinity lies between fresh and fully marine conditions and often varies over each tide, season, and position along the estuary. Density differences can create a salt wedge or partial stratification, while tidal currents transport sediments, oxygen, nutrients, plankton, and organisms. Estuaries support salt marshes, mangroves, mudflats, seagrass beds, nursery grounds, and productive detrital food webs. A river is primarily flowing fresh water, although its mouth may enter an estuary. Lakes and ponds are inland standing waters and normally lack regular marine tides and salt mixing. Some coastal lagoons also become brackish, but estuary specifically captures the combined river-sea connection and tidal forcing stated here. Organisms living there must tolerate rapid changes in salinity and turbidity. Thus, “brackish and tidal” identifies the physical mixing process that distinguishes an estuary from ordinary freshwater bodies.