level muddy surface bordering an estuary, alternately submerged and exposed to the air by changing tidal levels. The tidal waters enter and leave a tidal flat through fairly straight major channels, with minor channels serving as tributaries as well as distributaries. The minor channels meander and migrate considerably over periods of several years.

In addition to the alternating submergence and exposure, the varying influences of fresh river water and saline marine waters cause physical conditions, principally temperature, salinity, and acidity, to vary more widely than in any other marine environment. Waves are generally very small because of the shallow bottom and short fetch. The mud of a tidal flat is characteristically rich in dissolved nutrients, plankton, and organic debris, and it supports large numbers of smaller mobile and burrowing animals, notably crabs, pelecypods, gastropods, and worms. Vegetation is generally sparse; if wave erosion is minimal, however, felts and mats of blue or blue-green algae may be present.