Fresh water is lighter than salt water, so en estuaries when rivers meet the ocean, the fresh water floats on top and the denser salt water stays near bottom. The interface where fresh and salt water meet is called the salt front. Currents that bring water down from the river and up from the ocean also meet at the salt front and form a near-bottom convergence zone, trapping sediment and zooplankton. This cloudy, or turbid, area is known as the Estuarine Turbidity Maximum, or ETM.

Activity: Water Density

By definition, estuaries are influenced by tidal forces. In fact, the word estuary comes from the Latin word for tides, aestus. Tides cause the water to rise up on a beach and then retreat, usually twice in the course of a day. The rising and falling of the water also creates currents that run into (flood) and out of (ebb) the estuary (link to tides in the OOS module for more info). The strength of tidal currents is an important factor controlling suspended sediment in the ETM. Tidal currents sweep back and forth like a broom that brushes up (resuspends) a dust cloud. On flood tide, the dust cloud gets carried up-estuary; on ebb tide it gets carried down estuary; on slack tide it settles to the bottom. Sediment that is too light does not settle during slack tide and sediment that is too heavy does not get resuspended and carried during flood and ebb tide.

Activity: Tidal Rhythms