The word estuary is of sixteenth century origin and originated from the Latin aestuarium, which means marsh or channel, and this is derived from the Latin aestus, meaning tide or billowing movement. Estuaries are transitioning environments between the land and the ocean, where fresh water coming from the rivers mixes with saline oceanic water. This river inflow need not be perennial. There are several definitions of estuaries. For freshwater scientists the main thing is to define the head of an estuary; in one definition, this is the salinity limit; in another definition, this is the tidal limit; and in still another definition, it is the source of the fluvial sediment. For coastal scientists and oceanographers, the mouth of an estuary, i.e., the point where an estuary ends, is also ill-defined. It can be a geographic feature or the seaward edge of a tidal plume in the open ocean. Whatever the geographical definition, an estuary is a zone of transition with gradients in salinity, sediment characteristics, turbidity of the water column, chemical composition including nutrients, dissolved gases, and trace metals and in diversity and productivity of species of animals and plants. Estuarine waters are commonly more biologically productive than the oceanic waters. In addition, estuaries are strategic locations for human development; nearly 69% of the largest cities in the world are located on river estuaries. The scientific understanding of estuaries is thus of great practical importance. An estuary is never at a steady state. Its bathymetry is continuously evolving. Forced by tides, waves, and river inflows, estuarine water is continuously moving and with it, its ecology is also continuously evolving. Some largest estuaries may have the additional direct influence of winds on the local circulation such as the Amazon River. The resulting physical and biological features of an estuary and its ecosystem health are determined by the time scales of the processes controlling the movements of water and sediments and the ecological processes in the water and on the bottom.