An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which seawater mixes with fresh water. The key feature of an estuary is that it is a mixing place for sea water and a stream or river to supply fresh water. The mixing of fresh and salt water creates a different environment, but estuaries are still home to a lot of plants, animals and bacteria! When looking at estuaries, scientists quickly realized that these areas were extremely nutrient-rich because of sediment deposit of rivers, creeks or streams feeding into the salt water environment.

There are usually three overlapping zones in an estuary: an open connection with the sea where marine water preponderates, a middle area where strong salt water and fresh water mix, and a tidal river zone where fresh water preponderates. Tidal forces create variable estuarine characteristics in sea inlets. Variation in the seasonal discharge of rivers causes the limits of these zones to shift, and this increases the overall ecological complexity of estuaries. Estuaries are highly productive ecosystems, accounting for one-half of the living matter of the world`s oceans.