The places where rivers empty into the sea are areas of wildly fluctuating environmental conditions. Depending upon the local tides, the entire habitat may be underwater for part of the day and completely exposed for another. As fresh river water encounters the marine water of an ocean or bay, a habitat is created that is neither fresh nor marine, but somewhere in between. The actual salinity of the water may vary widely each day, as saltwater enters the river during high tide, and then retreats to the ocean as the tide falls.

Brackish water environments are found throughout the world, in temperate and cold places as well as in the tropics. Wherever freshwater encounters the sea, a unique habitat supporting a diverse array of organisms is formed. The influx of saltwater into a river at high tide may extend for miles in some cases. The denser saltwater sinks to the bottom, resulting in a stratified environment where, for a time, freshwater animals dwell at the surface while marine creatures live below. In the tropics, estuaries are often populated by mangrove trees. The long, exposed roots of the mangroves anchor the trees in shallow water. There they trap mud and detritus and, in the process, help to form new land. The trees create a complex environment populated by a unique assortment of fish, invertebrates, and birds. Temperate estuaries are also populated by plants and animals uniquely adapted to fluctuating levels of water specific gravity.

Estuaries are important nursery areas for the young of a number of commercially valuable fish and invertebrates. Also, especially where mangrove trees predominate, estuarine environments act as important buffer zones between land and sea and are vital in preventing erosion.

Sadly, estuaries and the coastlines adjacent to them are also highly desirable areas for real estate development. As the protective trees and grasses are removed, soil readily washes into the sea, often smothering offshore coral reefs in the process. Changes in the physical nature of the habitat also affect the salinity and other characteristics of the water itself, rendering it unsuitable for the myriad animals that dwell there. The destruction of mangrove forests and estuaries is an extremely serious but often overlooked environmental concern.

Animals that thrive in an environment where the pH and water levels fluctuate on a daily basis can be quite hardy in captivity. Indeed, many seem to benefit from environmental changes that would kill creatures of a more delicate constitution. That being said, it is still safer to introduce fluctuations gradually, even as concerns estuarine species.

A good number of brackish water animals are equally at home in freshwater and marine habitats. Populations living in different environments face varying challenges and in many cases follow quite unique lifestyles. The diet of animals living in a shallow marine environment would, for example, likely be quite different than that of populations of the same species inhabiting a freshwater or tidal river.