Estuaries are semi-enclosed areas where fresh water and seawater meet and mix, forming one of the most productive environments on earth. They serve as protected habitat and vital breeding and feeding grounds for many species. Cities were often built near estuaries due to the abundance of seafood. There are four main types of estuaries that form from different geological processes: drowned river valleys, bar-built, tectonic, and fjords. Estuaries have varying salinity levels and substrates that influence the distribution of species and ecological processes within them.