Region of interaction between rivers and near-shore ocean waters, where tidal action and river flow mix fresh and saltwater. Therefore estuaries mainly consist of brackish water. Estuaries at first were arisen due to rising sea levels, more specifically as the sea grew, river valleys dissipated, and glacial troughs were engrossed, which formed estuaries. As they continued to form, estuaries began becoming confines for particles and sediments. This was seen in the form of streams, rain, runoff, sand from the ocean and rain pushing down gravel, sand, and mud into the trappings of the estuaries.