An estuary is defined as?

An estuary is specifically characterized as the area where a freshwater stream, such as a river, meets the ocean. This unique environment is notable for its mixing of freshwater and saltwater, which creates a diverse habitat that supports various species of plants and animals. The transition zone between land and sea in an estuary typically features a mix of aquatic and terrestrial ecosystems and plays a crucial role in biological productivity.

In contrast to other options, an estuary is not merely identified as a place with abundant salt water, nor is it confined to a freshwater lake in the mountains or categorized as a type of salt marsh, although salt marshes can be found in estuarine environments. The defining feature of an estuary is this interplay between freshwater from rivers and saltwater from the ocean, which results in distinctive ecological dynamics.