What is the coastal area called where freshwater mixes with seawater, typically formed by deposition?

The area where freshwater from rivers or streams meets and mixes with saltwater from the ocean, typically characterized by unique ecosystems and high nutrient levels, is called an estuary. Estuaries are essential environments that serve as nurseries for various marine species and support diverse wildlife. They are often found at the mouths of rivers where these bodies of water deposit sediments, leading to a rich mixture of freshwater and seawater.

This mixing creates a brackish environment that is different from both the open ocean and freshwater systems, facilitating a unique habitat for both aquatic and terrestrial organisms. The tidal influences in estuaries also contribute to their dynamic nature, with the water levels and salinity fluctuating regularly.

In contrast, other options refer to different geographical features. An oceanic basin refers to a large underwater depression. A delta is a landform that forms at the mouth of a river where it deposits sediment, creating new land, but does not specifically refer to the mixing of freshwater and seawater. A lagoon is a shallow body of water separated from a larger body of water by a barrier such as a sandbar or coral reef, but it does not necessarily involve the mixing of river water and ocean water in the same manner as an estuary.