What describes the wide part of a river where fresh and salt water mix?

An estuary is defined as the area where a freshwater river meets and mixes with saltwater from the ocean. This unique environment is characterized by brackish water, which contains a mix of saltwater and freshwater. Estuaries are typically found at the mouths of rivers, providing a critical habitat for various species of fish, birds, and other wildlife.

These areas are also important for their ecological functions, including serving as nurseries for young aquatic organisms and acting as natural filters for pollutants. The mixing of fresh and saltwater creates a diverse range of habitats and contributes to high biological productivity.

The other options describe different concepts: an ocean trench refers to deep underwater valleys, flora signifies the plant life in an area, and terrestrial pertains to land environments, lacking the specific reference to the water mixing characteristic found in estuaries. Knowing this helps distinguish the significance of estuaries in both ecological and geographical contexts.