An estuary is the term used to describe the boundary where fresh and saltwater mix, such as in the lower Mississippi River.

Estuaries are important ecosystems where rivers meet the ocean, creating a unique environment with a mixture of fresh and saltwater. These areas are crucial for biodiversity, serving as nurseries for many fish and other aquatic species. In the lower Mississippi River, the estuary is vital for both wildlife and human activities, as it supports a variety of habitats, including wetlands, marshes, and tidal flats, while also playing a key role in water filtration and flood protection.