An estuary is a partially enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries are formed when a river meets the ocean, and the saltwater from the ocean mixes with the freshwater from the river.

There are three main types of estuaries: coastal plain estuaries, bar-built estuaries, and fjord estuaries. Coastal plain estuaries are the most common type of estuary. They are formed when a river meets the ocean on a gently sloping coastline. Bar-built estuaries are formed when a sandbar or barrier island forms across the mouth of a river. Fjord estuaries are formed when a glacier retreats and leaves behind a deep, narrow valley that is flooded by the sea.

Estuaries are important for a variety of reasons. They provide habitat for a variety of plants and animals, including fish, shellfish, and birds. They are also important for fishing and recreation. Additionally, estuaries help to filter pollutants from the water, and they play a role in the cycling of nutrients.

Estuaries are threatened by a variety of human activities, including pollution, habitat loss, and climate change. Pollution can come from a variety of sources, including sewage, industrial waste, and agricultural runoff. Habitat loss can occur when wetlands are drained or filled in for development. Climate change is also a threat to estuaries, as it can lead to sea level rise and changes in precipitation patterns.

There are a number of things that can be done to protect estuaries. These include reducing pollution, protecting and restoring habitat, and addressing climate change. Reducing pollution can be done by reducing the amount of waste that is discharged into waterways, and by improving wastewater treatment. Protecting and restoring habitat can be done by restoring wetlands, creating buffer zones around estuaries, and controlling invasive species. Addressing climate change can be done by reducing greenhouse gas emissions and investing in renewable energy.