Final answer:
An estuary is a coastal body of water where fresh water and salt water meet. It can form when land sinks due to earthquakes allowing the ocean to cover it. The San Francisco Bay is an example of a great estuary.
Step-by-step explanation:
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the ocean. It is where fresh water and salt water meet. Estuaries can form when earthquakes cause the land to sink, allowing seawater to cover it. The San Francisco Bay is an example of a great estuary.