Final answer:
A glacial estuary is formed when a glacier cuts a deep valley into the coastline that then fills with water. These estuaries have U-shaped valleys due to erosion caused by the glacier.
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. Estuaries can be thought of as the most biologically productive regions on Earth, with very high biodiversity. Estuaries are zones where land and sea come together, and where fresh and salt water meet.
When a glacier cuts a deep valley into the coastline, and that valley then fills with water, it forms a glacial estuary. Glacial estuaries are characterized by their U-shaped valleys, which were created by the erosion caused by the glacier.