Final answer:
A spit that extends across the mouth of an estuary, separating it from the open sea, becomes known as a bar. Estuaries are unique environments where freshwater and saltwater mix, leading to varying salinity levels.
Step-by-step explanation:
If a spit grows as it is deposited and extends completely across the former mouth of an estuary, separating it from the open sea, it has become a bar. An estuary is an area where freshwater from rivers or streams meets and mixes with salt water from the ocean. The mixture of salt water and fresh water creates a unique environment known as brackish water. Estuaries are typically characterized by their salinity gradients, which can vary due to tides and freshwater input. The formation of a bar is significant in coastal environments; it can lead to the creation of a bay or a lagoon, which is an area of relatively shallow water separated from the ocean by the natural barrier.