Final answer:
Fresh river water is primarily found at the surface of a salt wedge estuary, above the denser saltwater that forms a wedge shape underneath. Estuaries provide crucial habitats for many species, with salinity levels that vary with tidal movements and river flow.
Step-by-step explanation:
In a salt wedge estuary, fresh river water is primarily found at the surface of the estuary. This occurs because freshwater is less dense than saltwater and therefore floats above the denser seawater. The saltwater, being heavier, forms a wedge shape beneath the freshwater, hence the name 'salt wedge.'
Estuaries are dynamic environments where the salinity can vary greatly based on the tidal cycle and the flow rate of the river. These ecosystems are crucial for many species, particularly young crustaceans, mollusks, and fish, which rely on the estuary's protected waters for the early stages of their lives.
The mixing of fresh and saltwater creates brackish water conditions which can fluctuate rapidly, posing a physiological challenge to the organisms that live there.