They might be food for the fish we eat. As the saltwater tides flow into the estuaries it is heavier than the freshwater coming down the rivers. The fresh water flows out over the saltwater forming a wedge. How well these two waters mix at the interface is dependent on temperature, river flow, tidal flow, wind, and the animals that live there. As the moon travels around the earth the gravity of the moon pulls a wave of water around the planet.
Centrifugal force explains that there is another bulge or wave of water just opposite the first.
This causes the water to rise and fall twice a day. In North Carolina the change in water level is about four feet. Considering the lack of elevation at the coast, the tides make a significant difference on where the water is located. A change in tide occurs about every six hours, however, based on the lunar cycle the time advances each day. When the sun, earth, and moon are in line we have very high tides, spring tides.