Freshwater amounts will vary with variations in rainfall. During periods of heavy rainfall the estuary becomes less salty, since more freshwater is added. Animals living in the estuary must tolerate wide ranges of salinity and, therefore, are called euryhaline biota. If you travel from the top of the estuary toward the sea, salinity, as well as types of animals and plants will change. The salinity of an estuary may change on a daily basis due to tides and winds .
Tides are the daily or twice daily movement of water in and out of an estuary or coastal area.
There are high and low tides. High tides are determined by the high water mark on the shoreface, the sand on the beach. High tides bring high salinity water and add nutrients to the estuary, flushing away waste products, impurities, or even pollution. As the high tide gradually falls to its lowest point, it becomes the low tide, and the cycle starts over again. Winds are movements of air which blow from offshore, moving seawater into the estuary.