When the sun, earth, and moon are in line we have very high tides, spring tides. When they are at sharp angles to one another we don't have as much change, neap tides. Tides are predictable. You can obtain a tide table to let you predict the tide times on a given day. When high tide comes to our coast, salt water moves in through the barrier island inlets, through our sounds and up into our rivers.
Spring tides move the water further up the rivers.
Fresh water flows from the rivers and off the land out through the sounds and inlets as the tide recedes. The wind also acts to push water one way or another. If there is a large rainstorm there will be more fresh water flowing. As you can see the amount of salt and fresh waters in the system is highly variable. As low tide occurs the water in the estuary is shallow.