Question

what twice daily event would cause water levels around jamestown to rise and cause wells and fresh water streams to become brackish

Answer 1

Is it the tides? That's the only thing I could think of that happens twice and the water rises and falls.

Answer 2

The tide rising would cause water levels to rise and thus supply Jamestown with brackish water.

Explanation:

Tides are created by the gravity of both the satellite and the sun “pulling” at the water in the sea. Because the Earth is continually changing, the “pull” of gravity changes various places as each day goes on so when the tide is “out” in your area, it is “in” somewhere else. But the water level at large tide changes too.