Question

what twice daily event would cause water levels around Jamestown to rise

Answer 1

Jamestown was a city located near the seashores.   The rising and falling of tides twice in a day caused water levels around Jamestown to rise.  The tides were a result of the gravitational force of the moon acting upon the sea.   The gravitational pull of the moon is constantly changing at different places.

Answer 2

The tides rise and fall twice daily.

These rising and falling increased the water levels as the wells are becoming salty due to the incoming tides.

These falling and rising tides are affected by the fresh sources of water.

The shallow wells which were dug by the colonists became brackish owing to this.