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

The rising and falling of the tides would cause water levels around Jamestown to rise and cause fresh water streams to become brackish.

Explanation:

The rising and falling of the tides is likely to make the fresh water salty. This is because ocean water is salty and the rising and falling of tides will cause the fresh water to mix in the ocean water. in the given case, the event is likely to occur twice daily so it will increase the chances of fresh water streams turning salty.