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

The twice-daily event is the tides (the rising and falling of the tide caused mainly by the moon’s gravity).

When the tide comes in around Jamestown, saltwater from the ocean moves up the James River and nearby creeks, raising water levels and mixing with fresh water. This saltwater intrusion makes nearby wells and streams brackish —a mix of fresh and salt water that is unsafe to drink.