The twice daily event that would cause water levels around Jamestown to rise and cause wells and fresh water streams to become brackish is the tidal cycle, specifically the tides influenced by the moon. These tidal fluctuations cause saltwater from the James River to intrude into freshwater sources, making them brackish

. This tidal saltwater intrusion leads to the contamination of wells and fresh water streams with saltwater, which mixes with the freshwater and increases salinity levels. This phenomenon is exacerbated by the island's low elevation and proximity to the river, allowing tidal waters to penetrate inland and affect groundwater quality

. In summary:

• The twice daily event is the rising and falling of tides caused by the moon's gravitational pull.
• These tides cause water levels around Jamestown to rise.
• Saltwater from the James River intrudes into freshwater wells and streams, making them brackish