Complex sentence: Within the next few days and weeks, they would find and explore the harbor known as Hampton Roads and sail upriver on its biggest tributary, the James River, eventually setting on Jamestown to begin their first settlement.
Split into simpler sentences: Within the next few days and weeks, they found and explored the harbor now known as Hampton Roads. Sailing upriver on its biggest tributary, the James River, they eventually settled at what they would call Jamestown to begin their first settlement.