The source of the Hudson River is the Adirondack Mountains. Rain water collects into mountain ponds and lakes which then flow into streams and rivers which then empty into the Hudson River. The River flows 300 miles down to NYC where it then empties into the Atlantic Ocean. However, the lower Hudson, roughly from NYC to Troy is actually an "estuary" which ebs and flows with the tides of the Atlantic. This lower part of the Hudson is "brackish" or partly salty.
The dominent indiginous people of the Hudson Valley were the Iroquios. Their rivals, the Algongin, were to the North. In 1524 Giovanni da Verrazono sailed into New York Harbour. He sent a scouting group up the Hudson. (They made it as far as present day Alpine but panicked when they sighted a party of Iroquoi.) It was Henry Hudson, who in 1609 first sailed up the River opening the Valley for European settlement.