The Patuxent River, the longest river located entirely within Maryland, extends 115 miles from the Piedmont near the junction of Howard, Montgomery, Frederick, and Carroll counties to the Chesapeake Bay. Passing through seven counties, it crosses the heavily urbanized Baltimore-Washington corridor and gathers runoff from Laurel, Bowie, and Columbia. Near the colonial seaport of Queen Anne in Prince George's County, it broadens into a navigable tidal estuary, and from Queen Anne to the Chesapeake Bay it spans more than 2 miles in some places. Captain John Smith's 1608 voyage upriver first recorded its name as "Pawtuxunt," and he reached the Lyons Creek area, 40 miles from the Chesapeake. By the mid and late 1600's, colonists had moved upriver to Mt. Calvert and Billingsly Point, two colonial mansions about 44 miles from the Chesapeake. By the early 1700's, the Snowden iron ore furnace was sending pig iron downriver from near the future site of the 1783 Montpelier Mansion. In the mid 20th century, the Washington Suburban Sanitary Commission built two dams on the main branch: Brighton Dam, completed in 1943 96 miles from the Chesapeake and forming Triadelphia Reservoir, and the T. Howard Duckett Dam, completed in 1952 outside Laurel and creating Rocky Gorge Reservoir.