... of the Merrimack must fill the canal in its course through the Middlesex towns to the Charles, in Charlestown. Eleven streams of varying size flowed across, and all but one below its course. The Concord River at North Billerica crossed it at grade and being at its highest level, would supply it in either direction with water. Here, in the seventeenth century, a mill was erected and thither came the colo...