Concord, river, c.15 mi (24 km) long, NE Mass., a short tributary of the Merrimack, which it joins at Lowell. On Apr. 19, 1775, colonial militia fired some of the first shots of the American Revolution at the British over a bridge across the river at Concord, Mass. Henry David Thoreau's first book, A Week on the Concord and Merrimack Rivers (1849), records a boat trip with his brother.
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