Merrimack, river, c.110 mi (180 km) long, formed at Franklin, S central N.H., by the junction of the Pemigewasset (rising in the White Mts.) and Winnipesaukee rivers. It flows S past Concord and Manchester into NE Mass., where it flows NE past Lowell and Lawrence to the Atlantic Ocean at Newburyport. With its numerous tributaries, the river drains most of S New Hampshire. The towns along the Merrimack have used the river as a source of power for their textile mills from c.1850 to 1930. Although the river still provides waterpower, most of the mills and factories are gone. The valley is used by tourists vacationing in the White Mts. area.