North of Nashua and south of Manchester, the Merrimack River provides good trout fly fishing opportunties. The Merrimack River is accessible from the confluence of the Winnipesaukee River and the Pemigewasset River to the NH/MA border. This river is known for its rainbow trout, brook trout, and brown trout. Keep in mind, there are some sections of the Merrimack River that are closed to fishing, such as near Garvin Falls Dam and upstream of Amoskeag Dam.
This river is managed by the New Hampshire Fish and Game Department for trout fishing. There is no closed season.
Overall, Merrimack River is the 8th most popular fly fishing river of all 19 fly fishing spots in New Hampshire.