Salmon Brook begins at the outlet of Lower Massapoag Pond near the center of Dunstable.[1] It then flows approximately 9.3 miles (15.0 km) north-northeast to the Merrimack River. The water body farthest upstream in the watershed is Martins Pond just east of the center of the town of Groton. The outlet, Martins Pond Brook, runs southeast to Lost Lake/Knops Pond, the start of a chain of lakes that includes Whitney Pond, Cow Pond, and Upper and Lower Massapoag ponds. All of these water bodies are dammed. Salmon Brook runs roughly parallel to the Nashua River for its entire course from Lower Massapoag Pond to the Merrimack River.