The Piscataquog River is a 34.7-mile-long (55.8 km) river located in southern New Hampshire in the United States. It is a tributary of the Merrimack River, which flows to the Gulf of Maine. The Piscataquog River begins at the outlet of Deering Reservoir, a lake in Deering, New Hampshire. The river takes a winding course north, then east, then south, then east again through Hillsborough County to reach the Merrimack River in Manchester. On its way, it passes through the town of Weare, where it is impounded in two reservoirs — Weare Reservoir (or "Lake Horace") and Everett Lake. Everett Lake is part of the watershed-spanning Hopkinton-Everett flood control reservoir — a canal crossing the drainage divide connects Everett Lake to Hopkinton Reservoir, which impounds the Contoocook River.