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 the 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.