Upper Merrimack and Pemigewasset River

All figures are available on the bottom

The upper part of the Merrimack is a key region for maintaining good water quality for those who rely on it downstream. Most of this region is heavily forested. The forests provide a critical ecosystem service in improving the water quality, keeping the water clean, and providing a habitat for threatened freshwater/terrestrial species, including brook trout. The water quality of the Upper Merrimack and Pemigewasset has been studied by the United States Army Corps of Engineers (USACE), the New Hampshire Department of Environmental Services (NHDES), and various communities in the watershed.

Study Area Map

The purpose of this 2009-2012 study was to extend the previous survey’s water evaluation and provide an accurate and representative picture of the current water quality conditions of the upper Merrimack. The study began at the main stem Pemigewasset and Merrimack Rivers south of Lincoln, NH to the New Hampshire-Massachusetts state line in Nashua, NH and includes the main tributaries listed below.

Tribs Chart

In order to conduct this type of study, a field sampling program had to be designed and implemented. The field sampling program consisted of the following components:

  • Impoundment studies (i.e. studying river segments within the area)
  • Continuous dissolved oxygen and temperature monitoring at the impoundment sites
  • Low-flow water quality surveys
  • High-flow water quality survey
  • Sediment oxygen demand and nutrient flux monitoring

The Army Corps measured different elements of the water such as dissolved oxygen, temperature, phosphorus, nitrogen, pH, and bacteria (click here for descriptions of what the elements are and here for bacteria information) to determine the river’s water quality conditions and health.


DDOissolved oxygen was used to determine if the river was appropriately classified as a Class B for water quality. Class B waters are of the second highest water quality and are considered acceptable for fishing, swimming and other recreational purposes, and, after adequate treatment, for use as water supplies. All readings for dissolved oxygen were above the state standard of 5 mg/L or 75% saturation, meeting the requirements for Class B rivers. The best areas for dissolved oxygen were near the towns of Lincoln and Franklin; the segments with the lowest dissolved oxygen were near the Ayers Island dam in Bristol and near the town of Hooksett.

Total phosphorus concentrations from samples taken in the impoundments were greater in the lower watershed (specifically from the city of Concord to the city of Nashua) and at the end of the summer, meaning there was an increase of phosphorus in the water over time. The areas with the highest concentrations were near the Garvins, Hooksett, and Amoskeag dams, near the cities of Concord, Hooksett, and Manchester.


The two low flow surveys and the one high flow survey helps capture a snapshot of the water quality conditions at a specific moment over the course of the summer. Low flow is the flow of water in a stream during prolonged dry weather and high flow is the flow of water in a stream after a period of precipitation. The low flow and high flow surveys showed increases in chlorophyll, total phosphorus, and nitrogen concentrations from upstream to downstream.

The total nitrogen concentrations were high in the river areas near Concord and Derry and extremely high near the city of Nashua.


In rivers, healthy PH levels normally range between 6.5-8.5. During the storm events, PH levels higher than 8.5 were located in Woodstock (8.7) and Nashua (9.0-9.2). PH levels lower than 6.5 were located in Campton (4.9) and Concord (5.6).

The E. coli levels of all three events were higher along the beach’s water edge than in the river channel. The highest E. coli concentration was collected during the second low flow event at the Souhegan River. This concentration was five times above the state standard. Here are graphs showing the percentage of samples of E.Coli levels above the state standard limits.




Graphs from above:

Study Area Map and Tributaries

Nitrogen, Dissolved Oxygen, and Temperature