How Healthy is the Merrimack Watershed Now?

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Volunteers have been collecting water quality data for the Merrimack and Souhegan Rivers. Many thanks to the Souhegan Watershed Association for providing their data for New Hampshire’s Lower Merrimack section, and for the Souhegan River. A special thanks to George May and Karen Mattor of the Souhegan Watershed Association for all their hard work in providing DO, pH, and flow data. Here are some descriptions of the variables we measure:

Conductivity: “Conductivity is a measure of the ability of water to pass an electrical current”. Conductivity can be affected by temperature because warmer water carries a higher conductivity.  Discharge and runoff to streams can change the conductivity. The conductivity of rivers in the United States ranges from 50 to 1500 µmhos/cm. Streams supporting good mixed fisheries have a range between 150 and 500 µhos/cm. Higher or lower conductivity could indicate that the water is not suitable for certain aquatic species. Significant changes in conductivity could indicate runoff or some other source of pollution.

Dissolved Oxygen (DO): Dissolved oxygen (DO) is oxygen that is dissolved in water. Water systems both produce and consume oxygen. Oxygen is gained from the atmosphere and plant photosynthesis and is lost by respiration of aquatic animals, decomposition, and oxygen consuming chemical reactions. If more oxygen is consumed than is produced, the DO levels will decline which will have adverse effects on sensitive animals. Dissolved oxygen is an indicator for good fish health.

pH: pH is a term is used to indicate how basic or acidic a solution is ranked on a scale of 0 to 14, with pH 7 being neutral. pH is an important factor that can limit the distribution of species in aquatic habitats. Different species thrive in different ranges of pH, but most aquatic organisms can withstand a range between pH 6.5-8. pH outside this range causes stresses in many species and can result in decreased reproduction, decreased growth, disease, or death.

Temperature: Temperature can alter water density and the solubility of oxygen and other chemicals. Fluctuating temperatures and extreme temperatures could be stressful to aquatic life.

TDS: Total Dissolved Solids is the concentration of organic and inorganic materials dissolved in water. These substances can be from runoff or pollution discharge.

Turbidity: The amount of light transmission due to absorption and scattering as affected by suspended sediments.

Nitrate: There are many common forms of nitrogen in aquatic ecosystems. The most important in terms of nutrient impairment is nitrate because of its abundance and mobility on land and water.

Phosphorus: Of the total phosphorus in aquatic systems, about 25% is considered to be biologically available. The organic and inorganic forms of phosphorus vary significantly in their reactivity and availability for biological productivity.

Ortho-phosphates: Dissolved inorganic phosphorus and organic phosphorus. Ortho-phosphates are the most biologically available form of phosphorus, but only counts for a small percentage of less than 10% of the total phosphorus in aquatic systems.


Too many nutrients can cause excess algal growth and even fish kills:

Below are trend graphs for the nutrients nitrate/nitrogen and phosphorus in the Merrimack River.  The measurements were taken by our volunteers  in the main stem of the river, from October of 2009 and October 2010.   An average and standard deviation were calculated for 8 regions:  Nashua, Tyngsborough, Lowell, Andover, Lawrence, Haverhill, West Newbury and Newburyport.

EPA drinking water guidelines for nitrate require that nitrate-nitrogen should not exceed 10 mg/L.

The natural baseline levels for total phosphorus in rivers is usually less than 0.03 mg/L. While there is no drinking water standard for phosphate, EPA guidelines for phosphorous regarding streams and rivers that don’t flow into lakes suggest that phosphates should not exceed 0.1 mg/L to help control algal growth (USEPA 1986).  The State of Massachusetts states that total phosphates shall “not exceed an average of 0.05 mg/L as P during any monthly sampling period” (EPA 1971) for any Class B (Public Water Supply and Recreation) or Class C (Fish and Wildlife) waters.

Nitrate concentration tends to increase at downstream sampling stations for any given month, while phosphorus measurements are more spatially erratic. Overall, the highest measurements for either nutrient were sampled in August 2010.




Here is a link to a .pdf containing both of the above graphs: Average Nutrients by Region, 2009-2010


In addition to the Merrimack we also have water quality data for two tributaries of the Merrimack:

Spicket River Data 2013

Souhegan River Data 2013

We also have more water quality data for the Merrimack River:

Upper Merrimack and Pemigewasset River 2012

Haverhill to Newburyport Water Quality Data 2013

Merrimack River: Long Term Temperature Data

E.Coli and Dissolved Oxygen Historical Data 2012