Souhegan River has more oxygen – and that’s a very good thing

The Souhegan River has been looking pretty healthy lately, to the point that salmon are spawning there for the first time in many decades, and now some official numbers can support that impression.

The state Department of Environmental Services says that five stretches of the river, most notably all of it downstream from Route 122 near the Milford-
Amherst border to the river’s mouth in Merrimack, are no longer classified as having too little dissolved oxygen – one of the most important measurements for freshwater systems.

“It’s an indicator of biological health. When dissolved oxygen decreases, there’s a greater stress on all the organisms in that river,” said Kenneth Edwardson, Surface Water Quality Assessment coordinator for New Hampshire. “Cold water species, such as trout, need a lot of dissolved oxygen to survive.”

Oxygen enters rivers when the water interacts with the air, from wind or rippling over rocks. When levels are low and a river becomes stagnant, less oxygen enters the system.

More important, however, is the rate at which oxygen is used up by algae and bacteria in the water, leaving less of it for insects, fish and other life. When bacteria populations explode because nutrients have washed into the river from adjoining land or development, oxygen levels can plummet, which is why measuring dissolved oxygen gave give a good picture of river conditions.

“It’s about reducing nutrient loading in the river,” Edwardson said of the improvement in the Souhegan River’s numbers since 2002 measurements. “Maybe people using less fertilizer, maintaining septic systems better; wastewater facilities releasing less; on the agricultural side, people managing manure piles.”

The upgrade of the status on the five sections of river, including several small stretches through Milford and Wilton, also reflects something else: The value of

The only way to be sure of the oxygen content in some 20 different sections of a river is to go out and get early morning water samples every two weeks all summer long, which isn’t something that New Hampshire taxpayers are likely to pay to have done.

“The state doesn’t have the staff, doesn’t have the people to go out and do it,” said George May, of Merrimack, who has organized and overseen Souhegan Watershed Association
water-sampling efforts on the Souhegan and lower Merrimack rivers for almost 20 years. “They certainly can’t do it every two weeks; if they do it every two years, that is a lot.”

Edwardson said the sections of the Souhegan where the oxygen status changed last had official measurements taken in 2002, when they didn’t meet the threshold of 5 milligrams of dissolved oxygen per liter of water, or roughly 5 parts per million.

The new healthy status, he said, reflects the realization that SWA samples which indicate good oxygen levels can be used alongside officially gathered numbers.

Biologists always make oxygen readings in the early morning, before levels rise as oxygen is produced by plants, so they can accurately compare figures over time and among sites. Readings made later in the day could be misleading, like measuring a person’s heartbeat after exercise and unwittingly comparing it to the resting heartbeat from weeks earlier.

Edwardson said that he realized, while going through the data, that SWA almost make their collection by 8 a.m. and thus its results are usable.

SWA volunteers gather two water samples at about 20 sites in Merrimack, from Manchester to the state boarder, and on the Souhegan from Merrimack through New Ipswich. One sample is tested for oxygen and one for E. coli bacteria at local wastewater treatment facilities.

“The stuff that gets attention is the bacteria, because that affects human beings, but that really doesn’t affect much else,” May said. “For the tests that we do, dissolved oxygen is probably the most important test for the health of the river.”

SWA used to gather samples that were measured for phosphorus, but stopped because they couldn’t afford to pay laboratories to do the tests.

“We just didn’t have the money,” May said. “I’d love to do phosphorus again; that is pretty significant.”

Other tests he’d like to run include electrical conductivity, an indirect measurement of how much sediment is floating in the water, and testing for heavy metals.

“We know that there’s mercury, that’s all over the place, but one time when we tested it what we did see in the Souhegan is lead,” he said, pointing to a stretch of the river along Route 31, around Wilton and New Ipswich.

“I assume it probably has to do with leaded gasoline, but that’s just a guess,” he said.

As for the river’s most glamorous inhabitant, a major sign of the river’s improved health came in 2011, biologists found freshwater salmon laying eggs in the Souhegan River, the first time this has been recorded in many decades.

Schoolchildren are no longer filling the Souhegan River with salmon fry, or babies born in fish hatcheries and raised in classrooms. The salmon return was shifted to Manchester so these raised fish wouldn’t compete with the ones being naturally born and raised in the river.

However, this success may not last.

A decades-long federal program that attempts to help Atlantic salmon return to the tributaries of the Merrimack River has been cancelled because of to lack of success and government cost-cutting.

David Brooks can be reached at 594-6531 or dbrooks@nashua