Low flows continue on area rivers
An inch of rain overnight Sunday, Aug. 21, brought the Souhegan River back up over the Protected Critical Instream Flow that would require water users and towns along the river to curtail outside watering, but the low flows are still the topic of concern.
Flow in the upper section of the Souhegan was only 25 percent of its historic expected flow. This is a slight improvement from two weeks ago when the Souhegan Watershed Association water quality monitoring last took place, but unless more rain continues, the river will shortly return to critical flows.
The Merrimack River is ponded by the dam in Lowell, Mass., up through Nashua and so the level is less evident here, but the flows measured in Manchester are down about 25 percent. At the time of the tests the flow should have been 1400 cubic feet per second, but it was measured at 1090 cfs.
What this means for the E. coli tests is that there is less water to dilute pollution. Ironically heavy rains raise the E. coli levels temporarily – about three days – as pollution from storm drains and river banks gets washed into the rivers. This is especially true after a dry period preceding the rain.
This week E. coli levels reflect that. Both the Souhegan and Merrimack Rivers showed high E. coli counts – but for different reasons. The Merrimack is a large river that can dilute pollution that would overwhelm the much smaller Souhegan. Seldom do E. coli counts rise to dangerous levels on the Merrimack, but this week they did. The heavy rain caused the Manchester sewage treatment plant to release raw sewage directly into the river. E. coli counts rose above the 406 dangerous level all the way to Merrimack. But as an indication of the power of dilution, the E. coli count was back down to acceptable levels by the time the river reached Nashua. The E. coli level at the Greeley Park boat ramp measured only 43.
The much smaller Souhegan River did not have the very high numbers that showed up on the Merrimack, but the entire river was above the 88 level that indicates clean water. All of the swimming holes tested above the 88 level but most were below 126, which is acceptable swimming for healthy adults. Weston Park at Turkey Hill Bridge in Merrimack was slightly higher at 147, too high for swimming. The Amherst Canoeport at Boston Post Road was too high at 206. All other swimming holes were OK.
E. coli monitoring is for human use of the rivers only and a good indicator of pollution levels, but SWA also monitors for dissolved oxygen in both rivers. DO levels give the best general indication of the health of the river itself. And monitors found generally good levels of DO again this week. This has been true all season. The rivers are clean for fish and other river creatures and for the environment. More information about DO and the actual readings for each site are listed at www.souhegan river.org.
This is the final test for this season. We hope to continue next June to begin our 21st testing season. Tests are all done by Souhegan Watershed Association volunteers who collect samples and data from 30 specific sites on both rivers and transport the samples to wastewater treatment plants in Manchester, Greenville, Milford, Nashua and Merrimack, where they’re picked up, moved around, and processed by certified lab technicians at the wastewater treatment plants.
– Submitted by Souhegan Watershed Association