​Rain Raises E. Coli Levels On Souhegan, Merrimack Rivers Again

MERRIMACK, NH — Once again, heavy rains just before the latest water monitoring on the Souhegan and Merrimack Rivers raised the E. coli levels to less than favorable conditions for human use. Rainstorms wash bacteria into the rivers and raise E. coli bacteria levels. Several of the biweekly water tests this summer were influenced by rainstorms within the three day lifespan for bacteria. Both the Merrimack and Souhegan Rivers had dangerously high E. coli levels, the highest seen this summer.

The upper stretches of the Souhegan were fine until the river reached Greenville. Then E. coli levels became questionable for swimming. That continued until the levels became dangerous for swimming beginning in Wilton and continuing through Milford and Amherst. Near the mouth of the river in Merrimack the levels improved but still were never acceptable for healthy swimming. None of the usual swimming spots on the Souhegan were acceptable for swimming this week.

The Merrimack River normally has very acceptable E. coli levels. Not this week. The highest reading possible using this test was measured upstream of the Tyngsborough Bridge. The reading here maxed out at 2419.6. Readings below 88 are acceptable for swimming at lifeguarded beaches, over 126 is questionable, and over 406 is dangerous. Only tests taken in Manchester were acceptable. Thorntons Ferry in Merrimack also had a very high reading – 1299.7.

The unusually high E. coli readings on the Merrimack River may be due to a lowering of the water level at the Amoskeag Dam in Manchester. The water level above the dam is being lowered five feet for repairs and this flow of water from the pond may be raising the bacteria levels.

Rain has an opposite effect for dissolved oxygen levels in the rivers and this is a basic test for the general health of the river, its fish, insects and plant life. Generally the higher the oxygen level, the healthier the river. Rain splashes oxygen into the river and raises the flow over and around rocks. The dissolved oxygen levels in both rivers are excellent and have been all summer.

Clearly the drought of last summer is gone. The Merrimack River has had a higher flow than its historic average all summer. Generally the flow has been about double what it has been all summer. The Souhegan River flow, however, is about average this summer. Because it’s a smaller river (it would take 30 or 40 Souhegan Rivers to make one Merrimack River), it responds more quickly to rainstorms than the Merrimack. The Merrimack River is dammed up in Manchester and flow is moderated there.

The heavy rain the day before our tests brought out lots of fishermen. Monitors reported a number of people out fishing early Tuesday morning on both rivers after the heavy rain the day before. One of the monitors also noted the presence of mayflies, one the fishes favorite foods. The water may not be great for swimming right now, but it’s perfect for fishing.

The rivers will be tested again on Aug. 8. Thirty volunteers, many of whom have been with the program for many years, adopt a site and test every other Tuesday morning and carry the samples to local wastewater treatment plants in Greenville, Milford, Merrimack, Manchester and Nashua where the samples are processed by certified professionals. More information is available at souheganriver.org.

Here are the individual results for E. coli taken on July 25, 2017:

Souhegan River Sites

1. Billy Ward Pond, Ashburnham #1 40.4; #2 5.2
2. SoR 333 Water Loom Pond, New Ipswich 24.9
3. SoR 320 Highbridge, New Ipswich 111.2
4. SoR 309 Above Greenville Mill Pond, Greenville 167.4
5. SoR 296 Downtown Greenville upstream of WWTP 74.4
6. SoR 291 Green Bridge off Rte 31 below Greenville WWTP 140.1
7. SoR 218 Below the Horseshoe, Wilton 137.9
8. SoR 210 Downtown Wilton 488.4
9. SoR 201 Pine Valley Mill, West Milford 261.3
10. SoR 170 Behind Hayward Field, West Milford 517.2
11. SoR 155 Souhegan Valley Boys and Girls Club, Milford 547.5
12. SoR 146 Swing Bridge, Milford 258.9
13. SoR 133 Riverside Cemetery, Milford 686.7
14. SoR 130 Behind Lorden Plaza, Milford NA
15. SoR 122 Amherst Country Club 547.5
16. SoR 116 Amherst Conservation land Fairway Road, Amherst NA
17. SoR 095 Boston Post Road Canoeport, Amherst 613.1
18. SoR 057 Indian Ledges, Merrimack NA
19. SoR 034 Turkey Hill Bridge, Merrimack 228.2
20. SoR 001 Watson Park, Merrimack 137.6

Merrimack River Sites

1. Mer 600 Above Amoskeag Dam, Manchester 29.5
2. Mer 590 Arms Park, Manchester 52.0
3. Mer 580 Upstream of Piscataquog River, Manchester NA
4. Mer 570 Goffs Falls, Litchfield 86.0
5. Mer 560 Depot Street, Merrimack 461.1
6. Mer 550 Upstream of Souhegan River, Merrimack NA
7. Mer 540 Thorntons Ferry, Merrimack 1299.7
8. Mer 530 Greeley Park, Nashua 307.6
9. Mer 520 Taylors Falls Bridge, Nashua 172.3
10. Mer 510 Sagamore Bridge, Hudson 160.7
11. Mer 500 Pheasant Lane Mall, Hudson NA
12. Mer 490 Upstream of Tyngsboro Bridge 2419.6

Flows

• Flow on Upper Souhegan was 46CFS. Expected flow for that date is 32
• Flow on Lower Souhegan was 64CFS. Historic flow for that date is 55
• Flow on Merrimack was 2660CFS. Historic flow for that date is 1520
• RAINFALL: Heavy rain on Monday 0.86”
• Water Temperature: Souhegan 18c, Merrimack 21c

Submitted by George May.