​Heavy Rain Influences Souhegan And Merrimack River Levels

By George May

Flows on both the Souhegan and Merrimack Rivers had been coming down to their historic average flows until the heavy rain on Monday jumped them up again. Both rivers have been higher than their historic averages pretty much all summer. The Souhegan had just about come down to the average until the Monday rains; then it jumped almost straight up on the chart.

On Sunday, the flow on the Upper Souhegan was about 30 CFS (cubic feet per second); on Tuesday morning it was more than ten times higher at 321. The upper stretch of the river is smaller and flashier, but even measured in Merrimack near the mouth of the river, the flow jumped from 60 on Sunday to 1:15 a.m. this morning.

At least one of the usual sites in Wilton could not be monitored because it was not safe to do it.

The Merrimack River is much bigger than the Souhegan but flows here jumped also. On Sunday the flow was about 1700 CFS and on Tuesday morning it was over 2,500. To put things in perspective, one cubic foot is about 7.5 gallons. If you were standing on a bank of the Merrimack on Tuesday morning, almost 20,000 gallons flowed past the spot you were standing on everysecond.

The Merrimack is back up to where it was two weeks ago and the Souhegan is double what it was two weeks ago.

Normally heavy rains increase E. coli levels by washing pollutants into the tributaries that may have been dry or low and into the river. This week proved that to be correct.

Watson Park in Merrimack is tested every week and the results posted at the park and on Facebook for swimmers. Until now the water has tested as excellent all summer. This week it is dangerous for swimming.

No spots that people use for swimming were at acceptable levels for safe swimming – even using the more generous state standards. Goffs Falls in Litchfield tested higher than the test can indicate at 2419.6. This level suggests that raw sewage was coming directly into the river – possibly CSOs from Manchester or Derry.

The Horseshoe in Wilton was 1732.9. Watson Park in Merrimack was 410.6. The state considers the river unsafe for swimming above 406.

In an attempt to understand the rivers and their relationship to heavy rainstorms, I went back over the last five years of data to find another time when the intensity of rains and the timing was comparable to this week’s. I found another test exactly two years ago (7/25/2017) with similar conditions. I would suggest anyone interested reread that release information easily obtainable on our website at http://www.souheganriver.org/results.html and see how similar the conditions and results are. Flows on the Merrimack are almost identical; flows on the Souhegan, however, were much higher this week – at least double. Two years ago we found that E. coli levels were very high; this week we find levels are even higher on both rivers.

The test was maxed out just downstream of Greenville and the wastewater treatment plant there. E. coli levels stayed very high all the way to Merrimack. No sites were less than 406. Interestingly, the test just downstream of the Milford Oval was only 727 – still polluted, but only half that found at many other sites. Usually this site tests much higher than surrounding sites.

The Merrimack River also tested much higher than usual this week. This stretch of the river usually tests fairly clean for E. coli. There were two sites, however, at the Taylors Falls Bridge and the Sagamore Bridge in Nashua, that tested clean by SWA standards (less than 88). The level jumped up again by the time it got to the Tyngsboro Bridge – to 686.7. The entire stretch from Manchester to Tyngsboro averaged 641 – the highest seen in a long time.

The average for all the sites on the Souhegan was 936... off the charts.

Dissolved oxygen levels are a good indication of the overall health of the river and its ecosystem, probably the best single indicator – and why we use this test. Oxygen increases when the water splashes over rocks and when it rains. So whenever we get heavy rain, E. coli levels may go up (which is bad for swimmers), but DO levels go up (which is good for fish).

This week we saw that. DO levels decrease as the summer goes along and the water heats up. But this week DO levels remained at about the same level as two weeks ago. Good for the fish.

All in all, both rivers were severely impacted by pollutants brought into the rivers by the heavy rain that preceded the testing. Luckily these bad results come after a break in the very hot swimming weather recently when the E. coli counts were excellent. Keep out of the water for a while – the bacteria have a lifespan of three days – and hope for a return to normal before the next test on August 6!

I will report the Watson Park test next week to give an update.

Thank you to Jane Hemmingsen for collecting and compiling all the data this week.

Here are the individual results for this week’s tests:

E-COLI MONITORING RESULTS FOR TESTS TAKEN ON July 23, 2019

SOUHEGAN RIVER SITES:

1. Billy Ward Pond, Ashburnham #1 10.9; #2 16.1
2. SoR 333 Water Loom Pond, New Ipswich 5.2
3. SoR 320 Highbridge, New Ipswich 648.8
4. SoR 309 Above Greenville Mill Pond, Greenville 1553.1
5. SoR 296 Downtown Greenville upstream of WWTP 579.4
6. SoR 291 Green Bridge off Rte 31 below Greenville WWTP >2419.6
7. SoR 218 Below the Horseshoe, Wilton 1732.9
8. SoR 210 Downtown Wilton 1553.1
9. SoR 201 Pine Valley Mill, West Milford
10. SoR 170 Behind Hayward Field, West Milford 1046.2
11. SoR 155 Souhegan Valley Boys and Girls Club, Milford 1119.9
12. SoR 146 Swing Bridge, Milford 727
13. SoR 133 Riverside Cemetery, Milford 1553.1
14. SoR 130 Behind Lorden Plaza, Milford1203.3
15. SoR 122 Amherst Country Club 1732.9
16. SoR 116 Amherst Conservation land Fairway Road, Amherst
17. SoR 095 Boston Post Road Canoeport, Amherst 816.4
18. SoR 057 Indian Ledges, Merrimack 1203.3
19. SoR 034 Turkey Hill Bridge, Merrimack
20. SoR 001 Watson Park, Merrimack 410.6

MERRIMACK RIVER SITES:

• Mer 600 Above Amoskeag Dam, Manchester 98.4
• Mer 590 Arms Park, Manchester 488.4
• Mer 580 Upstream of Piscataquog River, Manchester
• Mer 570 Goffs Falls, Litchfield >2419.6
• Mer 560 Depot Street, Merrimack 1413.6
• Mer 550 Upstream of Souhegan River, Merrimack 106.7
• Mer 540 Thorntons Ferry, Merrimack 410.6
• Mer 530 Greeley Park, Nashua
• Mer 520 Taylors Falls Bridge, Nashua 78
• Mer 510 Sagamore Bridge, Hudson 67
• Mer 500 Pheasant Lane Mall, Hudson
• Mer 490 Upstream of Tyngsboro Bridge 686.7
• Mer 600 Above Amoskeag Dam, Manchester
• Mer 590 Arms Park, Manchester
• Mer 580 Upstream of Piscataquog River, Manchester
• Mer 570 Goffs Falls, Litchfield
• Mer 560 Depot Street, Merrimack 8.2
• Mer 550 Upstream of Souhegan River, Merrimack
• Mer 540 Thorntons Ferry, Merrimack 6.26
• Mer 530 Greeley Park, Nashua
• Mer 520 Taylors Falls Bridge, Nashua
• Mer 510 Sagamore Bridge, Hudson 6.66
• Mer 500 Pheasant Lane Mall, Hudson
• Mer 490 Upstream of Tyngsboro Bridge

Flow on Upper Souhegan was 321 CFS. Expected flow for that date is 27.

Flow on Lower Souhegan was 115 CFS. Historic flow or that date is 56.

Flow on Merrimack was 2520 CFS. Historic flow for that date is 1770

RAINFALL: 2.5” Monday

Temperature: Souhegan 20C Merrimack 20C

CERTAIN PH RESULTS FOR TESTS TAKEN ON July 23, 2019

SOUHEGAN RIVER SITES: (pH)

1. Billy Ward Pond, Ashburnham #1 6.58;#2 6.56
2. SoR 333 Water Loom Pond, New Ipswich 6.59
3. SoR 320 Highbridge, New Ipswich 6.79
4. SoR 309 Above Greenville Mill Pond, Greenville 6.64
5. SoR 296 Downtown Greenville upstream of WWTP 6.85
6. SoR 291 Green Bridge off Rte 31 below Greenville WWTP 6.80
7. SoR 218 Below the Horseshoe, Wilton 6.54
8. SoR 210 Downtown Wilton 6.94
9. SoR 201 Pine Valley Mill, West Milford X
10. SoR 170 Behind Hayward Field, West Milford 6.51
11. SoR 155 Souhegan Valley Boys and Girls Club, Milford 6.76
12. SoR 146 Swing Bridge, Milford 6.73
13. SoR 133 Riverside Cemetery, Milford 6.41
14. SoR 130 Behind Lorden Plaza, Milford 6.40
15. SoR 122 Amherst Country Club 6.86
16. SoR 116 Amherst Conservation land Fairway Road, Amherst X
17. SoR 095 Boston Post Road Canoeport, Amherst 6.55
18. SoR 057 Indian Ledges, Merrimack 6.9
19. SoR 034 Turkey Hill Bridge, Merrimack
20. SoR 001 Watson Park, Merrimack

MERRIMACK RIVER SITES:

• Mer 600 Above Amoskeag Dam, Manchester
• Mer 590 Arms Park, Manchester
• Mer 580 Upstream of Piscataquog River, Manchester
• Mer 570 Goffs Falls, Litchfield
• Mer 560 Depot Street, Merrimack 8.2
• Mer 550 Upstream of Souhegan River, Merrimack
• Mer 540 Thorntons Ferry, Merrimack 6.26
• Mer 530 Greeley Park, Nashua
• Mer 520 Taylors Falls Bridge, Nashua
• Mer 510 Sagamore Bridge, Hudson 6.66
• Mer 500 Pheasant Lane Mall, Hudson
• Mer 490 Upstream of Tyngsboro Bridge