Fecal Matter Swimming and Boating Exposure Risk in the Souhegan River

Recent testing results by the Souhegan Watershed Association indicate that only a few areas of the watershed have bacteria levels above the recommended swimmable levels. Most of the E. coli bacteria in the Souhegan River originate from inflow from the watershed land after heavy rainfall or sewer overflows (CSOs). The recent dry weather is ideal for minimizing the E. coli colony levels.

The Souhegan watershed received some rain over the three days preceding the recent test day. The soil moisture level is considered medium. Therefore, the 0.25 inches of rain that may have fallen were absorbed, rather than flowing into the river. The river level itself is low, but about at its seasonal median flow of about 60 cubic feet per second as measured in Merrimack. This is too low for kayaking.

With no recent rainfall, the lower section of the Merrimack River within New Hampshire had E. coli levels low enough to be considered safe for swimming. In the Souhegan River upstream of Milford, the coliform levels were also regarded as low enough to be safe for swimming, boating, and fishing.

As the Souhgan meanders into the relatively flat plains of western Milford, bacterial levels remain safe for swimming. But as the water slows and backs up against the Goldman Dam in central Milford, the E. coli levels increase. NH DES considers these levels only acceptable for boating and fishing. NOT swimming.

A review of the online data sheet 2025 summary indicates that bacterial levels typically build up before the dams and remain elevated as the river flows into Amherst and past the Boston Post Road crossing. This crossing area attracts wading and swimming activities, which pose a potential risk of E. coli exposure. We don’t have a reasonable hypothesis about the source of the bacteria in this area. It could be animals pooping in the river or river bordering septic system failures.

The Souhegan Watershed Association (SWA) will continue to investigate potential source(s), including inflow from Purgatory or Hartshorn Brooks.

Further downstream, in the town of Merrimack, there is often swimming activity at Wildcat Falls and Watson Park. Due to recent dry weather conditions and decades of investment in reducing sewage and industrial inflow into the Souhegan River, the fecal bacterial levels in this river segment are acceptable for swimming.

Investigation of High Bacteria Levels in Beaver Brook

Over the last few years, we’ve observed that Beaver Brook has higher levels of warm-blooded animal fecal bacteria than other areas of the watershed. So this year, we decided to gather data from additional sites along the brook to develop a more detailed understanding of the levels at various locations. Please review the map of the brook sub-watershed. Our sampling sites are indicated, and the E. coli levels are referenced on the E. coli data sheet by site number.

Reviewing the Topographic map, one can see that Beaver Brook feeds and runs through meadowland (marsh) for most of its route through Amherst. See the photo upstream of site BeB 710.

So I was pond-dering, what warm-blooded animal could be pooping in the marshes of Beaver Brook? Suddenly, it came to me. Beavers.