Souhegan River assessment presented

MILFORD – Rivers like to meander, to set their own channels and wander over their flood plains. Left to themselves, river courses are changed by storms, by temporary blocking of a channel, and by natural erosion. Such changes, of course, do not fit in with humans’ use of the river. They need the water to stay in one place and that containment effort has caused problems in the past and continues to do so.

On Sept. 11, Shane Csiki of the New Hampshire Geological Survey, a division of the Department of Environmental Services, presented an overview of an intensive study made of the banks of the Souhegan River and its major contributories last year. He was assisted in the study by Nick Miller of Field Geology Services.

That study, officially called a “fluvial geomorphic assessment,” covered 32 miles of river from New Ipswich to Merrimack and included Furnace Brook in New Ipswich, Stoney Brook in Wilton, Baboosic Brook in Merrimack, and confluences with others.

Miller noted the area’s long legacy of mills. “They were everywhere, and many of them straightened the river artificially” to provide for millraces and ponds, and built dams to provide waterpower. In fact, he said, of the areas they studied, “64 percent had been straightened.”

Prior to going into the field, they looked at topographical maps, aerial photographs, and historical archives. One big help was the discovery of a map of Milford from 1904.

Superimposing a new map on the old one showed how the river had moved in the past 100 years, leaving abandoned curves known as ox bows.

The first recorded flood on the Merrimack River was in 1740, Miller said. Two major events since then were the great flood of 1936 and the hurricane in 1938. After that, the Army Corps of Engineers began the building of flood control dams.

The study was made basically to determine the condition of riverbanks, erosion sites, and possible trouble spots. They found that 18 percent of the banks were eroding and that 9 percent had been “armored” with stones or riprap in the past, and berms had been created to keep the river from agricultural lands.

Causes of erosion are channel constrictions such as old bridge abutments, undersized culverts, encroachment of development, and previous straightening of the river. The major cause of local flooding and bank erosion is caused by undersized and inadequate culverts which force the water to back up above the culvert and wash out the stream bed below it causing build up of sandbars.

Both men showed photographs of various culverts and the erosion around them, using pictures from Brook Road in Mont Vernon, Wire Road in Merrimack, and Stoney Brook Drive in Wilton.

The study was sponsored by the Nashua Regional Planning Commission, Csiki said. “Our job is to work with Emergency Management officials, help them understand what is going on and how best to respond. The information is placed in towns’ hazard mitigation plans.

“We are working with NRPC to (help them) gain technical expertise in order to assist towns.”

They also helped an upstate town create a map of all of their culverts and the conditions of each, to help them form a plan for replacement and upgrade.

For more information contact the Milford Conservation Commission, or check the N.H. Geological Survey website, or call Lee Wilder, Outreach Coordinator, at 271-1976.