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New Hampshire

Restoring Rivers & Streams


Culvert Operations

Assessing Aquatic Continuity in the Ashuelot Watershed

By Eric Aldrich

Somewhere in a small stream near you, an Eastern brook trout is trying to swim upstream, seeking cool, fresh waters, food and cover.

It races against the current, darting around rocks and resting behind a boulder before bolting upstream again. The speckled beauty repeats the process, a seasonal exercise ingrained in its genetic makeup and practiced over the eons in small streams like this one.

But what is keeping this trout from reaching ideal habitat is the voluminous spout pouring from a poorly placed or designed culvert. In this particular culvert, the problem is called “perching,” when the outlet drops water so far and fast that fish can’t swim upstream. Culverts can also become barriers by causing low flow depth or high velocity.

This trout’s dilemma is a scenario repeated in countless brooks throughout New Hampshire, the Northeast and beyond, wherever roads cross streams. While some road-stream crossings let fish, salamanders, mussels, turtles, and other organisms pass freely up and downstream, others are barriers to aquatic connectivity.

Getting the Big Picture in the Ashuelot River Watershed

“Fragmentation in forests is caused by humans creating an edge that wildlife can not cross,” says Doug Bechtel, the Conservancy’s director of conservation science in New Hampshire. “In streams, fragmentation is similar, except the water’s edge is a dam or a culvert that aquatic wildlife can not cross.”

For these reasons, The Nature Conservancy wanted to examine just how fragmenting those potential barriers are at the watershed scale – and possibly replace ones to restore good stream habitat.

Rubber Boots and Measuring Tape

To study the aquatic connectivity throughout a 425-square-mile watershed across two dozen southwestern New Hampshire towns, the Conservancy had four essential elements:
• Funding from the N.H. Department of Environmental Services’ Watershed Assistance Program, the State Conservation Committee’s “Moose Plate,” and the U.S. Fish and Wildlife Service;
• Volunteers, more than 80 of them. To help recruit, train, organize and deploy them, we contracted with the Ashuelot Valley Environmental Observatory (AVEO), a non-profit citizen-science organization based at Keene State College. An ambitious summer intern for the Conservancy – Kate Kerivan – measured 128 dams in the watershed.
• Protocol to consistently asses each crossing, which we adapted from the River and Stream Continuity Partnership in Massachusetts; and
• GIS analysis, the science of Geographic Information Systems. By analyzing layers of spatial data, we could sort out priorities for restoration.

In 2006 our “culvert operatives” donned orange safety vests and rubber boots to scramble down road embankments, through poison ivy and into streams to measure some of the least glamorous pieces of civil infrastructure. Along with AVEO, they all deserve big kudos.

From Problems to Priorities

With their invaluable data in hand, the Conservancy determined that, of the 663 crossings evaluated, more than two-thirds were “moderate” or “severe” barriers to aquatic organisms.

While that alone is good information, it’s not enough. If a town were to replace a severe barrier culvert with one that restores connectivity, which would restore the most miles of high-quality stream habitat?

To answer that, we used the science of GIS to look closely at the streams themselves. We analyzed the number of stream miles; the upstream watershed water quality; and the quality of up- and down-stream forest buffer. This let us rank which culverts -- that if replaced with better ones -- would restore the most miles of good stream habitat.

Good Culvert!

This all begs the question: “What’s a good culvert?” Well, a culvert that lets  fish, turtles, mussels, salamanders and other organisms move freely up and downstream has these characteristics: It spans the stream and banks; doesn’t change water velocity; has a natural streambed; and creates no noticeable change in the river. They can include bridges, open-bottom arches, and culverts that span and are sunk into the streambed.

Now that we’ve identified priority culverts, the next steps will involve working with communities on restoration plans in the Ashuelot River watershed. Over the coming months, the Conservancy will collaborate with the Southwest Regional Planning Commission on a series of outreach steps with those communities.

Who knows? In a year or so, maybe that Eastern brook trout will be able to reach prime habitat where it will survive and thrive.

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