Mill Brook fish passage project complete

An innovative project to restore fish passage on Pomfret’s Mill Brook is complete.

The WRP, working with the Vermont Fish & Wildlife Department, US Fish & Wildlife Service, Greater Upper Valley Chapter of Trout Unlimited, and town of Pomfret, installed 29 rubber baffles inside the first stream-crossing culvert on Mill Brook in late-August to provide fish passage upstream.

This is the first, large-scale installation of these extruded, rubber baffles in the eastern United States.

A focus on Mill Brook

The first stream-crossing culvert on Mill Brook is located under Pomfret Road near the intersection with White River Lane. The site is just upstream of the brook’s confluence with the White River main stem, which passes under the West Hartford Bridge about ½ mile downstream.

Access to the culvert is easy. There’s a small parking area on White River Lane adjacent to the culvert outlet. And there’s a trail from the parking area to Mill Brook across land protected by The Nature Conservancy as part of their White River Ledges Natural Area.

On any given day in the summer time you can find at least one car in the parking area. Local fisherman know this is a great place to fish – for good reason.

According to WRP Watershed Restoration Manager Greg Russ, Pomfret’s Mill Brook is an important spawning tributary in the White River watershed. “Its 12-square-mile drainage boasts cold, clean water and a diversity of feeding and spawning areas, ” says Russ. “And its location in the lower portion of the watershed makes it accessible to wild rainbow trout moving upstream from both the White River main stem as well as the Connecticut River.”

For these reasons providing fish passage to Mill Brook has been a high priority for local, state, and federal conservation partners. But ensuring fish passage has proven challenging in this location.

The culvert problem

The first stream-crossing culvert on Mill Brook is long, tall, and steep. 184-feet-long by 16-feet-high by 15-feet-wide, the corrugated steel pipe was installed at a 6 percent slope.

Despite its large dimensions, the culvert is considered “under-sized.” This means the width of the culvert is smaller than the width of the stream. Mill Brook is 39-feet-wide at its confluence with the White River, making the 15-foot-wide culvert only 38 percent as wide as the stream channel.

The under-sized culvert restricts water flowing downstream and the result is similar to putting your thumb over the end of a water hose: the water moves at extremely high speed. This creates a “velocity barrier,” preventing fish and other aquatic species from swimming upstream.

Velocity barriers are especially problematic for rainbow trout, which need to move upstream in spring – during seasonal, high water flows – to access spawning areas.

Why baffles?

Luckily there are options for addressing velocity barriers. Replacing an under-sized culvert with a larger structure is one option. But when replacement isn’t feasible, “retrofitting” an under-sized culvert can provide for fish passage.

Baffles are one retrofit option. Installed on the bottom of an under-sized culvert, perpendicular to the water flow, baffles interrupt the fast-moving water and create pockets of quiet water at regular intervals.

Fish can then move upstream, swimming from one pocket of quiet water to the next. In this way baffles allow fish to navigate through an under-sized culvert that would otherwise be impassable.

The extruded, rubber baffles installed in the first stream-crossing culvert on Mill Brook are low profile – just 6-inches-high by 5-feet-long. Project partners installed them every 6 feet, inside the corrugations to protect the attachment points, creating 30 pockets of quiet water along the 184-foot culvert length.

Developed originally for installation in New Zealand river systems, the rubber baffles are sturdy enough to interrupt the water flow, but flexible enough to bend over when hit with debris.

According to Russ, “Large rocks and boulders moving through the Mill Brook culvert are the biggest threat to the retrofit project, so partners are excited to see how the rubber baffles fare.”

Investing in fish passage

Retrofit projects are not permanent solutions and the Mill Brook baffles are no exception.

The Greater Upper Valley Chapter of Trout Unlimited installed the first baffle system – 12 wooden baffles attached to thick metal plates – in 1995, which lasted until Tropical Storm Irene flooding in 2011. The White River Partnership worked with partners to install a replacement system – 18 metal baffles attached to steel expansion rings – in 2013, which lasted about 1 year.

Despite the potential for failure, funding partners are committed to investing in providing fish passage at the Mill Brook site, largely due to the success of these former baffle systems.

“Data gathered upstream of the Mill Brook culvert shows a 500 percent increase in young-of-the-year trout after the installation of the baffle systems,” says Russ. The results are clear: baffles work.

Used with success in the Pacific Northwest, the rubber baffles installed in the Mill Brook culvert are the first of their kind in the eastern United States. And these baffles cost only one-third as much as comparable wooden or metal baffles options.

If successful the Mill Brook baffle project will provide a replicable model for economical, culvert retrofit projects throughout New England.

A team effort

Many partners contributed to the Mill Brook fish passage project. Six Greater Upper Valley Chapter of Trout Unlimited volunteers joined seven White River Partnership, Vermont Fish & Wildlife Department, and US Fish & Wildlife Service staff to install the baffles over a two-day period in late-August.

“We were very excited to help install the baffles and watch the culvert transform from a long, steep chute to a series of steps and pools that fish could navigate more easily,” said VT Fish & Wildlife Habitat Biologist Will Eldridge, one of the state biologists who helped install the baffles.

“We will continue to monitor Mill Brook for adult fish migrating upstream again and young fish being born. We will also keep an eye on the baffles to see if they can withstand repeated buffeting by large boulders and ice floes. If this experiment succeeds, anglers will find more healthy trout in the stream, but the fish and other aquatic organisms that can once again spawn, avoid floods and find cool water in Mill Brook will be the biggest winners.”

Baffles were designed by ATS Environmental in New Zealand and distributed in the United States by S. Scott & Associates. Funding was provided by the Vermont Habitat Stamp Fund and the National Fish Passage Program. And access permission was granted by the town of Pomfret and The Nature Conservancy.