In both New Hampshire and Vermont, landowners and state officials are discussing the benefits of removing old dams and restoring rivers to their historical banks. Many of these dams are small, though some, like Hinsdale’s McGoldrick Dam, which was removed from the New Hampshire’s Ashuelot River in 2001, straddle major rivers.
There are consequences to setting a river free, ranging from impacts on aesthetics, history, and recreation, to the possible reduction of hydropower (a non-fossil source of energy), to changes in property values. The ecological repercussions of removing a dam are far-reaching and profound.
There are more than 75,000 dams in the United States that are at least 6 feet high. Vermont has about 1,200 dams – one quarter of the number found in New Hampshire. Most of these dams were originally built from the late 1700s through the 1800s in order to supply water power gristmills or sawmills, provide for recreation, and, later, to generate hydroelectric energy.
Dams transform river environments into ponds or lakes – still waters that form layers with different levels of temperature, clarity, and oxygen content. Water lilies and pondweed grow in the shallows, with arrowhead and cattails near the shore. I’ve angled these environments for sunfish, perch, pike, and the elusive lunker bass, casting the limpid idylls of historic millponds while dragonflies patrolled, ducklings wove through fragrant water lilies, and yellow warblers sang, Sweet, sweet, sweet, I’m so sweet!
But some old dams have outlived their original purposes, may be too expensive to repair, and may no longer be wanted by their owners. What happens when a dam is removed? Most dramatically, removing a dam physically re-opens a river to migratory fish, perhaps including American shad, Atlantic salmon, and blueback herring.
Removing a dam also tends to cool a river’s temperature, favoring the many species of fish that require consistently cool water in order to migrate. Warm water signals to many migratory fish that the spawning season is over. Even where dams are equipped with fish ladders, unnaturally warm water above the dam can create a thermal barrier that triggers an end to migration and spawning behavior in American shad.
On a smaller scale, insects such as stoneflies, water pennies, and certain mayflies, craneflies, and caddisflies thrive in a free-flowing river. Gravel bottoms are ideal because water flushes through the minute spaces, providing oxygen and nutrients while removing wastes.
Spring floods scour the river to maintain gravel, cobble, and rocky bottoms. Water tumbles and mixes over rocks and riffles, which balances the temperature from top to bottom and blends in oxygen. Shade from overhanging trees and shrubs keeps water cool, enabling it to hold more of the dissolved oxygen that is so essential for species such as trout and alewives. Some species—like the endangered cobblestone tiger beetle of the Connecticut River—require the scouring action of springtime flows to maintain critical habitat.
Removing a dam restores water flow to within the historic channel and reestablishes interconnections between stretches of river and bordering riparian habitats. Gradually, plants and animals of the open river return, and the ecological community harbors a growing diversity of species.
The free flow of water also re-establishes the process of nutrient production and recycling. With natural flow, the algae that grow on rocks in riffles produce most of the ecosystem’s energy and become food for other species. Nutrients, meanwhile, settle out and decompose in quiet pools. This cycle of production and decomposition occurs repeatedly along the course of a free-ranging river.
Dam removal does pose risks, like opening the upper reaches of a river to invasive species, where newly exposed shorelines provide fertile and available habitat. Planting native species will stabilize these areas. During the breaching itself, the impounded water needs to be released slowly to mitigate downstream flooding and riverbank erosion. In addition, the silt behind old dams may contain toxic sediment left over from our industrial heritage, and this silt may need to be dredged or captured with filters to reduce the possibility of poisoning habitat downstream. Finally dams must be removed at times of year that minimize the effect on migratory fish.
The Peterson Dam along the Lamoille River and the Dufresne Dam on the Batten Kill are two Vermont structures currently being considered for removal. Taking down the Peterson Dam could restore the Lamoille’s migratory runs of sturgeon, walleye, and landlocked salmon, while the Dufresne is the only dam along the main channel of the Batten Kill, one of the region’s celebrated trout-fishing rivers.
Done carefully, the long-term benefits to aquatic ecosystems that accrue from removing dams can outweigh the short-term disruptions of the removal process, especially when the dam in question is no longer being used for its intended purpose.