Some of the best-known aquatic features of the Vineyard’s “natural” landscapes are not natural at all. But economic and environmental incentives increasingly entice owners and communities to return these spots to their original condition. This was the gist of a presentation given Saturday afternoon at the West Tisbury library by Beth Lambert, river restoration coordinator for the Massachusetts Division of Ecological Restoration.
Statewide, Ms. Lambert told an audience of about 50 attentive Islanders, at least 3,000 dams plug the channels of rivers and streams. Some dams serve important functions such as controlling floods, generating power, or feeding municipal water supplies. But the vast majority of these dams were built to power mills that no longer exist. In place for decades or centuries, most of these dams are taken wholly for granted, and the ponds they form show little sign of their artificial origin. But many dams are deteriorating, posing a risk to residents and communities downstream. And dams come with a steep ecological price.
The Vineyard’s most significant streams, the Mill Brook and the Tiasquam River, each feature a handful of old dams and their associated impoundments. Some, like the Mill Pond in West Tisbury, rank among the Island’s best-known landmarks, valued for their scenic, recreational, and historic value. Discussions and related research are under way to determine whether to dredge this prominent pond, which is slowly but steadily filling in with silt. While not addressing the Mill Pond specifically, Ms. Lambert’s presentation implicitly added another option to the table: the possibility of removing the Mill Dam and restoring the pond site to its original condition.
One major financial incentive for dam removal, said Ms. Lambert, emerged following torrential rains in 2005 that nearly burst a geriatric dam in Taunton. In response to the near-catastrophe, changes to the state’s dam safety law were enacted, clarifying that dam owners are responsible for dam maintenance and liable for damages caused by dam failure. Individuals, businesses, and towns that owned dams found themselves saddled with structures serving no clear purpose but posing an expensive liability.
Coincidentally, aquatic biologists had focused increasingly on the effects that dams and other obstructions have on a stream’s ecology. Perhaps most importantly, dams block the passage of fish and other aquatic life. River herring can’t journey from the sea to the pools and headwaters where they spawn. Brook trout and other fish lose access to reaches of streams with characteristics they need at particular seasons, or at particular points in their life cycle. Predictably, populations decline or disappear.
More subtly, dams alter the physical characteristics of streams. They capture sediment that would ordinarily continue on to replenish downstream wetlands. And the broad, exposed surface of a pond heats up in summer. It is the warmest of this water, at the pond’s surface, that goes over the spillway; downstream reaches, as a result, can be far warmer than they would be were the stream undammed.
In response to a question, Ms. Lambert said that dam impoundments definitely harbor valuable wildlife. But the species that use ponds tend to be common and adaptable ones that can find other resources if a pond disappears. Free-running streams, in contrast, tend to host uncommon and specialized species that require cool, moving water. Dam removal, then, tends to favor species of higher conservation concern over widespread generalists.
Given the combination of economic and environmental incentives, removal is becoming a steadily more common response to an aging dam. The state’s Department of Ecological Restoration is currently assisting seven removal projects across the Bay State, Ms. Lambert reports.
Removal is a meticulous process that can involve several years of planning and permitting before any actual work is done, Ms. Lambert emphasized, in order to protect or relocate other infrastructure and historical features, or (at some industrial sites) contain toxins built up in pond sediment. Perhaps surprisingly, the mud exposed when a pond is drained revegetates quickly: photos presented by Ms. Lambert showed dam-removal sites that looked like mature wetland after just a couple of growing seasons.
The cost of removing a dam varies widely depending on the complexity of a project, Ms. Lambert said, from a low of about $70,000 to…well, a great deal more than that. (The road across the Mill Dam is a good example of a complicating factor.) But in response to the incentives to restore natural streams, financial support and planning assistance is available from a range of federal and state agencies, and even from nonprofit conservation groups.
Whether dam removal, dredging, or some other solution is best for the Mill Pond, or for other artificial ponds on the Vineyard, is a question that demands careful thought. These ponds are all familiar, a few receive considered public use, and some are highlights of our beautiful landscape. But stream restoration has emerged as a viable response to unneeded dams. Dam removal re-creates “a self-maintaining system that follows the trajectory of a natural stream,” according to Ms. Lambert, supporting threatened wildlife while saving maintenance costs over the long run.