Collapsed Long Island dams leave a choice: Whether to rebuild

As a heavy rainstorm swept across the North Shore last month, 9 inches of rain swelled a 110-acre pond at Blydenburgh County Park in Smithtown and overwhelmed a dam that held back the water.

When the dam burst during the Aug. 18 storm, the pond emptied into the Nissequogue River and then to the bay. Now, acres of mud extend from one wooded shore to the other, crossed by a meandering, swift-flowing stream.

The wasteland of muck and marooned fish — at Blydenburgh and at a dam in Stony Brook that came down in the same storm — inspired calls to rebuild the dams and let the ponds refill.

But many local conservationists and some park visitors favor a different approach: Let nature take its course.

"We always favor returning ecosystems to their most natural form," said Katie Friedman, the ecological restoration program manager for New York at Save the Sound. "The Blydenburgh dam breach in particular," she said, should be "left to naturally transition back to its historic wetland and river ecosystem."

Some locals have been quickly won over to that view.

Nancy Simone, who was walking her dog in the park two weeks after the breach, said she initially was "100%" in favor of bringing back the pond, which she had visited often when she lived nearby.

But when asked her thoughts on an alternative proposal to restore the creek, she immediately changed her mind. "Oh, they should do that!" she said. "The world is filled with all these unnatural things. Let’s just let it happen."

A movement for dam removal

All along America’s 3 million miles of rivers, tens of thousands of small dams block the free flow of water to the oceans. Originally built to power the grist and textile mills of the early industrial revolution, the dams remain long after the old mills closed.

Ecologists argue that these barriers have degraded habitats and tipped native species that depend on them into decline. Efforts to dismantle them have accelerated since the 1990s.

Last year, at least 80 dams across the United States were removed, according to the conservation group American Rivers — from a 173-foot-tall construction on the Klamath River in northern California to dozens of small, defunct mill dams up and down the East Coast. Destroying those barriers reconnected 1,160 miles of river.

Increasingly, environmentalists are also arguing that even low dams on small brooks can be dangerous. They were not built to withstand the intense rainstorms that have become more common with global heating, which puts downstream communities at risk.

The dams that collapsed last month in Smithtown and Stony Brook were classified as low hazard by the state Department of Environmental Conservation, the agency that monitors dam safety, as are all but a few of the remaining 96 dams in the DEC’s inventory for Nassau and Suffolk counties — a number that is almost certainly an undercount.

According to Enrico Nardone, director of the Seatuck Environmental Association in Islip, there are more than 140 rivers and streams in Nassau and Suffolk. "Almost all of those streams but for a few were dammed, most of them more than once, sometimes as many as a dozen times," Nardone said.

These low-hazard dams are not regularly inspected. Though the agency says it aims to visit them once every 10 years, DEC data shows that many have not been inspected since the 1970s.

When a dam doesn't meet the agency's safety standards and the owner doesn't want to spend the money for repairs or maintenance, the DEC says it encourages owners to remove it. And the agency offers grants to municipalities and nonprofits for dam removal projects that improve aquatic habitat connectivity and reduce flood risk, which can cost anywhere from a few thousand to several million dollars.

An unnatural environment

The ponds and lakes that form behind low-head dams can be popular recreation spots and pretty to look at, but conservationists point out that they are artificial environments and generally not healthy, diverse ecosystems.

The fish, amphibians, crustaceans and insects that are adapted to cool, fast-running streams don’t do well in warm, still ponds, which tend to support a more limited range of species, often nonnative. Warm, stagnant water also encourages the growth of harmful algal blooms, which almost never grow in streams. 

Barriers trap sediment that otherwise would flow downstream, and as the sediment accumulates and the ponds grow shallower, invasive aquatic plants such as water chestnut take hold. The plants can grow so dense that fishing and boating become difficult.

Dredging and herbicides, the usual methods of control, are temporary fixes, and neither is good for wildlife.

The dams also block river herring and other migrating fish that form a crucial part of Long Island’s coastal food web from reaching their spawning grounds. Conservation groups and government agencies have built at least a dozen fish ladders to help, but it’s an imperfect solution.

In some cases, fish struggle to navigate these structures. Kellie McCartin, a professor of marine biology at Suffolk County Community College, said that at some of the sites she monitors, hundreds of fish congregate at the bottom of the fish ladder, apparently unsure how to proceed.

How stream habitats recover

In the past 20 years, four small dams on Long Island have collapsed (not counting Stony Brook and Blydenburgh), and in each case the failure stirred an ecological revival; accidental experiments that could provide a model for the recent breaches.

Five years ago, the weir boards on the West Brook dam in Great River rotted and the pond drained in a matter of days. Plans to rebuild it were abandoned after conservation groups such as Seatuck suggested letting the area rewild.

Within a short time, Nardone said as he looked out over the brook, native grasses and wildflowers emerged from the exposed muck. "It was all native seed bank that had been under water for 120 years and just started growing," Nardone said.

Today, cattails, yellow-flowered jewelweed and goldenrod grow beside the brook. According to the crowdsourced database eBird, since the dam collapse, the number of bird species observed in this small piece of wilderness has more than doubled.

Other undammed landscapes include Sunken Meadow, where an earthen berm gave way during Superstorm Sandy, transforming a degraded marsh severed from the natural flow of the tides into a flourishing coastal wetland. At Hempstead Lake State Park, where a dam failure drained a pond in 2011, the revived wetland is now one of the most important stopovers for migrating birds in Nassau County.

New beginnings

On a cool September morning, three weeks after the Blydenburgh dam tumbled down, East Islip residents Jannine Thorp and her 8-year-old daughter, Hailey, watched a long-legged egret wading in the creek where the pond used to be.

Thorp told her daughter the stumps scattered around were the remains of an Atlantic white cedar forest that once thrived there, before the trees were cut down and the pond spread across the flood plain.

"I would love to see this become a natural area," Thorp said — a river habitat instead of an artificial pond. "I think it could be just as beautiful and just as useful to the environment, if not more."

Then they noticed that all across the drained pond floor, pale green seedlings were sprouting from the mud. The seeds had been slumbering under the pond water all along, Thorp told Hailey. "They’ve been waiting for their time."