Deconstructing dams

Going with the flow solves problems, creates others

An old joke among river ecologists goes like this: “What does a fish say when it hits a cement wall?” Pause. “Dam!”

Today, fewer fish are banging their heads against these barriers, due in large part to Wisconsin’s efforts to tear down dozens of deteriorating dams. Razing these structures may alleviate many wildlife headaches, but it also may create new problems, according to UW research.

Emily Stanley, a river ecologist at the Center for Limnology, has found that dam removal allows not just fish and canoes, but also damaging nutrients, to barge through the water system. Results of the study, which focused on dam removal sites along the Baraboo River and Koshkonong Creek in Wisconsin, were recently published in the journal BioScience.

Beginning in the mid-1800s, communities nationwide built dams. While these structures generated power, controlled floods and stimulated economic growth, they transformed the ecosystem by blocking the movement of organisms, worsening the water quality and altering downstream flow and channel formation. As Stanley sums up: “Dams do bad things to rivers.”

Today, there are nearly 4,000 dams in Wisconsin, and several million can be found throughout the United States. Fewer than 60 rivers in the country retain more than 62 miles of free-flowing channel.

Yet, these numbers are beginning to change. “Many of the dams are getting old,” explains Stanley. “Time has taken its toll on these structures and transformed them from productive sites of commerce to safety risks.”

Tearing them down, rather than restoring them, may seem like the best option, both environmentally and economically. In fact, 120 dams were razed last year in the United States. Over the years, more than 60 have been torn down in Wisconsin.

“It’s a moment in history to see water flow again for the first time in more than 150 years and to watch fish go places they haven’t been for generations,” says Stanley.

Despite these good outcomes, she cautions, “Very few quantitative studies have been done on the effects of dam removal. It’s surprising how little we actually know about how the system will respond.” She says that two years ago only one scientific paper had been published on the physical and biological effects of dam removal.

Since then, Stanley has been busy collecting data at dam sites, before and after their removal, along the Baraboo River and Koshkonong Creek. She says her findings show that dam removal “is not all cakes and ale.” Removing dams allows excess nutrients that run off from the land to drift downstream, where they empty into lakes and oceans.

“When the nutrients used to fertilize crops enter these systems, they end up fertilizing them, too,” says Stanley. Too much phosphorous and nitrogen can create algae blooms, which turn the water green.

Dams, however, prevent most of these nutrients from flowing downstream. Each structure forms a reservoir of water behind it that often fills with sediments, which carry the nutrients. In other words, damming the sediments dams the nutrients.

Stanley’s paper states that “the sediment-trapping ability of reservoirs means that topsoil and nutrients lost from farm fields are now stored behind dams.”

On the other hand, dam removal enables sediments, along with the nutrients, to flow freely downstream. For example, the reservoir along the Koshkonong Creek retained 15 to 20 percent of the total amount of phosphorous carried downstream. After removal, however, the nutrients poured into the water system. Phosphorous concentrations downstream jumped from 0.3 to 2.7 milligrams per liter. Two years later, the amount has decreased, but Stanley says it’s still about 30 percent greater than the amount found in the water entering the former reservoir.

Based on these findings, Stanley concludes, “Removing dams may not be the best way to manage how rivers handle nutrients.” But, she does add, “There are always going to be tradeoffs, and what we want to do is maximize the gains. When the day is done, I’d much prefer to see the dams go.”

The next dam removal in Wisconsin will be along Boulder Creek in the Baraboo River basin, starting in February.

Stanley’s work is supported by the Bradley Fund for the Environment and the National Science Foundation.