Nonprofit, independent journalism. Supported by readers.


Bit by bit, the dams are coming down and streams are regaining their health

Minnesota ranks No. 3 among the states for dam removals in 2016, but the pace here and elsewhere is far from furious.

California’s Oroville dam, which has been teetering at the edge of catastrophic failure
REUTERS/Jim Urquhart

Ever on the lookout for good news, I was drawn to Thursday’s report from American Rivers on U.S. dam removals last year and their potential to be, in these especially troubled times, “our country’s best ongoing environmental success story.”

Also, by reference to Minnesota’s ranking No. 3 among the 50 states this year for dam removals that, nationally, “restored 2,100 miles of streams to benefit public safety, local economies and our nation’s natural heritage.”

I came to learn that American Rivers’ stirring claims of restoration’s benefits are hardly overstated – quite the opposite, according to a Minnesota expert you’ll meet in a moment. But both here and across the country the pace of this work remains, shall we say, relaxed.

The 72 dams removed last year may sound like a lot – six per month! – until you consider that the latest U.S. inventory, conducted in 2013, found more than 87,000 extant dams. A mere 1,383 have been taken down since 1912, according to American Rivers statistics. (A cool interactive map of them all can be found here.)

Article continues after advertisement

Minnesota’s rivers and streams have more than 1,200 dams, two-thirds of them in public ownership; last year’s third-place showing was earned with just six removals, four of them in a single project on the Sand Hill River, near Fertile in northwestern Minnesota. The others were on Kanaranzi Creek near Adrian and the middle fork of the Zumbro near Oronoco.

According to the Minnesota Department of Natural Resources’ page on the subject, about 50 dams have been removed or modified for ecological purposes in the state since 1984, with modifications slightly outnumbering removals.

As for pace, things haven’t actually changed much since the 1980s, when dams were removed at the rate of one or two in a typical year – although the agency’s graph of cumulative removals and modications seems at a glance to show a dramatic acceleration. (On the other hand, the stats don’t count an additional dam or two per year that are taken down for safety reasons, according to Jason Boyle, the state’s dam safety engineer.)

Curious to know more, I called the DNR’s Luther Aadland, who has studied dam removals from the standpoint of ecological restoration and found them to be a simple, cost-effective tool that may be unmatched in potential to quickly restore the native biodiversity of dam-blocked streams.

Bad for fish, worse for mussels

Aadland is a Ph.D. river scientist in the agency’s Fergus Falls office, and two years ago he produced a paper based on his analysis of how dams affect species diversity in streams.

In general, he found that dams are hard on most fish and even harder on most mussels, while being highly beneficial to a wide range of invasive species (and neither particularly helpful nor harmful to invasive zebra mussels).

Dams that formed complete or near-complete barriers to fish and mussel movement had the strongest negative impacts, but even lower dams that formed incomplete or intermittent barriers were a problem. And when the dams came out, the ecosystems quickly recovered.

His core conclusion: Barrier dams “are among the most profound and definitive causes of native biodiversity losses in Minnesota waters.”

So I asked if the converse would also be true: Is dam removal therefore the most profound and effective tool for restoring natural biodiversity in our streams? He said it is.

Article continues after advertisement

Even more important than, say, reducing agricultural runoff of soils, nutrients, pesticides and so forth? He laughed and said, “Well, I guess I wouldn’t really want to go that far.”

Although water quality and biodiversity are intimately connected, and benefits to one tend to benefit the other, scientists and planners tend to think of them separately, and the state is making important and essential investments in water quality through the buffers program and other measures.

“And obviously it doesn’t do any good to remove a dam and allow free movement of fish back into parts of a stream with crap water quality,” he said.

But dam removal is a pretty easy thing to do, too, compared to a lot of other restoration work. It’s quick, and its cost-effectiveness is huge. And the watershed-level improvements you can achieve are also huge.

The major barrier to removing more dams, Aadland said, is community resistance to removing a piece of infrastructure that has standing as a landmark or a sort of monument, followed by suspicion that dams must be holding back invasive critters, although the science says otherwise.

We had one in Appleton, Minnesota, for instance, that had been there since the 1870s and at one time the reservoir was their lake – there was an island in it, where they had weddings, so there were a lot of lengthy memories of that thing. But it filled in with sediment.

The old-timers remembered jumping off the railroad bridge, diving into 15 feet of water behind a 16-foot dam, but now they’d be going into 15 feet of muck. And like a lot of these old dams it had started falling apart, and the city was faced with, well, do we pay the money to bring it up to current standards, which would be substantial, or do we remove it.

So it was probably more economics on their part than anything else, and in January of 1997 the city council voted to remove the dam. Then a few months later we had the 500-year record flood and it failed the dam. And I’ve had that happen multiple times – we’re talking about removal and a flood takes it out first.

Dam modifications with fish ladders and such are useful, he says, and have an interesting history going back to 17th-century France, not to mention the “shad wars” along the U.S. Atlantic seaboard in the middle 1800s – but if you’re serious about restoring a stream’s ecological health, full removal is the only way to go — “and I do feel we’ve been making good headway on this.”

Article continues after advertisement

How to re-carve a stream

After a dam comes out – whether by planned removal or sudden failure – some restoration work is likely to be needed, Aadland said, but the degree can vary considerably. Typically there’s no need to dig or dredge out the sediments, or to dam the stream above the worksite to keep things dry.

Flowing water will cut a new gully through the muck, and engineers will shape a new channel off to one side, “using the better parts of the natural river that remain to guide us,” then direct the flow into the new course. Riffles can be built on the bottom with boulders and gravel, and the stream can be reconnected to its natural floodplain.

I thought that sounded like a lot of work, and probably expensive, but Aadland said it typically is not, and sometimes the costs are offset by savings on fish-stocking and other management efforts that are no longer needed. Also, the costs are sometimes shared with partner agencies and local governments, so the DNR’s own spending is leveraged into larger outlays.

As for ecological rebound, Aadland’s paper analyzed the impacts of dams at 32 places on the Mississippi, Minnesota, St. Croix, St. Louis, Missouri and Red River of the North, and of removals at 11. Key findings, slightly paraphrased:

  • Complete and near complete barriers reduce upstream species richness by an average of 41% and 37 % respectively; moderate barriers that may flood every year or two also reduced species richness, by 20%.
  •  Loss of species richness due to barriers extended watershed-wide, with imperiled species especially vulnerable and barrier-tolerant species, like common carp, least affected.
  • Of species that had disappeared above barrier dams, 66% returned after the dam was removed.

And while a lot of attention is focused on maintaining diversity of fish species – especially game species – Aadland thinks people ought to be thinking more about native mussels, their protection and their recovery. In the paper, he wrote

North America is analogous to tropical rainforests in terms of mussel species richness, with more species than any other continent, but 71.7% are listed as special concern, threatened, or endangered. The ecological implications of mussel declines are extensive due to their roles in stabilizing stream beds, increasing diversity of other benthic invertebrates, and water filtration.  

It’s a topic he returned to toward the end of our interview:

People don’t really have much appreciation for their huge role in water quality. We’ve shown they can basically destroy harmful bacteria, like E. coli, by filtering them out. Also, removing what we’re now calling contaminants of emerging concern, like pharmaceuticals.

So the implications are really far-reaching.

Article continues after advertisement

A looming problem

Given Aadland’s insight that impending dam failures may be the main driver of removal efforts – and that those efforts can be downright cheap compared to repair bills – I need to mention this other sobering statistic from the American Rivers report.

According to the Association of State Dam Safety Officials, America’s dams are degrading faster than they are being repaired, the number of high hazard dams has increased over time, and the cost to rehabilitate dams continues to rise. By 2020, 70 percent of dams in the United States will be more than 50 years old. Aging dams can pose a serious safety threat for individuals and entire communities.

* * *

Luther Aadland’s paper can be read here [PDF] without charge, and a book he has written about restoration strategies and techniques can be found here.