“Set the river free” is a cry heard more and more often. It started in New England, when old, unused water diversions and containment structures were removed, often for good reasons, from smaller streams. The goal was to let the stream or river return to as close to original as possible, or to prevent a dam failure and the subsequent floods and damage. If the thing doesn’t serve a purpose, and might even have become a hazard, why not take it out? And again, in some cases, it made good sense. In other cases people took sediment samples from behind the dam and said, “Hold up a second! We don’t want this in the river.” And so the dam stays for now.
However, there’s a difference between a little mill-dam or a small diversion on a stream in what has reverted to forest, and a good-sized power dam, or flood control structure. And you can’t just “rip it out and let the river heal.” Depending on the dam, the river, and what’s down stream, that can lead to disaster, at least for the very aquatic life that people are trying to save/reintroduce.
So, a quick primer on what happens behind the dam when the water flow is greatly slowed or stopped. A large puddle forms, growing into a pond or lake. If the river carries lots of sediment (silt, sand), then the heaviest of that begins to settle out into the bottom of the lake. As the lake fills, the weight on the ground increases, as does pressure on the dam and the sides of the lake. If excitement is going to happen, this is when the first signs appear (Teton Dam is the horrible warning). A little seepage is pretty common, and should be clear. If muddy water starts appearing on the wrong side of the dam, then Things Need to Be Done. If all goes as planned, a lake fills in behind the dam. Hydropower dams use the pressure of that water to turn turbines that generate electricity, so they are always releasing water, or do so intermittently to meet peak demand. Irrigation dams draw-down water levels in the dry season for farmers to use, then allow levels to rise again in the wet. Recreational dams hold water at all times, unless there is a compelling reason to release water (controlled flooding as opposed to uncontrolled “it ate the dam!!!” flooding. See Johnstown for the horrible warning.)
Over time, if the stream carries enough silt, the lake might just turn into a mudflat with a waterfall on one end. That is very, very rare, at least in the modern era. You can see it with ancient structures in Arabia and a few other places. Eventually the dam wears away and you have to look very carefully to see that a water control structure ever existed. Most modern US dams have a life-time of about a century or so before something needs to be done about the silt, it silt deposition is a problem. Something like Hoover Dam or Glen Canyon? Not such a problem.
So, let’s say you want to tear out a mill-dam on a rocky stream that never had much sediment. You do a core check of what’s behind the dam, find nothing toxic and no huge slugs of sediment, and decide to get rid of the structure. Some sludge and gunk will flow downstream, so ideally you allow a slow release and everything keeps going until the sediment reaches the sea (or the next lake downstream) and you have a nicely restored river. You are happy, the land owners are happy, and the fish are happy. Good deal.
However, let’s say you look at a dam downstream of an industrial area. It needs to go. So you take a core of the yards’-deep sediment behind the dam and Whhoooooooah Nellie! Arsenic, dyes, lead enough to rearm the Ottoman Empire, and a few other things have accumulated. Do you really want that going downstream and settling all over the stream? Probably not. The better thing to do is to lower the dam, allowing increased flow-through but still keeping that “stuff” penned up and out of circulation until someone decides they want to pay for the removal and remediation.
So, what if there’s been no, zip, nada industrial development in the area, ever. Your core comes out clean, but full of sticky, silty, sludge. The flow in the stream is not what it used to be, for a number of reasons. If you rip out the dam, it will take a very, very long time for that material to get through the stream. Most likely, the stream will aggrade, getting shallower and wider, leaving the sediment over the current bed. It can also cause the shallower water to be warmer for a while, until the balance starts to return after more sediment is carried downstream. If you have really good trout fishing downstream, this is probably not the scenario you want. Now you start talking about dredging out the stuff, taking it somewhere else, and then opening the dam after the water settles again. $$$,$$$,$$$.00 can be involved.
So, OK, what if we just rip out Hoover, or Grand Coulee? Well, first, where is that electricity going to come from? Nuclear is the only close replacement, because wind and solar are not going to work. Second, that will be an enormous slug of very, very cold water racing down the Colorado River valley and taking a lot of stuff with it. Like downstream dams and diversions. Like any water-supply intakes. The fish will be in for a surprise with that temp, although the modern Colorado is far colder than it once was. And eventually the sediment will start to move, slowly, and you will get back to closer to the old Colorado, brown, wild or sluggish, and meandering. If you get/got drinking water from downstream of Hoover, well, buy a lot of filters and plan on changing them regularly. And get ready for floods, as the bed, once scoured, starts to rebuild with that sediment.
Did I mention floods? Annual or semi-annual floods will return. Any valuable infrastructure will have to be relocated, or turned sacrificial. You’re going to lose habitat for some creatures and gain it for others. Given the sediment that’s built up behind the dams of the 1910s-1950s, anything that needs a rocky bed with well-oxygenated water might have difficulty for a while, depending on how quickly the sediment is redistributed and filtered out as the balance in the stream resumes.
Note: All these are controlled removals. Nightmare fuel is if one of the big dams on the Colorado goes all at once, because then the downstream dams will likely go as well. The Colorado will reach the sea again, perhaps by a different route. Or it might refill the Salton Sea and a few other areas, then head out via LA or even double back and then go out to Baja. The loss of life would be tremendous, the loss of infrastructure eve more so.
So dam removal can be done, and done right. However, “rewilding the Columbia and the Colorado!” is probably an undertaking best left to fiction writers for now. Until nuclear reactors become far more common, and we know a lot more about what’s behind those structures and how to release the contents slowly, we could do far more harm to the environment than we ever did by building the dam.