What’s So Bad About Dams, Anyway?


In our line of work removing old dams to restore rivers, one of the most common questions people ask is “what’s so bad about dams, anyway?” It’s a great question and could be a short or long answer, depending on how detailed you want to get. I’ll attempt to summarize the key issues here.

1.  Dams block movement of fish and other species. What’s so important about this? Reproduction, for one. Some fish migrate from oceans to rivers (and the other way around) to spawn – salmon are the best-known example of this, but there are many others that might surprise you. Dams also isolate fish and other aquatic species from each other, which can result in bad genetic impacts over time. Think about the town you grew up in – if there was a big wall permanently separating you and your town from other towns, over time people would end up marrying their relatives! You might think, “well there would have to be a lot of dams for that to be a problem” – consider that in North Carolina alone, there are 5,000 known dams, and likely an equal number of undocumented dams.

2. Dams change river habitat to pond-like habitat. By impounding water behind them, dams take a free-flowing river and change it into a completely different type of habitat– mainly, that of a lake or pond. This is a problem because most of the fish and other species living in the river are adapted to free-flowing conditions, and do not fare well in ponds. That’s why we regularly see substantial decreases in the number and type of fish behind a dam. And for many low-head dams, we’re not just talking a few hundred feet –the slow-moving, pond-like section of river behind a dam as small as 10 feet can be miles! Removing the dam restores the natural, free-flowing habitat, often in a fairly short period of time.

3. Dams degrade water quality. By slowing water and trapping sediment and nutrients, dams create conditions favorable to algal growth. Town’s using dams to create drinking water reservoirs often battle these water quality issues at a high cost to taxpayers. Slowed water and algal growth can also significantly deplete the oxygen in the water behind a dam, leading to fish kills.

4. Dams change the flow of the river, and can impact downstream users.  With fresh water becoming more of a precious resource and drought increasing in many areas, any unnecessary blocking of water should be critically evaluated. While often dams are “run of river,” meaning what goes in comes out, sometimes they can reduce or even stop a river’s flow, leading to bad consequences for people, fish and wildlife downstream.

5. Dams are a danger to people and impede river-based recreation. Every year dozens of people are injured or die from drowning at low-head dams. Additionally, with many of our dams far past their intended lifespan, dam failure is not uncommon. This can result in flooding, loss of property, or loss of life downstream. Aside from safety, dams block safe navigation on rivers for boaters, and degrade the river habitat and quality of fish for anglers.