February rains brought trouble to the small town of Oroville, California this year. In a region so focused on a lack of water, it seemed cruel that the opposite, too much water, could impose an equally terrifying situation. Femina Sci chatted this week about the Oroville Dam crisis and speculated on the ecological and environmental impacts of dam removals and failures.
Sheila: A series of large rainstorms in Northern California caused the water level in the Oroville dam to rise uncharacteristically high. In situations like this, engineers and dam operators need to manage the water flowing out of the dam carefully to avoid causing structural damage and risking failure. Operators started letting water down the main spillway to lower the water level behind the dam but the power of the water going down the spillway caused a large hole to form. The operators were forced to slow the flow in the main spillway and use the emergency spillway. The crisis reached its peak when, despite using both the main and emergency spillways, officials became concerned about the the 188,000 people living below the dam. Evacuations were ordered for the city of Oroville and a few nearby communities.
Erin: I would add that a proposal to update the primary spillway didn’t get funded, and some folks were looking into creating a secondary spillway. But because California was, and continues to be in a drought for so long, spillway failure was seen as an unlikely scenario. Repairs were neglected.
Annise: This isn’t an isolated problem. Aging infrastructure like roads, bridges, and dams, are going to continue to be a problem we face in the U.S. and internationally.
Sheila: The dam generates hydropower too, some of which is used on site for regulating the reservoir water level above the dam. It has the capacity to generate as much power as 275 onshore wind turbines (assuming each turbine contributes 2.5-3 megawatts). The dam was built to provide flood control to Butte County, which includes the city of Oroville, and irrigation water to agriculture in the San Joaquin Valley downstream.
The images of water pouring over the emergency spillway are pretty stunning. A massive amount of sediment was carried off the slope of the emergency spillway. I imagine this is going to have an incredible impact downstream. Let’s talk about the possible ecological problems that might arise after the event at Oroville.
Start this video at 0:58 to see what it looked like when the emergency spillway was in use.
A: It’s not just sediment that will be a problem, but also the change in substrate. The stream is going from rocky riffles to the silty texture we see in those pictures. Fish are selective about where they spawn and this change might turn fish away in those streams. I’m not sure of the spatial extent of the sediment release and whether it will significantly impact the population but there will definitely be local impacts to spawning ground.
E: When I looked at the images of the spillway failure, I was thinking about the huge amount of debris that washed into the Feather River below the dam. If the dam were to fail completely, then there would be an even larger pulse of sediment and debris moving into the Feather River that drains into the Sacramento River and into the San Francisco Bay. It's hard to know where that sediment ultimately ends up. You can imagine severe repercussions for the land and species living there in this terrible scenario.
A: I agree. Large changes in the amount of sediment will be harmful to all types of life. With sediment and debris coming through, you also need to think about changes to aquatic plant and algal growth. Without light, it’s going to be greatly diminished, which will have huge ecological consequences.
E: Nutrient concentrations will increase in streams too, including the availability of otherwise limiting nutrients. I’m not sure about the Feather River specifically, but often nitrogen and phosphorus are limiting nutrients, meaning they control the amount of microbial and plant growth, which are on the lower levels of the foodweb. If growth is usually restrained by the amount of nitrogen or phosphorous, these organisms will grow quickly with a sudden burst of nutrients in the water. It’s hard to know the exact outcome in this scenario because there are so many variables, but it’s a concern in a lot of streams and lakes in the U.S.
Sedimentation and water clarity will also impact the uptake of carbon dioxide by primary producers who need to have access to light. A huge increase in the sediment getting washed into the river could impact the level of productivity of these lower food-web organisms and will trickle into higher trophic levels. In my work, that relates to driving down oxygen concentrations. If there is a finite amount of oxygen available, and organisms require oxygen, then these over productive microbes will outcompete fish and other larger streamlife.
A: It would probably get super oxygenated by the dam breaking….
E: Yeah, you’re right. In the case of Oroville, it seems the water will become reoxygenated and oxygen availability won’t really be a problem. I think the oxygen levels would stabilize and primary producers will re-colonize after the disturbance.
S: Nutrient impacts reminds me of the biofilms that live on the rocks in this stream. Biofilms are an ecosystem in themself. Stream biofilms are made of a sticky, carbohydrate matrix. As sediment particles or other nutrients go by, some of that will collect in the biofilm. It makes a nice little house for diatoms, microbes, and macro-invertebrates. When the spillway failure sent larger rocks flowing down the stream, it’s likely that this debris scoured away the biofilms that organisms in higher trophic levels (like fish) rely on for food, as Erin said. If biofilms are scoured out, the nutrients will be lost to the downstream.
Dam failures might also lead to some of the same problems faced when we intentionally remove dams. Can we talk about some of the better documented impacts of dam removals?
A: Sure. I think that’s an important point because there is an option to not fix our failing dams, but to take them down. We read this review article on the short and long term ecological impacts of dam removal. Overall, research indicates that biotic diversity increases when you take a dam down by enhancing the spawning ground and habitat for young fish. In the short term, the increase in sediment is going to affect the clarity of the water and will suffocate or damage the surface of fish and aquatic plants. But that’s pretty short term.
E: The authors of the article also talk about increasing fish passage and access. Six of the dam removals they reviewed showed increase in fish passage and access to more habitat previously unavailable to these species when the dam was in place.
A: When the review paper talks about increasing the diversity of species after dam removal this is not at all surprising. Removing a dam increases the connectivity of the stream and species will redistribute themselves. As ecologists, we don’t just talk about the number of species (the species richness), we also talk about the evenness of species. Previously, there was more beta diversity--more difference between different parts of the stream--but when you remove the dam then the total number of the species you find in one random sample would be greater. That doesn’t necessarily mean that’s a good thing.
S: Yes, I’m familiar with the idea in a different context. There was a recent paper out by Ashley Shade that talked about diversity not being the end goal for microbial ecologists.
A: Yeah, that’s also true in my field of invasion ecology. When you look at plant species richness in the Finger Lakes region, the number of invasive species we’ve gained (777) far outweighs the species we’ve lost (43, as of 2008). You have to ask yourself whether species richness is the right metric. Over time, it looks like we’ve increased our diversity, but we also should consider the homogenization of diversity as well.
On the other hand, when a dam is removed there are totally different species that show up upstream of the dam. The Nature Conservancy and Army Corps of Engineers removed the Cuddebackville dam on the Neversink river. After the dam was removed, the endangered parasitic dwarf wedge mussel showed up upstream of the dam, where there were previously none documented.
S: So the species that came in was the parasite?
A: Yeah, but it wasn’t framed as a bad thing. It was depicted as evidence of rescue of an endangered species.
S: Contaminated sediment also poses a problem in dam removal and failures, especially if the half-life of the contaminant is long.
E: True. One of the examples often cited is the PCB contamination in the Hudson River. Because officials didn’t remove the contaminant before removing the Fort Edwards Dam, they are still addressing the problem 40 years out. There are many people involved, like General Electric, the U.S. Environmental Protection Agency, New York Department of Environmental Compliance, and other State agencies, trying to figure out what to do about the PCB problem.
But I think the article also of spoke to the environmental impacts of installing a dam. When you dam a river, you change the physical properties of the river. There’s reduced flow and the temperature shifts. Different species will colonize a reservoir versus a river. When we remove a dam do we expect the original river species to recolonize? Is there a loss of life when the species that were thriving in the reservoir can no longer survive in the now river? What does this mean for restoration of vegetation and animals on a longer-term scale? The review paper on dam removal spoke about some of the short-term issues like supersaturation and reoxygenation of the water as it’s stirred up but didn’t really discuss long-term impacts including succession of organisms.
Well, that wraps up our thoughts about environmental and ecological impacts related to dam failure and removal. Look for our next installment on the Oroville Dam. We’re going to be talking about the political and social challenges faced in maintaining infrastructure in the U.S.
This week's moderator and panelists are Chelsea, Sheila, Annise, and Erin.