[Note that this article is a transcript of the video embedded above.]

Building a dam imparts a stupendous change to the environment, and as with any change, there are winners and losers. The winners are usually us, people, through hydropower generation, protection from flooding, irrigation for farming, and a stable water supply for populated areas. But, we've known for a long time, probably since we started building dams in the first place, that many of the losers are fish (especially migratory fish) through fragmentation of their habitat. Even in 1890, the state of Washington in the US had laws on the books requiring consideration of fish when building dams. And, not just consideration, but specific infrastructure that would allow fish around a dam if they were quote-unquote “wont to ascend.” I recently took a tour of McNary Dam on the Columbia River in Washington (operated and maintained by the U.S. Army Corps of Engineers, Walla Walla District) and an aquatic research laboratory at the Pacific Northwest National Lab to learn more about the ways we balance our own needs with those of the aquatic wildlife impacted by the infrastructure we build. You should check out that video after this one if you want to see the whole tour. But one of the biggest pieces to that puzzle was the enormous fish ladders that allowed salmon and other migratory fish to swim up and over the dam. And it got me wondering: how do engineers design a structure like this? So this video is a follow-up, a chance to dive a little bit deeper into the intersection between engineering and wildlife. I'm Grady, and this is Practical Engineering. Today we're talking about fishways.

You've probably seen a fish ladder before, but if you haven't, this one on the Oregon side of McNary Dam is just one of many designs. The way it works is simple in practice: adult fish swim upstream toward the dam. The goal is that they don't even realize the dam is there. They simply continue upstream through the fishway and out into the forebay on the other side. Water flows in one direction—fish flow in the other. But, designing a fishway isn't simple at all. In a way, it's like engineering life support systems for manned space missions: all the design criteria are biological. How fast can fish swim, and for how long? How are they motivated? How high can they jump? What temperature, dissolved oxygen, pH, and salinity can they handle? And how do all these factors vary across seasons and species? These are difficult questions to answer, and in fact, a big part of my tour at the Pacific Northwest National Laboratory was all about how scientists study exactly the limits and preferences of migratory fish. I saw the wide variety of tracking systems they use to observe the behaviors of fish in the wild and lots of different ways they study fish in a lab as well. From research like that and decades of trial and error with the fish passage systems in the real world, engineers and biologists have started to zero in on a few designs that work best.

All fish are different, which means every fishway needs to be specially designed for the particular species that they handle. At McNary, that mainly means salmonids, a group of fish species that spend most of their adult lives in the ocean but return to shallow freshwater headstreams to reproduce. Fortunately, NOAA Fisheries has a detailed Anadromous Salmonid Design Manual that boils a lot of this knowledge down. And I’ve built a scale model of a few fish ladder designs in the garage to show you how they work.

Salmon encounter all kinds of obstacles in natural streams and rivers, even ignoring the human-made ones. They're quite capable of moving upstream in a wide variety of conditions like rapids, small waterfalls and even the presence of hungry bears. Their species literally depends on it. So, the goal of a salmon fish ladder is to mimic natural conditions, to trick the salmon into thinking they're simply making their way up a section of the river, if a somewhat steep and concrete one, without delay, stress, or injury. Part of that trick is in the flow rate. In fact, the flow of water through a fishway is one of the most essential parts of the design. And like every engineering decision, it’s a balance. Every drop of water that flows through a fish ladder is a drop that isn't stored behind the dam, so it can't be used for hydropower or water supply. But the flow of water is obviously important to the fish, too. If the flow velocity is too high, the fish struggle to swim against it. And if it’s too low, there might not be enough water to swim through. But, fish not only need specific flow to swim through; they also need it to navigate. If flows through a ladder are too low, fish can become disoriented trying to find which way is upstream. And that’s especially true at the entrance. A dam stretches the entire width of a channel, but the entrance to a fish ladder usually doesn’t, so there has to be some way to draw them to the entrance. That’s called attraction flow. Salmon use the sound and turbulence of flowing water to know which direction to swim, so a big part of fish ladder design is simply encouraging the fish inside. In fact, the flow of water through the ladder itself is often not enough for attraction, so many fish ladders have auxiliary water systems. At McNary, two enormous pumps draw water from the tail race of the dam up and into the entrance channel just so it can fall back down, creating the sound and hydraulic conditions required for salmon to find their way in. In addition, a huge valve and conduit system under the fish ladder pulls additional water from the forebay and releases it at an intermediate point down the ladder. Both of these systems provide some redundancy (since no piece of infrastructure can operate 24/7) and operators some control over the conditions along the entire length of the fishway, ensuring it can always mimic ideal conditions for the fish. But once they’re in, another challenge begins.

Dams are tall. At least, a lot of them are. And most fish can't climb actual ladders. They can't walk upstairs, and although there are some fish elevators, they’re a lot more complicated and usually less efficient than a system that allows fish to swim in a somewhat natural channel. So, the overall hydraulic design of most fishways is to break up that elevation into manageable “chunks” that fish can navigate at their own pace. They need to go kind of horizontal, but still make their way upward over the dam. A steeper channel is shorter, but it can make the water flow too quickly. A shallower channel has slower flow, but it’s a longer distance, increasing the cost and complexity of getting up to the top. So, for salmon, at least, the engineers and biologists have generally settled on something in between (usually a 10-15% slope) that breaks up the total height into passable increments with some kind of baffle.

The simplest and oldest fishways are called “pool and weir” designs. The idea here is that fish can use a burst of energy to swim up the fast moving flow over the weir and then rest in the pool above. When they’re ready, they swim up the next one, and so on. Nothing like a grown man playing with fish in his garage. Lots of fish can handle this no problem, but not all species can manage the challenge of swimming up a high velocity jet of water over and over again. Pool and weir designs are generally considered one of the less effective designs because they can limit the species and fitness of the fish that can ultimately make it through. Many of the newer fishways use more sophisticated geometry to try and address that shortcoming.

The fish ladder at McNary modifies the concept a little bit by breaking the weir into two parts with a non-overflow section in the center and including submerged holes through each baffle, called orifices. This design provides a wider variety of flow conditions, allowing more types of fish to find their way to the top. McNary even sees a good number of Lamprey, a jawless fish species with similar migratory behavior to salmon, pass through the ladder each year. Most of the salmon prefer to use the submerged orifices rather than jump over the top. My model isn’t quite scaled to my toy fish, so I’ll demonstrate that here with some movie magic.

This particular configuration is sometimes called an Ice Harbor design because it was first implemented at Ice Harbor Dam on the Snake River, just upstream from McNary. Both the pool-and-weir and Ice Harbor designs have a major limitation in that they’re sensitive to the water level above the dam. Small changes in the forebay can significantly alter the amount of flow passing through the ladder just due to the hydraulics of weirs and orifices. So the designs only work when the reservoir or forebay above a dam is regulated to a tight margin. McNary has several large crest gates that can be used to control this, but that’s not always feasible. One type of fish ladder solves it in an interesting way.

Vertical slot fishways are exactly what they sound like. Instead of a weir or orifice, they use a slot along the entire height of the baffle. That makes it possible for fish to move upstream under a wide variety of flow conditions. When I remove some of the stoplogs in my model to lower the level, the vertical slot baffles continue working in essentially the same manner. Plus, the velocity is fairly consistent from the top to the bottom of the slot, giving fish ample opportunity to pass through each one. The protrusion on the upstream face creates a gentle area for fish to rest if they need it.

The big question with these designs and all artificial fishways is this: how well do they work? Again, a difficult question to answer, especially considering that many are designed with a specific group of species in mind. The fish ladders along the Columbia and Snake rivers are surprisingly good for salmon. One study found that 97% of Chinook, Sockeye, and steelhead that entered a dam tailrace made it up the ladder and into the forebay. But, millions of dollars of engineering, research, and testing were put into those structures because of the huge cultural and economic value of these fish in the region. The results vary wildly for other species or other dams.

The primary way we know this is through tagging fish, introducing them downstream of a fishway, and measuring how many make it to the upstream side. That type of study comes with all kinds of complications, and of course, it’s impossible to compare those numbers to how many fish might make it upstream if there were no dam in the first place. The Pacific Northwest National Lab showed me some of the mind-bogglingly tiny tags that can be implanted into fish and the fascinating tools they use to track them, so again, check out that other video if you want to learn more about the process. One of the simplest ways we use to measure the effectiveness of fishways is to count the number and type of fish that pass through them. Many of the largest of these structures are equipped with counting stations, often a simple window into one side of the ladder where someone watches and marks the fish as they pass by. This is extremely useful data, not just used to measure effectiveness but to keep track of overall fish migration year over year, and a lot of it is available online. But even this requires a little ingenuity.

Most fishways are too wide to see from one side to the other in the sometimes murky water, so it’s necessary to funnel fish toward the window. Sloped grates called pickets allow water to flow through while the fish are corralled to the counting station. Even these pickets can discourage fish from continuing upstream, so fish ladder designs have to consider whether they’re really necessary. A trash rack at the upstream exit is usually installed to keep debris from getting into the ladder and clogging up the baffles. Fish swim through the rack and into the forebay to continue their upstream journey.

Of course, this is a drop in the bucket of all that it takes to manage fish passage. You won’t be surprised that adult salmon migrating upstream is a small subset of the vast array of challenges in getting fish around the barriers we build. Even McNary has juvenile passage facilities for younger fish traveling downstream and another fish ladder on the Washington side with a totally different design. Dams worldwide, particularly those installed where migratory fish species live, often have significantly different systems custom-designed for the species needing through. This is important. It’s not just for biodiversity’s sake, although that’s pretty important on its own. We depend on these fish for food, for recreation, for cultural identity, and more. So, we’re constantly innovating. I mentioned fish elevators and locks. In some cases, we do the migrating for the fish by barging up or downstream. You may have seen viral videos of the Whooshh fish cannon that aims to make fish passage possible where traditional ladders aren’t feasible. We’ve even tried dropping fish from airplanes. It didn’t work very well, by the way. All this to say, it’s something we care deeply about. For better or worse, a big part of engineering is fixing the problems we created through engineering of the past when we either didn’t know or didn’t care about the impacts our projects could cause. Everyone has a different perspective about what it means for humanity to live harmoniously with all the other life we share the planet with. I think it’s fascinating how those ideas and endeavors trickle down through engineering into the real world.