We all know one. An ugly, impractical beast that you just can’t imagine was purposefully constructed. At the same time, you know Mother Nature would never do one of her streams that dirty.
That’s right, today we’re talking culverts. The pipes, concrete boxes, and rebar that tunnel streams under roadways and railroads. The idea behind a culvert is simple: it needs to be strong enough to support the road above ground, and big enough to pass the water below ground. But, turns out, after 150 years in the business, humans are still trying to figure out the right balance between the two.
Some of the earliest design criteria for a culvert were published in 1853 in the 6th Edition of A Manual of the Principles and Practices of Roadmaking. Simply put, they recommended that culvert “size must be proportional to the greatest quantity of water which can ever be required to pass, and should be large enough to admit a boy to enter to clean them out.”
Really, we’re using “boy” as the unit of measure?
Today we might scoff at the inadequacy of those basic requirements for culvert design. But, for 1853, that simple recommendation was revolutionary. It also spurred rapid advancement of basic hydrologic theory because, at the time, there was no way to measure the “greatest quantity of water” that would pass through a culvert. We could guess, but flows are tricky- there’s drought and wet years, hurricanes, and heavy snows. So, some brilliant mathematicians worked out the numbers, and by the 1900s they found fairly simple equations that could estimate flood recurrence intervals and peak discharge. Crazy enough, these equations were so good that they still form the foundation of those calculations today.
But, something was still missing. We might know how much water passes past a point (otherwise known as stream discharge), but what’s the most efficient structure for facilitating that stream flow? It wasn’t really until the 1920-1950s where scientists started considering the position of the culvert in the stream. Should the culvert be completely submerged? Mostly out of water? What if the inlet is completely submerged, but the outlet not? Vice versa? Things get complicated fast. And, while we’re now better at designing culverts, we still aren’t 100% sure the answer to some of those questions.
Complicating matters is that oftentimes the most efficient way to transport water isn’t the most fish-friendly design. It turns out, fish are really finicky when it comes to culverts. They like a very set amount of flow, substrate sizes, and shade. If the culvert is too long they won’t pass completely through. If the water depth on either side is too deep or shallow, they won’t pass. Some species are more divas than others, but all have a very narrow window of conditions they are willing to tolerate.
Do you know how scientists found out that fish weren’t passing through culverts? The hard way. After decades of data collected on millions of culverts and hundreds of studies on fish swimming and jumping abilities, we have refined our understanding of what makes a culvert “passable” or not by fish. Unfortunately, when we started looking at culverts with a critical eye, we started realizing that many need to be replaced in order to achieve adequate fish passage. Replacing a culvert is no easy feat. It’s expensive, requires a lot of work hours, can be a huge hassle with traffic, and could also endanger fish populations in the stream. Making matters worse, a lot of culverts that need replacing aren’t even that old. The really poorly designed culverts- the ones that dangle feet off the stream bed, or are crumpling- may be a few decades old. But, many culverts that score low on the fish passage test are less than 10 years old. Before we start tearing down was is essentially brand new infrastructure, we better be sure that the end result will be restored fish passage, increased population connectivity, and overall increase to stream health.
That was part of the motivation behind a study that researchers from West Virginia University recently undertook. Simply put, they sought to determine whether culvert restoration will restore brook trout connectivity. Using genetics, they found that before culvert replacement populations below and above two culverts in West Virginia were structurally dissimilar. Otherwise, very few, if any, brook trout were swimming through the culvert and the populations above the culvert were genetically isolated (to read more on why genetic isolation can spell bad things for brook trout populations, click here). After culvert replacement, they found immediate evidence that fish were swimming upstream and that population connectivity had been restored. Success!
But, let’s not go tearing out all the culverts just yet. This was an obvious case where skilled engineers and biologists worked together and installed a culvert that was designed better than the one that was previously in place. But, sometimes it’s not that easy. Sometimes, what should be a great culvert still doesn’t result in great fish passage. And, a culvert that doesn’t seem so great by design is biologically functioning just fine. Biology is oftentimes more than a numbers game, and it’s worth reiterating that there’s no one solution to every problem. That’s what is making the science of culvert design frustrating and at times slow. There’s so many variables, and nature can be so unpredictable.
Further, even if we did know the perfect culvert design for every stream, there is also the question of whether populations really should be reconnected. If the isolated population has great genetic diversity, large size, and is seemingly healthy, then maybe it’s okay to place that stream low on the priority list for culvert replacement. Or, if downstream of an impassable culvert there is a thriving population of a nonnative species, maybe we should start considering whether we want to purposefully prevent fish passage.
That’s right, I said it. Go against everything I, and science, have ever told you and PURPOSEFULLY keep populations isolated. But, that’s a story for the next blog…
*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent. I encourage you to read the manuscript, found here, so you can contribute to the discussion.
That’s one of those fancy words scientist love to throw around to make our sentences more sophisticated. And, for some reason, journals are more likely to publish “freshwater ecosystems are among the most imperiled worldwide”’ than “kiss ‘em goodbye, freshwater streams are tanking.” Beats me.
But, think about that (using whichever sentence you prefer). Freshwater streams are among the most threatened ecosystems on this planet and, on average, freshwater fish are going extinct far faster than marine animals and all the air breathers. That’s no joke. We hype up the polar bears (for good reason….), while in plain sight is an ecosystem that is crashing before us. To put this into perspective, if we consider local extinctions, otherwise known as population extirpation, from climate change alone, the frequency of local extinctions in freshwater ecosystems is about 20% higher than marine or terrestrial environments.
But, let’s play devil’s advocate. A local extinction simply means a species is lost from a specific ecosystem. For example, brook trout might go extinct from a specific stream reach. When a species goes missing, it leaves a hole in the ecosystem that usually gets filled by another species that, in many ways, operates in the same way. The new species will usually eat the same stuff, have the same population sizes, and seem to be a 1:1 replacement. But, is it really the same?
Think about it from a business perspective. If all the burger restaurants leave town, it opens up the market for another burger restaurant to move in. But, not all burgers are the same. Will the new place have the same menu? Will the food taste as good? Will they generate as many jobs as the old?
Will new species function just like their locally extinct predecessors? A study recently found that the answer is probably not. And, a species doesn’t necessarily have to go extinct for the ecosystem to fundamentally change after a new fish species move into down. Invasion fundamentally changes how an ecosystem operates.
Looking across continents, a group of researchers aimed to answer the question “how much does functional diversity change when a species invades a freshwater ecosystem.” You can think of functional diversity as the “menu” of the new burger restaurant. It’s basically how a species operates within an ecosystem- how far does it move, how much does it reproduce, what does it eat, how does it interact with other species, etc. Big changes in functional diversity can mean big changes to other fish species present in a stream, as well as the insect community, plant community, and the flow of nutrients through the environment.
What the study found was that species invasions increased average functional diversity by 150%. If you want to continue with the burger analogy- the menu of the new place is 150% larger. But, bigger is not always better. A pristine ecosystem evolved with a certain amount of functional diversity, and an increase by 150% means that the ecosystem is probably getting stressed in new ways. For example, they found a general pattern that invading species having larger, deeper body shapes. Species with this body patterns tend to live in slow-moving waters, and really excel in life in deep pools and impoundments. If streams and rivers are dominated by those species, and there are fewer fish living in swifter currents, then it could reduce predation on certain insect species which, big picture, will disrupt the food web.
And, 150% just represents the AVERAGE change in functional diversity. They also found that changes were higher than average when the invading species was truly nonnative (like, maybe from a different country, as opposed to from a neighboring watershed), and when the original ecosystem only contained a few species.
So, why bring this study up on a trout blog? I frequently like to imagine what stream ecosystems are going to look like in 200 years. Right now, we are already seeing rapid changes in the species diversity in stream ecosystems. We’ve stocked a lot of nonnative fish, to the point that it is sometimes difficult to know when a species is truly native anymore. Of course we know the history with brown trout. But, smallmouth bass? Nonnative. Channel catfish? Mostly nonnative. Bluegill? Guess what- mostly nonnative. A lot of these species mix with native species in cool and warmwater rivers and, as climate change advances, we continue to see these species creep further into the headwaters in search of cooler water. It’s now not that uncommon to find bluegill and brook trout together. These two species aren’t direct competitors, but how are those invasions going to change the stream ecosystem as a whole?
I guess only time will tell….
*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent. I encourage you to read the manuscript, found here, so you can contribute to the discussion