Forget Texas. From a minnow-eye view, everything really is bigger, and that can make even small, seemingly inconsequential obstacles in a stream nearly impossible to bypass.
To minnows, shiners, dace and other freshwater fish topping out at just a few inches long, a minuscule increase in flow rate or a sudden elevation hike of a foot — or even a few inches — could end their journey to more resource-rich waters or prospective mates.
Scientists at the Tennessee Aquarium Conservation Institute devote themselves to the study and protection of the incredibly diverse aquatic ecosystems of the Southeast. In pursuit of that goal, they devote almost as much attention to measuring the health and navigability of waterways as the animals that call them home.
Earlier this year, scientists ventured north from Chattanooga into the chill damp of a gray winter morning to wind their way up and onto Walden’s Ridge. Donning chest-high waders and high-visibility vests, they descended a brush-tangled bank into a shallow tributary of Soddy Creek.
This time, however, the target of their expedition wasn’t a colorful fish, an imperiled mussel or even a wayward amphibian.
It was a culvert.
A common feature of highways, streets and country lanes, these manmade structures are literally beneath most drivers’ notice. When properly installed, culverts enable the unimpeded passage of water under roads and do their job without calling attention to themselves.
But all culverts aren’t created equally. Any number of factors could turn an improperly installed culvert into an insurmountable blockade that fragments a fish’s habitat by restricting its movement upstream.
“Normally, fish passage isn’t on the minds of people installing them, but as aquatic scientists, that’s important to us,” says Recovery Biologist Shawna Fix. “If you have a culvert upstream that doesn’t allow for fish passage and another 100 meters downstream that doesn’t allow for fish passage, then those fish are stuck in just a 100-meter stretch of water.
“That’s really bad for their genetics and bad for their health. They can’t find food that way. Fish need to have free access to the streams that they live in.”
Fig. 1 While evaluating how well a culvert accommodates fish passage, scientists make a wide range of measurements of the structure, itself (left), and the waterway (right). Combined, these metrics tell biologists whether the culvert can function without fragmenting a fish's natural habitat.
On Walden’s Ridge, Fix and her fellow biologists are surveying and evaluating culverts to determine the ones that are functioning as intended and those that may need to be replaced to ensure native fish can freely move throughout their range to feed and spawn.
By taking measurements and making comparisons between the passage’s structure and the waterway, scientists can determine a culvert’s suitability. Some of the items on the scientists’ checklist include:
- Is the culvert “bottomless,” or does the water flow through and over a pipe or other manmade material?
- Is the culvert properly aligned with the natural path of the stream?
- Is the culvert at the level of the stream or is it “perched” over the waterway, creating a height difference with downstream stretches?
- Is the culvert wide enough to accommodate the stream’s maximum volume, or does it form a bottleneck that increases the water’s flow rate?
Fix and her team work spend about two hours surveying a pair of culverts on Walden’s Ridge. The first is deemed non-disruptive for fish passage. The second, however, has a number of issues — a perched exit, misalignment with the stream and a corrugated pipe bottom — that will negatively impact fish movement, lead to bank erosion and potentially shorten the structure’s lifespan.
“Well-installed culverts are important, because you want longevity in a culvert,” Fix says. “You don’t want to have to come and install a new culvert every five years because it was too small or installed incorrectly.”
Fig. 2 Some signs a culvert could negatively impact fish navigation include: a bottom made from a man-made material, such as corrugated metal piping (top left), being skewed out of alignment with the stream's natural orientation (top right) and being "perched" above downstream sections (bottom).
The second, problematic culvert will be added to a list of candidates for replacement. Fix and her team will reach out to land owners and local government representatives to help with addressing these less-than-ideal culverts.
Replacing a culvert, especially with the end goal of facilitating fish passage, can be expensive, but scientists seek funding from grants and other sources to blunt the cost of correcting the issue. Once financing is secured and permission from land owners granted, engineers are brought in to evaluate the culvert and draft plans to tackle a replacement.
Installing a new culvert is typically not a long-term project, with less-complicated replacements requiring a couple of days and more-complicated projects wrapping up in a week or two. Despite the temporary inconvenience of this work, ensuring these structures don’t unintentionally imperil native aquatic wildlife is an issue worth addressing.
“I joke that this is one of the least-glamorous things I get to do,” Fix laughs. “Usually, I’m in these beautiful streams collecting beautiful fish, but this work is still very important.
“We can monitor populations all we want, but until we put boots on the ground and put in an effort to enact change, these species aren’t going to benefit. Their populations won’t improve or become healthier until we fix the mistakes we’ve made by putting in these structures that fragment their habitat.”