fish | Crossroads

Roadways for humans can sometimes create roadblocks for fish, but researchers hope to establish a set of culvert design practices to help aquatic creatures get where they’re going.

Many fish depend on mobility along a river for feeding and spawning. Where roads meet rivers, however, culverts can block fish and other aquatic organisms that can’t navigate changes in current, lighting and other factors.

Waterway barriers threaten an already endangered species of minnow known as the Topeka shiner (pictured above). It can also be a big problem for economically important fish such as trout or northern pike. That’s why the Minnesota Department of Natural Resources prefers building bridges to culverts.

However, bridges are not always economically feasible, and so MnDOT is working closely with the DNR to develop culverts that protect both public safety and the environment.

Photo of boxed culvert — Culverts allow water to pass under roads. Occasionally, they can harm a stream’s fish habitat by inadvertently acting as a barrier to fish passage or migration. There are nearly 11,000 culverts in Minnesota.

Sediment Content

Recent research suggests that installing boxed culverts differently could greatly improve fish passage.

Culverts are typically placed a little below the streambed with the expectation that the stream flow will naturally fill them with sediment. Researchers tested that assumption and found it to not always be accurate.

“We found that pre-filling the culvert with sediment that replicates the streambed as part of the installation process helped prevent upstream erosion and the development of vertical drops that can become barriers to aquatic movement,” said Jessica Kozarek, a University of Minnesota research associate. “In addition, pre-filling the culvert helped ensure the sediment remained inside the culvert flows were high and water moved quickly during rainstorms.”

MnDOT has been working with the DNR to identify the conditions that determine whether a newly installed culvert will naturally fill with sediment, replicating surrounding streambed conditions, or whether a stream’s water flow will transport sediment out of a culvert.

Using an experimental flume at the University of Minnesota’s St. Anthony Falls Laboratory, researchers tested MnDOT’s standard box culvert design under a variety of stream conditions.

Laboratory simulations suggest that filling a culvert with sediment at installation, rather than allowing it to fill over time is, with some exceptions, generally the best approach for low- and moderate-grade streams. Additionally, steep, fast-moving waters require a filled culvert with structures such as larger rocks to keep sediment in place. These structures also create steps, pools and riffles that enable fish to rest as they move upstream.

MnDOT will use this latest research, along with conclusions from other recent studies, to create a guide for fish-friendly culvert designs.

“Of all the things we’ve studied, there are maybe three or four research projects. This manual will pull it all together,” said Petra DeWall, state waterway engineer at the Minnesota Department of Transportation.

Further research is underway to determine whether aquatic organisms are deterred by low light conditions in long, dark culverts. Researchers are also looking into whether mussel spat rope could be used to create a rough bottom to reduce water speed in culverts with no sediment.