The Chesapeake Bay watershed contains thousands of miles of freshwater streams and rivers. These waterways support a diverse range of aquatic wildlife, such as fish, salamanders, turtles and freshwater mussels.
Land use changes and development affect habitat and migration for aquatic wildlife. Of particular concern are barriers associated with roadways.
Fragmentation of aquatic habitats by dams, culverts and other infrastructure is a primary threat to aquatic wildlife. There is a need to improve aquatic connectivity. Dam removals and fish passage projects open miles of rivers to many migratory fish species. But smaller barriers created by road crossings also must be addressed.
When not properly designed, the culverts (usually large steel or concrete pipes) that allow a stream to flow under a road can obstruct the movement of fish, fragment populations of other aquatic organisms and degrade water and habitat by changing natural water flow and depositing sediment.
But stream continuity — the uninterrupted connection of a river network — is not always the primary consideration when road-stream crossings are designed. The priorities tend to be public safety, durability, moving floodwater and, of course, financial considerations. Until recently, the safe passage of fish and other aquatic life was of secondary concern — or of no concern at all. But they aren’t mutually exclusive interests; in most cases, a properly built stream culvert can be as nature friendly as it is practical for us humans.
Benefits to wildlife
Stream continuity is important for the reproduction of migratory and resident fish species. Spawning runs can range from short distances for resident fish to hundreds of miles for migratory fish. Stream continuity is especially crucial for striped bass, hickory shad, American shad, blueback herring, alewife, white perch, yellow perch and American eels.
Many fish species, such as brook trout, rely on cold-water habitats for refugeduring warmer months. These cold-water habitats include groundwater-fed headwater streams that maintain cooler temperatures during the summer, as well as deeper pool habitats found along cool and coldwater streams. Access to cold-water habitat is crucial for the survival and maintenance of these aquatic communities.
Varied stream habitats have different prey and feeding opportunities depending on the location and time of year. For example, large predators, such as striped bass, will often travel to prey on schools of baitfish during certain seasons or times of day. The types of macroinvertebrates also vary greatly along the stream network, providing different feeding opportunities, depending on location. The fragmentation of streams by road crossings can reduce access to feeding areas, impacting fish communities.
In addition to fish, other aquatic, semi-aquatic and even terrestrial wildlife rely on stream corridors. Aquatic and semi-aquatic salamanders, frogs and turtles use streams and streambanks for daily and seasonal movement. A barrier at a road-stream crossing may force these species to move over land and across roadways, exposing them to predators and vehicles.
Unlike reptiles and amphibians, which move freely on their own, freshwater mussels require a host fish for dispersal. Mussels reproduce by releasing larvae, or glochidia, into the water. The glochidia attach to the fins or gills of host fish and later detach to colonize new parts of a waterway. A stream crossing that blocks fish movements may also block the upstream dispersal of freshwater mussels.
Healthy populations of fish and other aquatic wildliferequire the dispersal of individuals to maintain genetic diversity. Road-stream blockages can isolate populations, leading to populations being eliminated, reduced or damaged by inbreeding. Maintaining genetic diversity helps wildlife adapt to changing environments.
Common problems at road-stream crossings
A so-called perched crossing — where the floor of the culvert is higher than typical water level on the downstream end — can be an impassable vertical barrier for aquatic fauna that are trying to get upstream but cannot climb what is essentially a small waterfall. Low water on the downstream end can make the situation worse.
Culverts that are simply not deep enough, relative to the stream bed, can also make passage difficult. During high streamflow, the water depth in the culvert may be sufficient, but not so in lower-water conditions.
Uniform-surface concrete or metal culverts provide no hiding or resting areas for aquatic organisms and are not ideal for those that travel along the streambed. Natural substrate, including rocks and finer sediments, should match substrate characteristics of the surrounding stream.
If a culvert is too small in diameter, high flows can increase the velocity of the water passing through, making the upstream passage that much more difficult.
Undersized culverts can also be prone to clogging by woody debris, leaves and trash. Debris jams at the upstream end of crossings can inhibit passage and often make costly maintenance necessary to avoid flooding or structure failure.
What makes a good road-stream crossing?
Here are the main characteristics of a well-designed stream crossing:
- Large enough for high water flows
- Open-bottom design that retains the natural stream bed
- Wide enough to reach from one stream bank to the other
- Water depth and velocity comparable to upstream and downstream conditions
For information about stream connectivity and improving road-stream crossings, visit the Chesapeake Bay Program website, Chesapeakebay.net, and search for “recommendations for aquatic organism passage.” That will take you to a page where you can download a document recently released by the Bay Program’s Fish Passage Workgroup.