Culvert Upgrades

Besides allowing us to drive over streams and rivers, why are culverts such an important part of watershed stewardship? Culverts are a vital facet of transportation infrastructure, play critical roles (both positive and negative) in fish passage, water movement, and affect many aspects of overall watershed health. Poorly designed culverts are passage barriers preventing fish and other aquatic organisms from moving freely through their habitat. Seasonal and heavy rainfall event flooding washes out many culverts across our region every year. With the emerging challenges of climate change and associated increase in flooding events and stream temperature warming, properly functioning culverts will play an increasingly important role in our watersheds’ ecology and the lives of the people living here.

Our Projects

Since 2006, the Superior Rivers Watershed Association’s Culvert Program has completed over 20 culvert upgrade projects, reconnecting roughly 40 miles of stream habitat through improved fish passage, erosion reduction, and improved flood resilience. These projects save local townships hundreds of thousands of dollars in recurring road maintenance costs. None of this progress would be possible without our collaboration with townships, counties, and agencies. Many of our partners now are engaged in ambitious culvert upgrade programs. We are proud to collaborate with them in these efforts, while keeping our membership and the community informed about projects, and spreading the message about the importance of culverts to watershed health. As local county and Forest Service partners continue to make progress in improving stream crossing passage and flood resilience, the challenge remains large-recent culvert inventory work by several partner agencies indicates that as many of 60% of culverts in the southern Lake Superior basin may still present passage barriers of some kind.

The Role of Culverts

Culvert flooding and washouts, erosion, and aquatic organism passage issues have plagued the southern Lake Superior basin since the road network was developed. Large-scale removal of forest cover during the early 20^th Century and subsequent conversion of the land for agriculture resulted in rapid acceleration of runoff across clay-dominated soils. The erosion that resulted significantly reduced the natural capacity of wetlands and stream floodplains to slow and store runoff, leading to a cycle of flooding, excessive sediment transport, road damage, and costly repairs.

Culverts in Floods and Infrastructure

Sizing culverts to handle episodic high flows is expensive and can be hard to justify for cash-short local governments. Historically, state and federal government disaster “replacement in kind” aid programs have failed to fund needed design and capacity upgrades that would break the cycle of washout culvert repairs.

However, emerging regional climate change impact trends with more frequent 500-1,000 year flood events underscore the importance of culverts. We now urgently need to design flood resilience into our road infrastructure and land use practices, and must use greater consideration of the restorable flood control potential of natural infrastructure such as wetlands and stream floodplains.

Culverts and Fish Passage

Aquatic organism passage is a key factor in sustaining biodiversity in the Lake Superior basin. The term “aquatic organisms” often solicits images of walleye, salmon, brook trout, and other fish species, but also includes any organism living in the water, including species such as the macroinvertebrates that are an important food source for fish.

The economic strain of replacing culverts often leads to installation of undersized or improperly designed culverts. These culverts can block aquatic organism passage by creating insurmountable jump barriers (perched culverts) or velocity barriers where constricted stream flow is too fast for smaller fish and other organisms to swim against. Culverts that are too large may not provide sufficient water depth for passage, especially during low-flow periods.

Passage barriers have closed off entire historic habitats for fish that require access to upstream cold water habitat for spawning, such as brook trout. This has resulted in genetic isolation of some populations.

The impact of passage barriers extends beyond brook trout. The Lake Superior Basin contains a diverse assemblage of fish that are impacted by passage barriers or culvert sedimentation, including lake sturgeon, walleye, yellow perch and many more. Many of these are listed as “species of management concern” by the US Fish and Wildlife Service, Wisconsin Department of Natural Resources, and the Great Lakes Indian Fish and Wildlife Commission. A US Fish and Wildlife Service predictive modeling study has indicated that a significant number of lower stream reaches in the basin will experience warming beyond the tolerable range for cold water species in less than 50 years. In that scenario, access to cold, groundwater-fed upper-watershed reaches will become increasingly important, serving as refuges for brook trout and other cold water-dependent species.

Culvert Inventories

In 2008, we published a Culvert Program Strategic Plan using a preliminary needs assessment from our culvert inventory data.