The Turtle Tunnel

Creating Wildlife Corridors in Washington County

Hannah Peterson

November 20, 2023

Threats to MN Wildlife

In Minnesota, much like the rest of the world, we are seeing a decline in biodiversity. A major threat for many species is habitat loss, degradation, and fragmentation by climate change and human development. The following are impacts on MN wildlife and their habitats:

Temperature change, precipitation level change, and increased natural disaster risk disrupt ecosystem function, simplify habitats, and lead to loss of resilience.
Growing human populations and their demand for natural resources destroys the land and resources wildlife needs to thrive.
Road development drives habitat fragmentation, vehicle collision mortality, barriers to movement, and degraded habitat quality.
Habitat stressors are compounded on long generation times and low reproductive rates of some species that limit population growth.

Threats to MN Turtles

Minnesota is home to nine freshwater turtle species, three of which are Species in Greatest Conservation Need (SGCN) in Minnesota’s wildlife action plan: the smooth softshell turtle, Blanding’s turtle, and the wood turtle. Like many other species, turtle habitats are under threat due to climate change and human development, as well as having other population-limiting characteristics. The following are impacts on MN turtles and their habitats:

As turtles travel seasonally between overwintering and nesting habitats, both for females to lay eggs and baby turtles to seek out permanent homes, they are vulnerable to barriers to movement such as roads, as well as being hit by cars.
Increased flooding frequency, duration, and intensity threatens the viability of nesting sites in flood-prone areas and the survival of turtle hatchlings, and adult wood turtles have been seen to drown.
Turtles also face threat from nest predators as well as humans taking them from the wild.
Turtles often don’t start reproducing until their late teens, and only about one percent of turtle hatchlings will make it to breeding age.

The Turtle Tunnel

While climate change is a complex issue to tackle, protecting habitats and promoting connectivity is a smaller-scale way to address biodiversity decline, especially as it relates to Minnesota’s turtles. One solution is to create crossings, such as a tunnel beneath the road, intended for these turtles and other wildlife to make safe passage.

In 2014, a turtle tunnel was installed in Washington County near Big Marine Lake in May Township. It was planned and constructed in partnership between the Minnesota Department of Natural Resources (MN DNR), Washington Conservation District (WCD), Carnelian Marine St. Croix Watershed District (CMSCWD), the Minnesota Herpetological Society, Washington County, and the University of Minnesota. It was an experimental crossing installed for Herptiles, and more specifically turtles, to help direct them to cross under rather than across the road as they travel from their overwintering habitats in the lake and wetland north of the road to the shallow wetlands south of the road to nest in the spring.

The tunnel, engineered by ACO Wildlife, includes three key components: the fencing, the tunnel entrance unit, and the tunnel itself. The fencing is curved to prevent animals from climbing over but allows them to drop to safety from the road. The entrance is made from recycled plastic, and the tunnel sections are made from freeze-thaw-resistant polymer concrete blocks that have been hollowed out and assembled to fit across a dug out culvert. Light is allowed through slots on top of the blocks, encouraging turtles to enter and cross. The Washington County turtle tunnel saw success through capturing photos of many creatures using it and reduced road mortalities.

Tunnel Photo Gallery

Below are a sampling of photos collected of the turtle tunnel throughout the years. As you can see, it is utilized by turtles as well as many other species of wildlife.

Herptiles

Mammals

Birds

The Tunnel

Photos of tunnel construction and what it looks like now

Future Crossing Opportunities

While new roads should avoid bisecting wetlands or be bridged because the vast majority of turtles can swim under bridges, it is not always possible. In these cases, wildlife crossing structures should be developed. The location of future crossings can be determined from identifying priority areas for a target species, often using data. The data used to determine priority location could include:

Turtle movement tracking
Road mortality incidents and lack thereof (Do not attempt to cross)
Where turtle nesting, foraging, and hibernacula habitats are found
Proximity to water on one versus both sides of the road
The type of water bodies present (wetlands, rivers, lakes)
Traffic conditions
The number of nearby culverts and bridges
Adjacent land usage

As for the type and size of future structures, it is important to reflect on the success of the current turtle tunnel. While it has been seen to create passage for turtles and many other species, it has also been said that turtles would actually prefer a larger structure, as Herptiles have been found to more easily cross through larger structures. If additional tunnels are developed, they should be at least three feet wide and the shortest possible length to be most welcoming to turtles. There is also potential for existing drainage culverts to be retrofitted to meet the needs of turtles. Because barriers and culverts work best in tandem, barriers should also be developed to guide turtles away from the road and towards a crossing culvert. Turtles are more likely to walk along barriers they can see through, so chain link or mesh fencing would be most effective. Once new structures are developed, they must also be maintained to keep debris out of the way and ensure they are functioning as intended.