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Denver Water makes way for toads

Amphibian-friendly culvert upgrades mean safe passage for small species of concern.

A nearly completed amphibian passage shown before the road is backfilled and regraded atop the structure. The more natural streambed along the bottom of the passage makes movement easier for aquatic species. Photo credit: Denver Water.


Why did the boreal toad cross the road?

Because it didn’t have a tunnel providing safe passage underneath.

But it does now.

Denver Water, as part of an expansive environmental agreement with the Arapaho-Roosevelt National Forest, recently completed its first “toad tunnel” under Vasquez Creek Road just outside Winter Park.

The passage replaces an old, undersized culvert and allows aquatic organisms — in this case beleaguered boreal toads — to more easily pass from one side of the road to the other. The tunnel most directly benefits a known breeding population of the toads that live just uphill from the area and reduces the chances of vehicle-toad encounters.

The amphibian underpass, designed by Denver Water engineers in conjunction with U.S. Forest Service biologists, was designed to closely mimic the natural grade and structure of the streambed.

This arched design includes natural rock and streambed on the bottom and an easier route for the toads via “toad benches” on both sides of the stream as it passes under the road. In addition, the passages are wider than normal and can accommodate flood events.

“As opposed to a traditional culvert, having a bottom with rocks slows the velocity of water, and the incorporation of benches gives the toads room to move,” said Jason Marks, an environmental planner for Denver Water. “Otherwise, for them, those culverts can become the world’s fastest waterslide, forcing them to use the road.”

These boreal toad borings are called “aquatic organism passages,” or AOPs. And they are increasingly common in Colorado and beyond.

Many aquatic species, including fish, amphibians and macroinvertebrates, can benefit if the route under the road is more like a typical stream, with flowing water that behaves as it would outside of a human-made culvert.

Coloradans may be more familiar with another, more visible, kind of animal road crossing — the crossings that funnel bigger wildlife over or under highways.

Mule deer move across a wildlife bridge over State Highway 9 south of Kremmling in north-central Colorado. Such bridges have significantly reduced collisions between wildlife and vehicles.


Those earthen-looking crossings along State Highway 9 between Silverthorne and Kremmling have significantly reduced collisions between cars and large wildlife such as deer and elk.

“Over the past five years, the average number of animal carcasses found after a vehicle (accident) dropped from 56 to six annually,” according to a story this summer in the Summit Daily.

Aquatic passages, often less visible as they are under the road, are replacing traditional culverts at a steady clip. The Forest Service and partner organizations upgraded more than 1,000 crossings with AOPs across the country between 2008 and 2015.

The toad tunnel near Winter Park may give a lift to a species in a tough spot.

Colorado Parks and Wildlife describes a “dramatic” decline in boreal toad populations in recent decades, blamed mainly on a fungus and perhaps exacerbated by habitat loss. The toad is considered a “species of special concern” by state and federal biologists.

Boreal toads, a "species of special concern" in Colorado, are expected to benefit from new, safer underpasses that make it easier for them to move beneath roadways. Photo credit: U.S. Geological Survey.


Field trials involving bathing young toads in a bacterial elixir, nicknamed "Purple Rain," a potion that may protect them from the fungus, put the toads in the spotlight earlier this year, with a burst of media attention on such efforts.

The AOPs could help, too.

Rick Henderson of the Forest Service told Denver Water that monitoring studies of AOPs have “generally validated their effectiveness,” in providing safe passage for various aquatic species.

Grand County recently installed a large AOP under County Road 3 near Williams Fork Reservoir. The previous culvert was in disrepair and the new structure dovetailed with other river restoration and habitat improvement work conducted by Denver Water and other partners in the same stretch of the Williams Fork River.

In October 2019, Denver Water crews installed the first of two AOPs on Vasquez Creek Road near Winter Park. A second will be installed on the road in 2020, along with a third near Denver Water’s Cabin Creek diversion structure above Fraser.

Crews with Denver Water and the U.S. Forest Service meet prior to removal of a traditional culvert that will be replaced with an "aquatic organism passage." Traditional culverts are harder for aquatic species to navigate, particularly during high flows. Photo credit: Denver Water.


The work is part of detailed agreement between Denver Water and the Forest Service to offset impacts to National Forest Service land on the West Slope from the planned expansion of Gross Reservoir southwest of Boulder.

The agreement extends to areas of the West Slope, where Denver Water operates facilities in the Williams Fork, Fraser and Blue river basins. Other elements of the agreement include improvements to fisheries and stream habitat as well as operational changes to improve stream flows at certain times.

Information about Denver Water’s environmental projects, such as recent work to restore aquatic habitat for fish and anglers, is available on TAP.