The High Line Canal reinvents an irrigation canal that shows off the region’s diversity.

By Haniya Rae

Stretching 71 miles from south of Denver into Aurora, Colorado, the High Line Canal is a constructed feat of the late 19th century. Originally hand-dug to supply irrigation to local farmers, the canal is now in the midst of transformation from a historical relic to a burgeoning greenway.

Plans for the High Line Canal’s transformation, with input from Sasaki, Agency Landscape + Planning, and Livable Cities Studio, call for clearly designating five zones of the canal based on their ecology while also linking the zones with a unified design and wayfinding system.

The plan also stresses the need for accessibility and basic amenities so that all communities along the canal can enjoy it. A newly formed nonprofit, the High Line Canal Conservancy, will oversee the implementation of the plan and promote the benefits for all who live near the canal.

“The canal is natural, connected, and continuous, and it’s not one thing from beginning to end,” says Gina Ford, FASLA, the principal landscape architect at Agency Landscape + Planning. “It’s not a system that was made for people. The High Line Canal Conservancy needs to do a lot of work to adapt it for people. And that’s a lot of what I think really came in the vision and framework plans.”

The canal meanders in and out of 11 jurisdictions, each of which may have slightly different experiences of the canal. Many areas of the canal sit in stark contrast to one another for a variety of reasons, some of which are natural barriers and others socioeconomic ones.

To understand the broader needs for the canal as a whole, a series of in-person open houses helped capture themes that could be applied anywhere along the canal. Each open house allowed attendees to select from an array of visual representations, mostly other well-known parks and facilities, to communicate what they wanted their canal experience to be like.

“Do people see this as an active space, or is it a quiet place for bird-watching?” asks Jill Dixon, an associate principal and planner at Sasaki, who helped guide the High Line Canal Vision Plan. “What’s the aspiration for each section? People wanted it to be connected and natural and to celebrate what it already offered. We wanted to build on the special character zones that it already had.”

This in turn helps create a sense of identity for each district, making it easier for design suggestions. With an identity, there can be playful building around both urban frameworks and the natural ones, Ford says.

Overwhelmingly, residents asked for the canal to remain natural, to be connected across jurisdictions, to maintain the character and integrity of each section, and for organizations to manage and enhance the canal for a more resilient future.

Not all of the communities will enact projects at the same time, nor do all communities need the same approach. A series of ruler drawings—mile-by-mile graphic representations of assets along the canal—shows trails, tree cover, and amenities. Areas with higher poverty and higher population density in the northernmost part of the canal also face a drier climate, fewer amenities, and less tree cover, but residents don’t necessarily want the same approach found in the lusher suburbs to the south. Some of their requests included a community gathering place and better bridges and walkways.

“When I came into this project as a Bostonian who’s worked on a lot of urban waterfronts, my mindset was, it’s going to be about improvements to activate and make this a great urban public space,” Ford says. “And it was so hard to shift out of that mindset and say, actually, this is small and strategic and that’s part of it—not alienating people.” Ford relays that, like a scalpel, small changes over time could keep the community engaged with the project as opposed to large-scale, quickly implemented solutions that leave little room for comment or feedback.

Following months of public meetings to gain feedback about the responses to the canal, many communities expressed a strong desire for safer crossings, more accessibility, and consistent wayfinding, especially since sometimes the canal loops through heavily trafficked roads. “A lot of those major crossings are where two jurisdictions come together,” says Meredith Wenskoski, ASLA, the president of Livable Cities Studio in Denver. “It came down to really figuring out how to help people cross at those locations so they can experience the canal continuously.”

For the busiest roads and some gaps in the trail, residents preferred underpasses for a safer way to cross. Two major underpasses have been completed along the canal so far, and another four are scheduled to be built within the Denver Metropolitan Area in the next few years.

For less busy areas, or developing areas, creating a visual connection and clear guidance on how to proceed is paramount for connecting the trail sections.

“It’s easy to miss the trail if you’re not familiar with the terrain,” Ford says. “The lushest parts seamlessly integrate with Denver’s southern suburbs. The driest areas on the northern end of the canal may have been filled in where water was no longer needed.”

Linking the various parts of the canal with trailheads, mile markers, and updated graphics and maps, all with the same design identity, will aid trail-goers as they progress. At some gaps, such as the beginning of the canal near Platte Canyon Reservoir, improved and reorganized parking, interpretive signage, and shaded terraces will make the canal trail entrance more inviting and comfortable. Other gaps that are not as easily remedied by new signage may require making new trails, improving trail surfaces, building bridges, and connecting existing trail systems to offer an alternative path.

Modern-day Denver doesn’t depend on irrigation in the same way that settler farmers did. But the city is dry; the typically arid mountainous area doesn’t get more than 12 to 15 inches of rainfall a year. One of the most ambitious projects of the High Line Canal is to convert the dam to an effective stormwater management system.

The canal was built as a natural drain, running perpendicular to a number of creeks and gulches, catching the watershed as it flowed from the foothills to the plains. Today, roughly 20 percent of the watershed drains directly into the canal, but more must be done to divert stormwater into the canal to make it a more robust system.

“There’s a lot of infrastructure in place to help the stormwater pass through, but that same infrastructure could be used to just stop it and hold it for a little bit,” Ford says. “When we got to the point where we explained how to catch stormwater and the community started seeing it, it was a game changer. It was scary for us because we thought that the public would think of it as sewage.”

In some areas, water-quality berms and constructed overflows will redirect runoff from roads directly into the canal. Forebays with sumps will be installed at the end of inflow points to the canal and will collect and store trash, debris, and sediment until maintenance crews can remove it, allowing the water to flow through to the canal. And control structures encased within an earthen berm can be placed between different stretches of the canal to aid with water intake and quality control before being released downstream after holding for up to 72 hours.

Because the stormwater is held in place rather than flowing into an underground drain, it leaves a wet canal bottom for an additional 100 days a year. The change in the hydrology will also change the ecology.

“We’re creating by stewarding,” Wenskoski says. “We’re creating an ecological link between all of these systems by doing this.”

At Mamie D. Eisenhower Park in Denver, a one-mile span of the canal makes up one of the first stormwater control projects. The site was chosen as a case study because of its strong focus on water-quality improvements. Three water-quality berms are currently being built into two of the four reaches of the canal along with two water-quality control structures, which will soon allow the city to track the effects of stormwater retention across economic, social, and ecological boundaries. Currently, an expectation for the canal is that longer holding periods of water will result in a denser tree canopy, as water-loving species start to grow. A more robust tree canopy could reduce the heat island effect and allow for more water retention. But a number of other species will be strongly affected by the change.

To better understand how the altered hydrology will influence the site, Christina Alba, an assistant research scientist and ecologist with the Denver Botanic Gardens, took soil cores to assess the quality of the soil. She also documented which plants currently grow along the canal in the park and established a seed bank to determine what seeds have flowed from upstream. Perennial grasses, such as smooth brome, reed canary grass, quack grass, and orchard grass, make up more than 50 percent of the current cover at the location. All except for orchard grass are rhizomatous, which may change soil porosity and density, and possibly allow for better stormwater retention over a long period.

“Whether things are long-lived or short-lived really matters,” Alba says. “Grasses and shrubs provide cover and stability. If you have an area that’s totally dominated by annual grasses, you need vegetative cover to anchor the soil.”

Alba is also concerned with the lack of wetland-loving species present at the location. “If most of the community that’s present there right now is upland, those species might fall out,” she says. “We might lose some of the upland species. We’re looking at what might replace them.” Luckily, much of the Denver metropolitan area’s native plant population is more resilient to changes in hydrology than ecologists first thought, and may in turn help the stormwater reduction plan and treatment even further.

“One of the more interesting results that I wasn’t totally expecting is that there was a clear functional difference between introduced and native species. In the native species pool, about 70 percent of the native species are mesic; they do well in moist locations,” Alba says. “Only 24 percent of the nonnative species were mesic, or wetland. The native potentially have a better chance with a change in the hydrology than the nonnative species.”

As the retained stormwater changes the vegetation along the canal trail, supplemental plantings will help increase tree cover and create a refuge for wildlife. The canal’s built environment will help sustain this natural environment, and create a space for residents to connect and relax. At every stage, the community will be encouraged to give more input to help shape future improvements.

“The first public meeting was really incredible because the whole thing was described as writing a story,” Ford says. “It’s this idea that the community has a role to say where it’s going to go.”

Haniya Rae is a freelance design writer in Brooklyn, New York.

CORRECTION: The fifth image in this post was incorrectly credited to Agency Landscape + Planning. The image should instead be credited to Livable Cities Studio. This error has been corrected.