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Denver health officials investigate mosquito surge along High Line Canal

In south Denver, the High Line Canal is morphing from an outdated piece of water delivery infrastructure to a recreational and environmental asset with the ability to filter contaminants out of stormwater.

The buzz in Denver’s University Hills neighborhood this month, however, has mainly focused on all the mosquitoes leaving aggravating — and potentially dangerous — marks on residents.

Emily Holben Walker, the president of the University Hills Neighborhood Association, lives within a few blocks of the canal where it winds through the suburban part of town.

Aside from rainwater runoff, the 71-mile-long ditch has sat dry this year as owner and operator Denver Water looks at alternative ways to deliver water to the customers that have historically tapped into it. Since city officials completed an infrastructure project last year designed to catch more stormwater for plants and wildlife, Walker has also noticed more potential habitats for mosquito larvae in the canal.

She and her family have a plot in the community garden at the Bradley International School, also within a few blocks of the High Line. Tending it has been a fraught experience at times this summer.

In one night of just going out to water my garden plot, I had, I think I counted 21 bites. And I was out there maybe 12 minutes,” Walker said of the mosquito problem. “I hope that it’s better because they really have been terrible. They do spread disease and we need to keep that under control.”

Denver Department of Public Health and Environment officials visited the High Line Canal and surrounding areas last week and confirmed heightened mosquito and larval activity, department spokeswoman Amber Campbell said. As of Tuesday, city crews were still investigating and applying anti-larval treatments in several places.

As of late last week, the city had received nine complaints about mosquitoes in and around the canal including at James A. Bible Park just east of University Hills. Most of those complaints came before rainstorms soaked the metro area on Aug. 15 and 16, Campbell said.

Water along the High Line Canal Trail on Thursday, August 18, 2022. (Photo by AAron Ontiveroz/The Denver Post)

The city created a mosquito control program specifically to limit the spread of West Nile Virus, the serious and sometimes fatal disease the insects can carry, Campbell said. The health department works with a contractor to monitor known mosquito hotspots and apply larvicide and also sends its own staff members to investigate complaints.

“We also recently investigated a complaint that came from (Denver Parks and Recreation) about portions of High Line Canal near the tennis courts at Bible Park,” Campbell said. “Our staff didn’t find any standing water or active larval mosquito activity.”

Anecdotally, Denver is having a bad mosquito season in many corners of the city. National Weather Service records show that the metro area is having a below-average year for precipitation but regular afternoon monsoons have driven up mosquito complaints and concerns.

“We have actually seen lighter mosquito seasons over the last few years,” Campbell said. “This year, we had a very dry spring, and the summer heat came quickly and stayed hot. Rain doesn’t necessarily mean a more active mosquito season across the board. Sometimes we will see increased activity in some places, but less in others.”

The city health department has not confirmed a human case of West Nile Virus this year, Campbell said. But city records show that 2021 was the worst year for infections since 2016. There were 19 confirmed cases last year after just 15 over the previous four years combined. The last time a Denver resident died of the virus was in 2018, according to health department records.

While the city has its own program, issues on private property fall to the property owners. In the case of the High Line Canal, that’s Denver Water. When the issues in University Hills bubbled over last week, Walker reached out to District 4 City Councilwoman Kendra Black. Black, in turn, reached out to Denver Water. By then the water utility had already gone to work on the bugs.

Using briquets of a bacterium deadly to mosquito larvae but harmless to other living things — the same treatment the city health department uses — Denver Water officials treated parts of the canal near Yale and Holly streets as well as near Bible Park and other parts of the University Hills neighborhood in early August, according to spokesman Todd Hartman..

“The treatment takes effect within 24-48 hours and lasts for 30 days. We will continue to monitor the situation and apply more briquets as needed,” Hartman said Monday.

Some people who live near the canal have called on Denver Water to do more. Specifically, they want the water utility to run water through the canal to wash out standing water and prevent stagnation.

In an email sent to The Denver Post tip line earlier this month, Denver resident Renee Best said it’s been impossible to sit outside in Univerity Hills this summer without being bombarded by mosquitoes and she holds Denver Water responsible.

“I would like to talk to somebody about filing a complaint in order to get the canal running,” Best wrote.

Running water through the canal has not been a “priority” this year, Hartman said. That means customers with more senior water rights have had first dibs on the water that might otherwise have gone down it. The utility didn’t receive any requests for water deliveries from customers that still tap into the more than 130-year-old infrastructure.

Black was not supportive of the idea of releasing millions of gallons of water into a channel known for high rates of seepage and evaporation during an ongoing drought. But she knows the mosquitoes have been a problem in 2022.

“I have lived by the High Line Canal almost my entire life. This is the first year I have ever noticed that we have bad mosquitoes,” she said last week.

A young dog plays in the water along the High Line Canal Trail on Thursday, August 18, 2022. (Photo by AAron Ontiveroz/The Denver Post)

Denver Water has a page on its website dedicated to how the High Line Canal is transforming from just an irrigation ditch to more of a recreational and ecological asset.

The city of Denver has been an active participant.

Last fall the Department of Transportation and Infrastructure completed a $1.38 millionc stormwater project along a stretch of the canal, DOTI spokeswoman Vanessa Lacayo said. The project area was located roughly between Colorado Boulevard and Interstate 25, the stretch of the canal where it runs through the University Hills neighborhood. That project involved installing three small berms meant to collect more stormwater for a slower release over time and two underground “trash vaults” for filtering out debris and refuse as the water enters the canal, according to the project website.

The idea, Lacayo said is to provide more water for plants and wildlife like water-hungry cottonwood trees.

“It’s important that we have that healthy flow of water going to that ecosystem,” Lacayo said. “From what we have seen so far, it is working really well.”

But neighbors wonder if the project could be having an effect on the mosquito population.

Walker said she has noticed more stagnant pools in the canal since the stormwater project was completed. She is grateful to Denver Water for the quick response to neighbors’ concerns this month.

The city’s stormwater project is the most exciting example to date of projects that are transforming the canal into a multifaceted part of the metro area landscape, said Harriet LaMair, executive director of the High Line Canal Conservancy. The conservancy is a nonprofit organization that was formed to work with governments and residents along the canal to shepherd it into its next phase.

LaMair said mosquitoes have been a problem along the canal for all of the 10 years she has been involved. That said, she hasn’t noticed more complaints up and down the 71-mile length of the channel this year than in years past.

Projects like Denver’s are critical for the future of the canal and for a future where governments and people will be asked to do more with less water.

“As years go on, the stormwater structures and the system will become more sophisticated and we may see less standing water but we’re not sure,” LaMair said. “It’s the beginning of a new life for the High Line Canal.”

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