Toss a Frisbee too far at the disc golf course in west Denver and you’ll hit water burbling at the bottom of Lakewood Gulch.
Your errant Frisbee shouldn’t be underwater. Not this time of year. The other stream that runs into the confluence that makes up the disc golf park has a more historically accurate name: Dry Gulch.
A new study from Colorado State University researchers shows that about 80% of the water running through Denver’s inviting stream parks late in the summer flows there from lawns drenched in Denver tap water and leaks from the agency’s intricate system, not from snow runoff or foothills rain.
By history, geology and hydrology, these greenbelts should be bone dry. It’s only the return flow from all your lawn watering that makes a Denver stream anything other than a dry gulch for much of the year, according to the study. Watering to excess — meaning the grass doesn’t need all of it or the sprinklers are hitting concrete instead of green space — makes up most of that 80%. Leaking pipes and system flushes flowing down into stream beds make up the rest.
“It was surprisingly high,” said lead researcher Aditi Bhaskar, assistant professor at CSU’s Department of Civil and Environmental Engineering. Water engineers know tap water flows have an influence on urban streams, but Bhaskar had not expected them to make up effectively the entire creek.
The potential implications for Colorado’s dicey water future are big and deserve exploration, Bhaskar and her colleagues said. Fast-growing Front Range cities all need to secure larger water supplies, a truism made even more urgent in the 22nd year of what climatologists have identified as a Western mega-drought that cuts deeply into mountain runoff.
Denver is in a fierce battle with Boulder County officials and environmental groups over expanding its Gross Reservoir Dam to hold more water diverted under the Continental Divide. If further research supports that too much drinkable water ends up greening neighborhood space, Denver and other cities could face far more pressure to conserve the existing supply instead of damming new sources.
Until now, a lot of municipal utilities have been focused on replacing high-flow toilets and shower heads, while outdoor water use hasn’t changed much, Bhaskar said.
“But there’s room for improvement,” she said.
Denver Water was not involved in the CSU study and so would not comment on the researchers’ calculations or methods. A Denver Water spokesman did say, though, that the agency’s calculations over time show that leaks from pipes and system flushes make up far more stream water over the course of a year than excessive lawn watering.
Since 2014, Denver Water has cut by one-third the amount of lawn water returning to local streams, even as the population has grown by the tens of thousands, the utility said.
If Front Range city residents want to know what their local stream would look like without the water from pipe leaks and excess lawn water, they could stroll backward in time along the much-drier creeks at Rocky Flats National Wildlife Refuge. Bhaskar said CSU researchers used those streambeds, to the west of developed, irrigated neighborhoods, as a sort of control to their Denver measurements.
Similar elevation, similar natural precipitation to the cities, Bhaskar noted, and yet “those grassland streams are dry 40% or more of the summer.”
Graduate students took bottle samples in 2019 from various Denver commercial and residential taps, and also in stream beds including Harvard Gulch and Lakewood Gulch. Water running downhill from high in the mountains, where Denver gets its tap water, comes with different natural, traceable isotopes than local rainwater. The isotopes remain in Denver water after it is treated for drinking, allowing researchers to measure where water in a given stream originated.
Western cities with even more immediate water shortages than Colorado are more aggressive getting rid of lawn grass to save water. Las Vegas pays lucrative amounts to homeowners to rip out grass, with ongoing payments for low-water landscapes.
Though it’s less generous, Aurora Water has a similar program reimbursing homeowners for low-water landscape design and construction. Denver has said such cash-for-grass programs wouldn’t save enough in the city to be worth it compared to other conservation measures.
A city like Denver could save much water by requiring or advising more landowners to stop overwatering, or redirecting sprinklers, Bhaskar said. And since much of Denver’s water is moved from the Colorado River Basin under the divide to the South Platte River Basin, Denver has the right to reuse that portion of water “to extinction,” meaning it can recycle it from streambeds into other uses.
Denver Water said it has indeed asked state engineers to formalize its right to some of the return flows from lawn irrigation, and uses them for exchanges or recycled water opportunities.
But as with everything involving water in the West, the best solution may not be so obvious, Bhaskar added. Neighbors and park-goers are used to year-round green spaces. Communities orient and improve themselves around water recreation opportunities from standup paddling to kayaking to fishing. In west Denver, they build disc golf courses alongside water features.
But in the future, people might visit a once-flowing stream in their local park and discover a different look. “If people are irrigating more efficiently,” Bhaskar said, “it might be dry for a good part of the year.”
This story was updated at 6:44 p.m. on Aug. 24, 2021, to add links to the water study and other relevant stories.