If climate change renders the Western Slope warmer and drier, and if historic growth rates keep up, then Aspen’s water utility could have trouble meeting consumer demand without depleting minimum in-stream flows in Castle and Maroon creeks over the next 50 years.
Aspen City Council on Monday heard a presentation from consultants hired to evaluate the adequacy of the municipal water supply. Wilson Water Group put together a report forecasting demand and available supply over a 50-year outlook, and found that in the worst-case climate change scenario, the city could miss in-stream flow targets on Castle and Maroon creeks by between 4 and 9 cubic feet per second during the “irrigation months” of June through September.
The city has committed to a 13 cfs minimum flow in Castle Creek, and 14 cfs in Maroon. Both creeks are tapped to feed municipal needs through diversion structures that send water to Thomas Reservoir, a holding bay for the city’s treatment plant.
Even if the worst-case scenario projections come to pass in terms of climate change and population growth — demands on the city’s water system historically have risen by about 1.2 percent a year, according to special projects utilities engineer Phil Overeynder — the city has other ways to shore up its water supply.
One project that has been on the drawing board for years would pump treated wastewater uphill from the sanitation plant to irrigate the city’s golf course.
The city also controls three wells in town drawing from the local aquifer. If irrigation for city parks increasingly relied on those wells, then more water could be left in Castle and Maroon creeks.
Combined with more water conservation, or restrictions in drought years, depletion of in-stream flows could be avoided, consultants report.
City council agreed to adopt the 2016 Water Supply Availability Study, and continue monitoring hydrologic conditions.
Council also heard a presentation on Monday from another consultant that analyzed threats to the water supply and water quality. Given that Aspen’s water originates in high mountain valleys, wildfire poses perhaps the most imminent and hazardous threat. A bad fire in the Castle or Maroon watersheds could be detrimental to water quality in those streams, and subsequent mudslides could also cause problems.
There is also the abandoned Pitkin Iron Mine above Ashcroft that drains into Copper Creek, a Castle Creek tributary.
The Colorado Rural Water Association conducted a study for the city assessing the best ways to mitigate these threats.
Creating a buffer zone against wildfire near the diversion structures on Castle and Maroon creeks, while continuing to develop plans to limit wildfire debris flow into Thomas Reservoir, were among the study’s top recommendations.
More work to control erosion at the Pitkin Iron Mine site was also recommended. However, the consultant noted that the Pitkin Iron Mine did not make the list of the state’s 200 most pressing mine cleanup needs.