American Rivers and Trout Unlimited have joined the chorus of opposition against the city’s application to extend its conditional water rights to build dams on Castle and Maroon creeks.

The two lobbyist organizations announced Wednesday they entered a statement of opposition in Colorado Water Court, where the city made its filing Oct. 31.

The city secured its conditional water rights to build reservoirs on Maroon and Castle creeks in 1965. It is required to renew those rights every six years.

Castle and Maroon creeks supply Aspen’s drinking water. The city has maintained it must preserve the rights to build the dams in order to be prepared for population growth and climate-induced droughts depleting the water supply. The City Council approved the extension application Oct. 5. The proposed dams would be 150 feet tall on Maroon Creek and 175 feet tall on Castle Creek.

The statement from American Rivers and Trout Unlimited emphasized that the conditional water rights the city wants to renew hinge on damming the two streams.

“Conditional is the crucial word here,” the statement said. “According to the Colorado Standards for Due Diligence and Colorado Water Law, the City of Aspen can only possess these rights on the condition they develop the dams. That is what the water right was granted for in 1965. If the city does not renew these rights they simply vanish. No one else can claim these water rights.”

Filing an opposition allows American Rivers and Trout Unlimited to participate in water court proceedings regarding the city’s application. Pitkin County and Carbondale-based conservation group Wilderness Workshop also have filed statements of opposition, which are due no later than Saturday.

“We hope that Aspen will take this opportunity to work with stakeholders on better solutions for its water future,” said Dave Nickum, the executive director of Colorado Trout Unlimited, in a statement. “Building dams on free-flowing streams in one of Colorado’s most iconic wilderness areas is the last approach we should be taking to meet water needs in the 21st century. It is time to look forward toward new strategies, instead of relying on flawed ideas from the past.”

Matt Rice, director of the Colorado River Basin Program for American Rivers, said: “Aspen does not need these dams for municipal water supply, climate resiliency or for stream protection — now or at any time in the foreseeable future. Why not come up with a solution that does not involve dams? If the city were to determine sometime in the future that new storage is needed, reservoirs that flood wilderness on Castle and Maroon creeks would be the last place in the valley they would consider. We believe the best time to get bad projects off the books is as early as possible and this diligence filing is that opportunity for the city to do so.”

The city has countered that it is simply preserving its rights in the event of a worst-case scenario that severely impacts Aspen’s water supply.

“The science confirms that Aspen’s climate is already changing and will continue to do so,” Ashley Perl, the city’s climate-action manager, said in a city-issued statement released Dec. 20 in advance of the county commissioners’ decision to file opposition. “Aspen now sees 23 days less of winter than in the years before 1980. This trend is projected to continue and Aspen’s current water storage — our snowpack — will diminish.”