Last summer's Fourth of July parade in the resort town of Aspen, Colo., was apple-pie middle America. There were Rotarians and librarians, prancing horses and dirt bikers. The mayor passed out flags. Cheers erupted as veterans passed, their signs like bookmarks in American history from World War II to Afghanistan.

Then came some unusual floats: "Aspen hydro fish-cally irresponsible," said one sign. Another: "When we divert, the stream gets hurt." Finally, on a convertible: "Protecting Nature is Patriotic."

Inside, draped in an American flag, sat Connie Harvey. The matriarch of Aspen's environmental community wore hiking shorts and a visor snuggled into her whitening hair. Since settling in Aspen in 1958, she's fought against miners, loggers, even the local ski company on behalf of wilderness and rivers -- often with local politicians on her side.

But those allies have become adversaries. In a move that splintered Aspen's enthusiastic environmental community, city officials began developing a small hydroelectric plant that would partially dewater short sections of two popular mountain streams next to town, Maroon and Castle creeks, one of which flows by Harvey's home. Harvey calls it a "devastating attack" that threatens a "thriving complex of aquatic and terrestrial life." Her views are shared by American Rivers, state and local chapters of Trout Unlimited, and a group called "Saving our Streams" made up of Castle Creek property owners.

The city holds that environmental impacts would be minimal, and that the 1.175-megawatt Castle Creek Energy Center is key to generating more energy locally from clean sources. The city utility, which delivers half of Aspen's electricity, will soon be 89 percent carbon-free, thanks to the imminent addition of turbines to an existing Colorado dam. Castle Creek would bring it nearly to its goal of 100 percent carbon-free by 2015.

In November, after a barrage of advertising, some of it from unidentified Castle Creek hydro opponents, Aspenites narrowly voted to abandon the project. The vote was advisory -- not legally binding -- but for now, hydro is stalled as the city considers its options.

It was supposed to be an easy part of Aspen's Canary Initiative, which laid out steps to improve energy efficiency and boost renewable energy to reduce local carbon emissions 30 percent by 2020 as compared to 2004 levels, and 80 percent by 2050. Aspen is vulnerable to climate change because its four ski areas depend on long, snowy winters; the resorts also give the city a pulpit. "We think that, while we can't turn around the situation globally, we can deliver a powerful message to an influential group of visitors," says Mayor Mick Ireland.

But as the fight over even this small hydro project shows, such ambitions are proving difficult to execute here and elsewhere as communities grapple with competing environmental values, institutional barriers, financial limits and general resistance to change. Aspen was supposed to have reduced its total greenhouse gas emissions 11 percent by 2011, says Canary Initiative director Lauren McDonell, but it's only about halfway there. And "it's getting increasingly challenging to move the needle."

Aspen was the first town west of the Mississippi with hydroelectric streetlights. The controversial new proposal would use the same footprint of existing reservoirs, dams and pipeline routes from a small plant on Castle Creek that provided nearly all of Aspen's electricity from 1892 to 1958, when the city began buying cheaper power from sources elsewhere in the West, including coal plants.

Aspen got back in the hydro business in the mid-'80s for environmental reasons, installing turbines in Ruedi Dam, 20 miles away. That project, another small plant on Maroon Creek several miles upstream of Aspen, and other dams in the West supplied Aspen's municipal utility with roughly 42 percent of its electricity in 2011; another 30 percent came from wind, primarily in Nebraska.

The city stepped up efforts to scrub its power supply in 2006; climate models had begun suggesting narrowed ski seasons by 2050, and perhaps no skiing by 2100. According to a city study, the number of frost-free summer days has expanded from 73 to 107 during the quarter-century before 2008.

Given the existing infrastructure and Aspen's hydro-savvy, reviving the old Castle Creek plant seemed the most immediately attainable goal in the city's climate strategy. In 2007, a large majority of voters authorized $5.5 million in bonds for the project. Steel pipes 42 inches across and stretching 4,000 feet would replace the old wood flumes. Water would be diverted upstream of Aspen into a small reservoir, then down through the pipes to a new power plant smaller than many Aspen houses.

Nobody disputes the availability of water during spring runoff. Even after existing diversions, peak Castle Creek flows average 650 cubic feet per second (cfs), and Maroon's average 750. The new plant would divert up to 52 total cfs. The problem lies in low-flow months, mostly during winter, when Castle averages 15 cfs, and Maroon 25. Despite holding more senior rights, the city promised to keep flows at or above 12 cfs in Castle and 14 in Maroon, in accordance with the state's instream flow rights. A city study found "no measurable impacts" were expected to stream health, the fish community or macroinvertebrates from the reduced flows, but two Pitkin County studies found the city's data too narrow to draw confident conclusions.