The low flow of water in rivers and streams combined with prolonged low temperatures have created what observers say is an unusually widespread buildup of anchor ice in the Roaring Fork basin.
Anchor ice occurs when there is super cooling of the river or stream bottom. Rocks under the surface of the water gather ice that takes on a greenish-gray tint.
Anchor ice started forming on large sections of the Roaring Fork River adjacent to Crown Mountain Park in El Jebel roughly two weeks ago. As the cold persisted and intensified, the ice layers grew until they covered some of the surface in areas where the river has rarely, if ever, iced over in the last 20 years. The water continues to flow over parts of the surface ice, creating a stunningly beautiful scene.
Rick Lafaro, executive director of Roaring Fork Conservancy, a nonprofit focused on river quality and quantity issues, said anchor ice is fairly common on stretches of the upper Roaring Fork River, particularly shady areas such as Snowmass Canyon. What's different this year is there is more anchor ice spread over a broader area, he said.
When anchor ice first starts forming on rocks, it feels like slush when a person walks on it with waders, he said. The slush solidifies and broadens with time. "It sort of starts stacking up like a layer cake," Lafaro said.
This winter has produced the perfect recipe for the formation of anchor ice. The streamflows are lower than usual because of the ongoing drought. The cold snap that typically comes at this time of year "sure feels like it's longer," Lafaro said.
John Armstrong, senior ranger for Pitkin County Open Space and Trails, said he's witnessed anchor ice forming in the Roaring Fork River beneath the Satank Bridge. "It's not uncommon to get at least a couple days of it per year," said Armstrong. But the anchor ice typically forms further upvalley. He's rarely seen it at Satank. He called it a "fascinating phenomenon."
"I do look at it as a treat," Armstrong said.
The Fryingpan River also is experiencing a greater-than-usual build-up of anchor ice. And while it's pretty, it also can be tough on the environment.
"I've been living on the Fryingpan for over 40 years and this winter has been the worst," former fishing guide Roy Palm wrote in an email.
"Anchor ice builds up from the bottom, scouring and destroying the invertebrates - the trout's food source," Palm said. "Bank ice comes in from the sides of the river and leaves a very small, fast-flowing channel for the fish to survive in. There is little food for them in the winter and these conditions force them to use more energy than they take in."
Palm contends the situation poses a possible threat to Basalt and residents along the lower Fryingpan River. When there is a thaw, there is a good chance ice dams will build up in pinch points of the river. Eventually the accumulation of water and ice will break the jam and send a torrent toward Basalt, he said.
Huge ice floes are common on the Roaring Fork River. One already occurred this winter, leaving a 2-foot high ridge of ice and debris along a midvalley section of the river.
Palm wrote to Basalt officials this week urging them to lobby the U.S. Bureau of Reclamation to change management policies at Ruedi Reservoir and dam. He wants the minimum streamflow in the lower Fryingpan River increased during the winter from 40 cubic feet per second to 80 cfs. The streamflow cannot be increased now, with the icing issues, because it could add to the ice dam danger, he said. He wants higher flows for future winters.
Lafaro said some scouring of a river bottom can be beneficial because it gets rid of vegetation. However, it also can kill insects and possibly some fish. The biggest danger, though, is from insects freezing on the river bottom when the anchor ice forms.
The scouring will also affect the eggs of brown and brook trout, which spawn in the fall, said Sharon Clarke, director of watershed action for Roaring Fork Conservancy. The eggs stay on the river bottom throughout the winter so scouring could destroy them, she said.
Clarke lives along the Crystal River and has witnessed substantial build-up of anchor ice there, too. Like others, she pointed to low flow as a contributing factor. The Crystal River near Penny Hot Springs is flowing at 35 cfs. The median flow for this time of year is 48 cfs, she said. In 2003, the winter following a severe drought, the flow was at 32 cfs, according to Clarke.
Lafaro said the flows on the Fryingpan River at this time of year haven't been as low as they are now for probably 10 years.