September flooding changed the Cache la Poudre and Big Thompson River channels #COflood

Ash and silt pollute the Cache la Poudre River after the High Park Fire September 2012

Here’s an in-depth look at the Cache la Poudre and Big Thompson Rivers in the aftermath of the September floods, from Ryan Maye Handy writing for the Fort Collins Coloradoan. Click through and read the whole article: Here’s an excerpt:

Record water levels churned the river’s bottom, washing a year’s worth of ash from its banks and stream bed. Residents of Fort Collins and Poudre Canyon say the iconic river looks “magnificent,” a stark contrast to the black sludge that choked it after the 2012 High Park Fire…

The flood also changed the Big Thompson’s course through Loveland, swallowing golf courses, bike paths and front yards. In many places, the river chose its natural path, from which it was diverted a century ago. This presents a struggle for city officials: Do they change the river, again, or let it run its chosen course?

Letting rivers be rivers is best, whether they are the pristine Poudre or the fetid Big Thompson, said professor Ellen Wohl, a geomorphologist at Colorado State University. While the September floods were devastating for cities along the Front Range, their high waters can reset a river’s cycle, encouraging everything from new trout habitat to new cottonwood saplings.

“There is no question that they are really good for rivers,” Wohl said of catastrophic floods.

Although Whol said there’s value in letting the rivers pick their own courses, Colorado residents are too dependent on the rivers’ dammed and diverted waters. In some places, both rivers will be rerouted to spare costly sewage and utilities systems that their new courses threaten…

Poudre Canyon resident Bill Sears has marveled at the river’s transformation since the floods. After 28 years of watching the river, he had never seen anything like the black water that coursed in it after the High Park Fire.

“It was like a slug,” he said of the river’s movement through the canyon. “It was more like asphalt — fresh black stuff. It coated everything.”

But after several days of September rains, the churning water — so powerful that it gnawed away roads and consumed bridges — scrubbed the river cleaner than it had ever been in Sears’ memory.

“In the long run, the flood is probably a pretty good thing,” said John Stokes with the Fort Collins Natural Resources Department. “It tends to turn the riverbed over, which cleans the river up. The bottom of the river is quite stable. It takes a big flood to turn rocks around.”[…]

On Sept. 11, the Big Thompson was not an idyllic trickle winding groggily through the hill country west of Loveland. Normally, the Big Thompson runs at 15 cubic feet per second (cfs) — like watching 15 basketballs float by every second. During the flood it pushed 19,000 cfs of water down the Big Thompson Canyon, more than the Poudre at its highest peak.

The river ruptured 6 miles of sewage lines in Estes Park, leaving about 2,000 residents without sewer service, Gunderson said. Since September, when Gunderson last sampled the river water, human waste has been dumped into the river, giving it high levels of E. coli.

Carlson’s Big Thompson “trickle” erupted in Loveland during the flood, flowing over 11 bridges and covering some of the city’s most important sewer pipes.

The river also dramatically changed its course.

Loveland Water Utilities Manager Chris Matkins had spent many years working along the Big Thompson and said he knew it as well as anyone could know a landscape. But when he saw the swollen river crash through Home Supply Dam, near the water treatment plant on County Road 29, he recognized nothing…

“It’s really unbelievable. I couldn’t get my bearing where I was,” he said. “I was standing in these river corridors, and it was like standing on the moon.”

The quiet creeklike trickle became a Poudre-sized river. It tore away rock cliffs and cracked portions of the nearby highway into splinters. Carlson remembered listening to the surging water and hearing a grinding, the sound of boulders rolling on the river bottom.

Worst of all, Loveland’s three main sewer lines were now stuck beneath the river’s new course — one had survived intact, two others were destroyed. The river had to be moved.

“You can’t just move a river,” Carlson said. “You have to train the river to move.”

The old channel had been all but obliterated, filled with “so much sediment you couldn’t even tell there was an old channel,” Carlson said. Using 18-inch pink limestone boulders — called “riprap” — city officials coaxed the river back to its former channel, nearly 300 feet away.

It was a colossal effort that cost $700,000. And it’s not something the city will do for the numerous other spots where the river left its old course. All told, Matkins thinks it will cost Loveland about $2.5 million to repair all the damages to its water treatment facilities.

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