About a half-inch of rain fell in about 45 minutes on the area that weeks earlier had been hit by the 276,000-acre Elk Fire Complex. It left bare slopes and gullies susceptible to sliding.
The mudslides hit above and below Anderson Ranch Dam, but are not expected to hurt the South Fork's prized trout population.
"I can't think of an instance in Idaho where we've seen any direct mortality from this kind of thing," said Jeff Dillon, state fisheries manager of Idaho Fish and Game.
Roads are closed in the South Fork canyon and on the west side of Anderson Ranch Reservoir while crews work to clear mud and debris.
The Boise National Forest reported mudflows on Evans and Castle creeks on the west side of the reservoir, and Fall Creek Road, also on the west side, is closed because of large boulders on the roadway.
There were four mudslides on the South Fork of the Boise River Road below Anderson Ranch Dam.
Mike Davison, highway district foreman with the Elmore County Highway District, said crews will concentrate on the section of South Fork road below Cow Creek Bridge so people can access Prairie from U.S. 20. The bridge is about 6 miles downstream from Anderson Ranch Dam.
"If everything goes well, it's at least possible we could have it open tomorrow," Davison said on Friday.
Two drainages slid between Cow Creek Bridge and the Danskin boat launch where the road leaves the river bottom.
Granite and Pearce creeks both slid, but Pearce did not block the road. It still dumped tons of mud and rock into the river.
All four slides below the dam moved enough mud, rock and debris into the river to partially block it Thursday night.
"I came down (Thursday) night and there was hardly any water coming down the river, and it was really eerie," Davison said.
A river flow gauge at Neal Bridge downstream on the South Fork dropped from about 600 cubic feet to about 250 cfs before spiking back up 650 cfs, the Forest Service reported.
The largest slide along the South Fork was upstream at Reclamation Village, where mud and rock covered several hundred yards of road.
"It's probably going to be weeks of work to get it all cleaned up," Davison said.
Slides at Reclamation Village and Rough Creek both appeared larger than Granite and Pearce creeks, but they appeared to dump less material into the river, according to Terry Hardy, soil scientist for the Boise National Forest.
Mudslides are relatively common after large wildfires, but it's fairly rare to have several occur in a popular recreation area. This may not be the last of them, Hardy said.
"Having that many go at the same time is a concern," he said. "You're dealing with the same landscape and repetitive drainage features," he said.
The areas that slid will naturally stabilize and are unlikely to again release massive amounts of mud. But they will likely be "chronic bleeders" that release mud in the river after rains or when snow melts.
The South Fork is a year-round destination for Treasure Valley anglers, and it features one of the healthiest population of large rainbow trout.
The river has been closed since the fire burned through the area in mid-August, and anglers are eager to get back on the river.
That may take a while. But when they return, the fish will still be there, Dillon said.
"Fish are very well equipped to deal with these things. They don't happen every year, but they happen on a regular basis in nature," he said. "By and large, we often have a hard time even documenting an effect."
Alan Percy of Boise saw the river on Friday and said it didn't look good for anglers in the short term.
"I have a feeling with all that burned it could be muddy all winter," he said. "I don't see this recovering with any decent fishing this winter."
The slides will also likely affect the popular whitewater stretch farther downstream, currently closed to rafting.
"We are very concerned about boaters starting a trip downriver in this isolated and limited communication section of the river," said Cecilia Seesholtz, Boise National Forest Supervisor.
Mudslides and flooding were also causing problems Friday night in Boise County.
Sheriff's dispatch said a mudslide forced the closure of Grimes Creek Road from Clear Creek Road to Centerville. A backed-up culvert shut down the Middle Fork Arrowrock Road at milepost 23. There was 8-10 inches of mud on the road, dispatch said.
Crews were expected to start cleanup Saturday morning, starting with Grimes Creek.
Roger Phillips: 377-6215, Twitter: @rogeroutdoors