Caution: Salmon crossing! Skokomish River flood brings salmon to the streets

Those weren't your typical pedestrians crossing the streets of Mason County during an atmospheric river Thursday afternoon.

It was salmon!

The Skokomish River is home to an annual salmon migration, but it's also the most flood-prone river in Western Washington.  And when you put the two together, the salmon get an unexpected detour and drivers get an unexpected sight.

"There's a road nearby (the river)," says hydrologist Brent Bower with the National Weather Service in Seattle. "So basically what's happened is (the salmon) are trying to go up the river. But when the river floods, they don't necessarily know what's the actual river channel, and what's not, so they just follow the water. And sometimes that water takes them across the road."

The surreal sight typically happens a few times each year, mainly during the autumn migration runs. FOX 13 reporter Franque Thompson had a front-row seat:

Late October, through November, into December, probably the most common month to see that," Bower said.

The Olympic Mountains, which feed the Skokomish, are one of the wettest locations in the lower 48. The southwest-facing side that takes the brunt of Pacific storms and atmospheric rivers receives over 200 inches of rain per year on average.

"It's not quite that much (rain) in the headwaters of the Skokomish," Bower said. Still, annual rainfall maps show that the basin gets around 100-120 inches per year.

"For some reason, unlike some of the other larger rivers off the (Olympic) Peninsula, this river never really eroded a stream channel that was capable of containing the heavy rains most of the time," Bower said. "There's forestry practices in there that might contribute, but it gets enough silt and gravel and stuff coming down (from the Olympics) that it's a shallow stream bed."

It's a shallow riverbed that receives a lot of rainfall off the mountains to where just about any flooding event in Western Washington includes the Skokomish River.

And for some unlucky salmon caught in traffic, a mysterious detour on the way home.

WATCH: Forecast and Radar 
READ: Closures and Delays 
CHECK: Latest Weather Alerts and Live Traffic Map 
INTERACT: Submit your Weather Photo
DAILY BRIEF: Sign Up For Our Newsletter
FOLLOW: Lisa VillegasErin Mayovsky, Brian MacMillan and Scott Sistek