No browns in The yak? Gotta give me a little wiggle room here, I'm a nebraska transplant and am not quite familiar with the river yet. It was a big dark trout seen from across the river; I must humbly apologize for my mistake!
There are some brown trout in the Cooper River system, mainly Cooper Lake, above Lake Cle Elum. So there is a chance that a brown might come down the river, through Cle Elum Lake, over Cle Elum Dam, but not many are found.