Too bad this is Washington state.
The slide blocked the North Fork of the Stillaguamish River, which prompted an evacuation notice because water was rising rapidly behind the debris. Authorities worried about severe downstream flooding if water suddenly broke through the debris.
That statement is much more serious than the story states. The Sheriff is in video posted online stating the water behind the debris dam 'was' rising 1.5 inches per 7 minutes. Interestingly, the article says 'forced evacuations', and later says 'evacuation notice', but the Sheriff's Dept. also stated, to almost to the words, "I can't force those that don't want to leave to go, but at least let your family know where you are should something happen." Quite a dichotomy. I do believe he asserted if the dam gave way suddenly there would be a wall of water 10-15 feet high washing down the valley. Ah...I guess I hadn't cleared my cache yet; here's the link with the video
(note: Looking at the post date on the video/story above, if the video was shot around noon, that's over 6 1/2 feet at the stated rate, at this posting, valley-wide...not just in the river channel. Overnight may bring a concurrent & more widespread tragedy)
I’m trying to envision what happened...did a long stretch of mountain side just slip off?