Looking at the tidal ChehaIis from say Montesano down, I've always thought it was an underfit river with lots more capacity.

But to think of all the south-face Puget/Vashon glaciers draining thru the tiny Black River really stretches the imagination. Current flows are a trickle in comparison.

Speaking of current flows, imagine what the Chehalis volume looked like back then. Modern society has built so close to the entire river corridor. Hard to imagine how big the historic flood plains were 10-15K years ago.

And we're all up in arms about modern day "flooding"... J F C!