The metric mentioned most often to give people who don't live in Cedar Falls IA an idea of how much of the city has been inundated by the Great Iowa Flood of 2008 is 438 blocks on both sides of the Cedar River.
In the past, without knowing how big a city block is in other cities (about 220 feet on a side in Portland, 1/8th of a mile in Chicago, extremely variable in Salem, &c) it's actually not all that helpful.
Thank silicon wafers we have the Google, and intrepid Google Maps mashers who made the following happen for the Cedar Rapids local paper, The Gazette: (use that textual link – I tried to link to the image and it just. wouldn't. work.)
Cedar Rapids is a city of about 120,000, just a little smaller than Salem or Eugene. That pool looks like it's about eight miles long, and as wide as 1.5 miles in some places.
A comparable flood in Portland would probably have to submerge downtown and the Pearl as far back as about 10th Avenue, and go east at least as far as Grand Avenue or maybe 6th or 7th, to get the same sort of spread.
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