Nothing shows the vulnerability of the older American city more vividly than the swamping of Chicago's Loop business district April 13 by the little river whose name the city bears.

Perhaps it couldn't happen here, quite that way. But every older city has its crumbling sewers, its rusting bridges, its broken water mains, its underground subsidence from untamed springs, its semi-abandoned tunnels.

Chicago is the city of the big gesture, and when something goes wrong there, it does so in a big way. And so the channelized stream that flushes Lake Michigan clean, and that is engineered to flow the wrong way, poured into old tunnels and sloshed around transformers and flooded the basements of the original skyscraper city.

The heart of Chicago came to a stop. A lot of commerce halted. Mayor Richard M. Daley fired people and feels better. But unless precautions are taken, something like this will happen again. Maybe not in Chicago. It's probably New York's or Washington's or Baltimore's turn.