15 miles

The Erie canal, completed in 1825, did more to improve the fortunes and standing of New York City than any event, invention, or accomplishment before or since. Once opened, goods from Michigan, Indiana, Illinois, Ohio and western Pennsylvania could be easily and cheaply transported along the Great Lakes to Buffalo, onto the canal, to the Hudson River, into New York City, and the wide world. By 1850, the population of the city had quadrupled, largely due to the influx of goods, and therefore jobs, on the canal.
All cities on the canal route benefited, Buffalo and New York the most. Therefore it's odd that the city of Rochester takes its canal heritage very seriously. It's even considering changing one of the bridges over the Genesee back into the original aqueduct.

The original [Erie] canal went right through downtown Rochester and crossed the Genesee River on a major aqueduct. The first enlargement of the canal replaced the original aqueduct, which leaked, with a new, improved aqueduct which still exists in the guise of the Broad Street Bridge.

And, in case you didn't know it, you can travel the canal; it is still in use. It's primarily recreational, but wiki says that 42 commercial shipments moved on the canal in 2008.