IN THE EARLY Teens and Twenties, airplanes were lightweight machines requiring comparative short takeoff and landing space. All that changed, of course, in the Thirties, when Passenger and Freight planes appeared in quantity, requiring long runways.
Until the runways could reasonably be extended, what to do? The answer, until the runways had the time and space to enlarge, was to use the rivers and large lakes surrounding many cities. Then sheer space was no problem, and a new class of airplane was invented called Flying Boats.