With six million tons of cargo produced and trafficked annually though the waterway after World War I, the Gowanus Canal became the nation's busiest commercial canal, and arguably the most polluted. The heavy sewage flow into the canal required regular dredging to keep the waters navigable. By the 1950s Brooklyn's fuel trade was already converting from coal and artificial gas to petroleum, which was served by the wider and deeper Newtown Creek, and natural gas, which arrived by pipeline. In 1951, with the opening of the elevated Gowanus Expressway over the waterway, easy access for trucks and cars catalyzed industry slightly. The expressway carried 150,000 daily vehicles, which unloaded tons of toxic emissions into the air and water beneath. Around this time, sewage going to the Gowanus Canal was redirected into sewage treatment plants near the Buttermilk Channel.