Complex sentence: In 1951, with the opening of the elevated Gowanus Expressway over the waterway, easy access for trucks and cars catalyzed industry slightly, but with 150 thousand vehicles passing overhead each day the expressway also deposits tons of toxic emissions into the air and water beneath.
Split into simpler sentences: In 1951, with the opening of the elevated Gowanus Expressway over the waterway, easy access for trucks and cars catalyzed industry slightly. But, with 150 thousand vehicles passing overhead each day, the expressway also provides the means for depositing tons of toxic emissions into the air and the water beneath.