Several months ago Dubbo City Council build a bioswale and directed stormwater from a large part of the town through it. Of course, it hasn't rained since March and people scoffed at the money being spent. They couldn't see what was wrong with just dumping stormwater into the Macquarrie River, like we'd always done.
Yesterday we had 1 inch of rain fall. The stormwater rushed into the swale, hit the reed beds, and meandered down to the river. Oil was trapped, sediment was dropped. The water that flowed into the river was as clear as tap water.