GIVEN THE PASTING PRESIDENT BUSH has taken over the government's response to Hurricane Katrina, one might have assumed the president's critics were in agreement about how to prevent such disasters. But for years now, the left has been deeply ambivalent about the most logical and time-tested mitigator against the threat of city-wide and regional floods: dams.How could dams, embraced by everyone from beavers to Franklin Delano Roosevelt, be a source of contention? Ask the environmentalists. Their campaign against dams has gained influence and stalled, decommissioned, or otherwise limited the construction of many dams and levees, including one project that could...