… The Sacramento region is in one of the nation’s most flood-prone metropolitan areas, and its levee system is vulnerable to “erosion, seepage and stability,” the U.S. Army Corps said on its website. The Corps will raze trees to install riprap, or rocks, along the lower American River, as well as fortify the Natomas East Main Drainage Canal, Arcade Creek and Magpie Creek. … But the Corps did not consider any other methods other than riprap, according to the lawsuit filed Thursday by the Save the American River Association, American River Trees organization and the Center for Biological Diversity in Eastern District of California.