California has ecological, cultural, and geographical diversity with 10 natural bioregions (Northern Coastal, Central Coastal, Southern Coastal, Great Central Valley, Cascade Mountains, Sierra Nevada Mountains, East of Sierra Nevada, Mojave Desert, and Sonoran Desert), and several urban centers. Its population exceeds 33 million, with the largest concentrations in the Los Angeles Basin, San Francisco Bay Area, and within the Central Valley. It has an area of approximately 420,000km2, with over 2000km of coastline, and 3000km2 of lakes, bays, and deltas. Areas within the San Francisco Bay Delta are below sea level and have levees, retaining walls, and drainage ponds, which along with the California coastal areas, are vulnerable to storm-generated tidal surges, flooding, erosion, and loss of property.