The desert floor and bajadas (convergences of alluvial fans into a single apron of deposits below a slope) of the Sonoran Desert are the mildest of North American deserts. They are also the wettest, experiencing the only bi-annual rainfalls of any North American arid land. Consequently these habitats are also the ecologically richest and most biodiverse. Soils are coarse sands riddled with dry streambeds and washes. Dominant megaflora vary from Saguaro cactus to Organ pipe cactus and various Fouquiera species in the Mexican reaches. Besides hundreds of other Cactaceae species, there are a wide variety of trees and shrubs such as Creosote bush, Ironwood and Palo Verde tree. Riparian zones support such species as Velvet ash, Arizona sycamore and hopbush. Wildflower species include Paper flower, Desert marigold and Mexican poppy.