The diversion of the Southern Emigrant Trail and the later stage road south of the Mexican Border was due to the Algodones Dunes, extensive sand dunes located west of Fort Yuma and northwest for over 50 miles that were impassable to wheeled vehicles of the era. The route followed the Colorado River south from Fort Yuma to beyond Pilot Knob where it turned west. Here the route passed south of the dunes, following the direction of ephemeral sloughs that fed the Alamo River to the south of the route, made by the variable overflowing of the Colorado River in the spring. When it overflowed this inundation flowed west on the sloughs of Alamo River to the vicinity of Alamo Mocho Well and via the Paradones River to the volcanic lake southeast of Cerro Prieto and down the New River, up to 60 miles west of the Colorado to the vicinity of Indian Wells. In years of extraordinary flooding or if the Colorado changed course down these channels it might flow north, to the Salton Sink. Following these spring flood events, water that remained in these channels formed ponds and lakes, where water percolated into the soil before they dried out. Here wells could be sunk to obtain water even in years the channels were not flooded, in the otherwise almost waterless desert during the rest of the year. Cottonwoods, fond of water, are found along the length of the lower Colorado River and in these sloughs also.