While most people easily understand that if the headwaters of rivers are glaciers, and the glaciers melt, then the rivers will have no headwaters.
Most people will also note that even in deserts, rivers may exist -- even if only part of the year -- because of seasonal precipitation.
The Southwestern US, for example, undergoes a recurrent "Mexican Monsoon". For those who live in the area, this isn't just a season where the desert blooms briefly after baking in the sun for months. It's a time of intense localized storms, and a lot of Flash Flooding (see photos).
Of course, if such localized storms are both recurrent enough in one location, and the flash-flooding isn't sufficiently strong as to carry away the hardy vegetation that may evolve to withstand both high temperatures and forceful inundation, you may wind up with the sort of seasonably variable waterway that is lined with trees that both help hold water in the soil, and help hold fertile soil near their roots.
If the flows are too variable, or erratically variable, you may wind up with a situation where no amount of vegetation can protect the soils from erosion. In that case, two things can happen once the soil has been rendered infertile and incapable of supporting vegetation that can help hold soil in place.
If the ground is high ground, and there is enough of a drop so as to enable rapid flows, over time you will see the erosion of massive chasms such as the Grand Canyon of the Colorado River.
If the ground is low ground, and there is not enough of a drop so as to enable rapid flows, the water may pool.
In either case, infertile soil adheres poorly, and wind will tend to pick up particles and transport them, usually transporting dust and other "fines ()PDF" farther than sand. Dusts tend to collect and adhere, and dusts which have little fertility are quickly absorbed into fertile soils.
Sand, on the other hand, tends to just stay where it lands, and where enough sand lands without there also being a deposition of fertile fines and dusts, what you get is a desert.
Brazil's Lençóis Maranhenses National Park should be a beautiful tropical paradise, full of lush vegetation and teeming with wildlife. Instead, there is this:
The Wikipedia article claims that this is not a desert, but the author has a mistaken definition of desert. The author seems to think that the primary characteristic of a desert is that it has no rainfall. Thus, in the opinion of the author, the Lençóis_Maranhenses is not a desert because it is frequently inundated.
Yet when you see the photo, despite the lagoons of significant quantities of fresh water, you know you are looking at a desert, because there is no soil that can support vegetation, no matter how much rainfall it gets.
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The location is south of the equator, not by much, but enough so that this isn't easily explained as fallout from the massive sandstorms that often sweep westward from the Western Sahara.
If one would wish to explain the deposition of sand as a side effect of upstream river erosion, the nearest significant estuary is the Bay of Saint Marcus (Baía de São Marcos). Interestingly, none of the rivers feeding into this bay have origins in the Andean glaciers.
Situated in the Brazilian State of Maranhão, the Lençóis Maranhenses are merely the extreme manifestation of a series of ecologies which are never far removed from desertification. Much of the State is tropical rain forest, but much is very close to the state of being a desert in terms of lack of vegetation or vegetation that is very well adapted to living on nearly pure sand, such as palm trees.
The wetter parts of Maranhão are those parts closest to the mouth of the Amazon... and so one has to wonder what might happen, and if the deserts would spread from the place where they already exist despite abundant rains, should the flows of the Amazon decrease as the end stages of Glacial Melt-Off are passed.
According to local lore, the region of that abundantly-watered desert was inhabited by Caeté Indians, who woke up one day to find their town covered by sand.
I guess they had to move away.
And if the Amazon can no longer provide the soil humidity to retain fertile fines and they blow away in the winds, and the desert expands despite the abundant rain, I guess everyone else in the region might wake up one day to find their town covered by sand. And I guess that they too, will have to move away.
More to come?
