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The situation is much the same[0] in many other parts of the world.

[0]: http://www.smithsonianmag.com/science-nature/world-facing-gl...

Does anyone know how much of it comes down to geology?

A lot of the accessible sand in the US is from glacial deposits.

Wisconsin is mentioned in the Wikipedia article as having a boom in sand mining and the surface geology there is nearly entirely a result of glaciers.

Ok we've sent the title above to Africa.

Isn't the river sand full of mud, in many cases? Can it really be washed that well?

Not only it can be washed perfectly but river sand (and gravel) tend to be much better for concrete when compared to "crushed" aggregates.

More or less the reason is that usually for a given size river ones are harder/more resistant, and have rounded edges (which also imply a bigger resistance) due to the cycles (wear, long time immersion, etc.) they were subjected to in the river.

And, as a final advantage, concrete made with rounded aggregates is usually easier to pump.

Mud is just soil in water, and drying and sieving soil is easy.