Storm Over Warlock by Andre Norton

The sun was a harsh ball of heat baking the ground and then, in some odd manner, drawing back that same fieriness. In the coolness of the eastern mountains Shann would not have believed that Warlock could hold such heat. The men discarded their jackets early as they swung to dip the poles. But they dared not strip off the rest of their clothing lest their skin burn. And again gusts of wind now drove sand over the edge of the cut to blanket the water.

Shann wiped his eyes, pausing in his eternal push-push, to look at the rocks which they were passing in threatening proximity. For the slash which held the river had narrowed. And the rock of its walls was naked of earth, save for sheltered pockets holding the drift of sand dust, while boulders of all sizes cut into the path of the flowing water.