On a river in the Haute Alpes region of the French Alps, near Briançon.
At first I thought it was a dipper, it was flying so close to the surface of the river, then I thought a grey wagtail, it was bobbing up and down above the surface of the moving water, I suppose catching flies.
When it flew to a rock near the river bank, its tail was bobbing like a wagtail's but its tail was short and tapered, it looked kind of conical. It was a uniform dun colour on top, a light tan colour in front. Its beak seemed longer than a wagtail's and straight and slender.