That sounds a bit like a driver issue to me. When playing dirt 3 with most earlier drivers I would get artifacts in rainy/wet tracks or with some of the drivers I would get only 50% GPU usage in wet tracks but decent FPS everywhere else. Latest driver fixes all those in game issues but slows down the loading time and FPS in the menu. So, the rivers are just not perfect!
i use the 12.11 driver. same results with the 12.10 driver