From Austin Topolnicki:

I went to college at CSU and while there I tried Horsetooth 2 times and would not sail there again. The water is cold and even in the summer it never gets warm. But the main reason people don't windsurf there is because the wind changes directions about every 100 yards due to the different canyons and there are often huge wind shadows that make getting to the wind and back to shore difficult especially if you want to ride a smaller board.