I spent the past two summers working at Great Sand Dunes. Medano is not runnable. Like Dan and Imyers said, too many beaver dams and fallen tree's. Even if you cleaned it all up, there is still nothing there. I've got I think the only 1st D of Medano and that constists of a 1/8th mile section along the Dunes when I got bored last summer, oh a a screaming 1st D of the Castle Creek sand field. playboats make great sleds, just remember to bail before you vertical piton into the 90 degree bend at the bottom.
Upper Sand Creek on the other hand has a 1st D possibility. But it requires a hike in and out and only about 2 miles of runnable water. Drops about 1500 ft too, but the biggest thing is a 30ft stair stepping falls into 2ft of water.