Ummm.... don’t know about the actual slope but that looks like a gully in the mid-ground of the photo.
http://edition.cnn.com/2007/TECH/science/06/21/missing.lake.ap/index.html
“One theory is the water disappeared through cracks in the lake bottom into underground fissures. But experts do not know why the cracks would have appeared because there have been no earthquakes reported in the area recently...”
(the cracks appear to be clearly visible in the image.)