Because of it’s location at the Paradox Valley.
Paradox Valley - Named in 1875 by surveyor A.J. Peale for a geologic “quirk”. He noticed that the Dolores River flowed across the middle of the valley, whereas conventional rivers flow down along the valley’s length, hence the name.
http://geonames.usgs.gov/apex/f?p=gnispq:3:0::NO::P3_FID:185690
There are places in Colorado where rivers seem to flow uphill. It’s just a optical illusion, of course.................