When the Yellow River flows through Togtoh, north China, a puzzling phenome-non occurs, one side is the Hobq desert, and the other side of the river is a lush landscape of gardens. No theory can explain the desertification beside a river. Especially the river might be the main factor in the desertification. We address this challenge by establishing a mathematical model that can depict the different effects of the river and mountain on the river's both sides. The model herein takes into account the different meteorological parameters and soil parameters of the river’s both sides and Earth surface's morphology. Moreover, though our focus is put on the Hobq desert, the present theory on mountain-river-desert relation can also be used for combating desertification. We anticipate that the mountain-river-desert relation is much more important than vegetation restoration and human activity.