| The erosion of a river works up-stream from the mouth to the source, and its ultimate goal is to give the riverbed the profile of a parabola. The erosive action is at its maximum where the slope is steepest. The Drugub river has a considerable erosive power which would increase during a moist period. It cuts down its bed in the direction of the Panggong threshold, and will finally cut through it, thus again capturing the Panggong Lakes and transforming them into a river. The ultimate goal of this action is to give the Tso-nyak — threshold Drugub line
in the upper profile of our diagram, the same appearance as the Hlagar—DemchokLungkung line in the lower profile. Lakes of the Panggong type are therefore very ephemeric phenomena on the earth. | 
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