The Han river above Tan-chiang-kou flows in a W-E direction, cutting obli-quely the NW-trending strata composed mainly of Sinian schists and limestones and tertiary red beds. The alternation of resistant and soft rocks allows the development of basin and gorge in this stretch of the valley, so that it may narrow at places to about 100 m and broaden at others 18 times as wide. The gradient, varying from 0.13呱to 0.48呱，is interrupted by rapids of all sorts. Favourable sites for reservoirs are numerous.