A general equation is proposed for the determination of water-surface slope of a steady, gradually varied openchannel flow at a critical section. Thereupon the different types of singular points and water-surface slopes are given at such a section caused by changes in (1) bed slope, (2) channel width, (3) surface roughness of the channel, and (4) discharge, or their combinations. Although these factors are very different they have similar hydraulic effects on the occurrence of a certain type of singular point and water-surface slope. It is shown that a saddle type singular point is the only one which can become a control. The water-surface slopes at a critical control are evaluated using the proposed equation and are verified by the experimental results of Wilkinson (1974) for cases (1) and (2).