From the inclination rates in Figure 1 and Table 1, it can be seen that the total runoff of the Minjiang and Dadu rivers exhibited a downward trend from 1961 to 2016. Additionally, the reduction extent increased gradually from upstream to downstream. The monthly average discharges at ZJG, DJY, SW, and GS were selected to undergo an M–K trend test (Figure 2). If the UFk and UBk curves exceed the critical boundary, then the upward or downward trend is significant. If the UFk and UBk curves intersect and the intersection appears between the critical lines, the time corresponding to the intersection is the time when the change begins. The interannual variation of runoff at each station was not significant, and the same result can be seen in Figure 2. However, the runoff at each station changed significantly, both in the flood season (May–October) and in the non-flood season (November–April) (Table 1). From this analysis, the inclination rates of runoff variations in flood and non-flood seasons were found to be significantly greater than those of average annual variations.