Located at the transition between monsoon and westerly dominated climate systems, major rivers draining the western Qilian Shan incise deep, narrow canyons into latest Quaternary foreland basin sediments of the Hexi Corridor. Field surveys show that the Beida River incised 135 m at mountain front over the Late Pleistocene and Holocene at an average rate of 0.006 m/yr. A steep knickzone, with 3% slope, initiated at the mountain front and has since retreated 10km upstream. Terrace dating results suggest that this knickzone formed around the mid-Holocene, over a duration of less than 1.5 kyr, during which incision accelerated to at least 0.035 m/yr. These incision rates are much larger than the uplift rate across the North Qilian fault, which suggests a climate-related increase in discharge drove rapid incision over the Holocene and formation of the knickzone. Using the relationship between incision rates and the amount of base level drop, we build a bedrock and foreland incision model for the Beida River system. We find that narrowing of channel width plays a key role, as important as increased channel slope, in enhancing the rate of river incision. Our model places the maximum duration of knickzone formation to about 600yr, and the minimum river discharge needed to trigger knickzone formation to be 1.7 times of the present discharge. This period of increased river discharge corresponds to a pluvial lake-filling event at the terminus of the Beida River and correlates with a wet period driven by strengthening of the Southeast Asian Monsoon.