We conducted coupled U-Pb dating and Hf isotope analysis of detrital zircon in modern sand of the Yalu River in southern Tibet. Our work indicates that the presence or absence of distinctive zircon populations in the Yalu main stream depends critically on the geometric configuration of the tributary rivers. The proportion of upper-stream zircon populations in the Yalu River sand decreases systematically in the downstream direction, which is caused mainly by zircon addition from new source areas in the downstream region. In some extreme cases, the upstream zircon signals can completely be lost in the downstream region due to this dilution effect. Analysis of sand modal composition reveals a downstream increase in the proportion of lithic fragments along the Yalu River, from ∼40% to ∼60% over a distance of ∼600 km. This may be attributed to the combined effect of an eastward increase in the topographic relief and an eastward increase in annual precipitation across the Yalu River drainage basin. Quantitative comparison of detrital-zircon ages between the Yalu River sand and Neogene sediments of the eastern Himalayan foreland supports a previous proposal that the Yalu River once flowed directly over the eastern Himalaya, without going around the Himalaya through its eastern syntaxis. The shortcut appears to have been transient, as it is only recorded in specific stratigraphic horizons of foreland sediments. The inferred Yalu River diversion may have been caused by past advances of glaciers or emplacements of giant landslides that temporarily dammed the Yalu River.