We investigate past and future trends in glacier runoff and the associated hydrological impacts on river runoff in the Hailuogou catchment, a highly glacierized catchment with extensive debris cover in the southeastern Tibetan Plateau, using a catchment-scale glacio-hydrological model. Past trends in various runoff components of the catchment indicate that glacier runoff has been a large component of total runoff, contributing ∼53.4% of total runoff during the period 1952–2013. Future changes in runoff calculated using the outputs of ten global climate models for representative concentration pathway (RCP) 4.5 and RCP8.5 reveal that glacier runoff plays different roles in the water supply of the catchment for the two RCPs, and the discrepancies between the two RCPs increase in the second half of this century, leading to a considerable difference in the hydrological regime of the catchment. In particular, changes are more remarkable under RCP8.5, under which all glaciers are projected to retreat dramatically and total runoff to decrease slightly by the end of this century. An experimental analysis, in which no debris cover is assumed on glacier ablation zones, indicates that excess meltwater from the debris-covered area provides an 8.1% increase in total runoff relative to the no-debris assumption case.