A hydrological model of Siberia's Lena River Basin is calibrated and validated against observed river discharge at five stations. Implications of the Representative Concentration Pathway 4.5 scenario for river discharge are assessed using projections from 41 Coupled Model Intercomparison Project Phase 5 General Circulation Models grouped into 12 genealogical-based groups as well as a group ensemble mean. Annual precipitation increases in all scenarios (1.7–47.4%). Increases in annual PET are of a similar range (6.0–45.5%). PET peaks in June compared to July for the baseline. All temperature changes exceed 1.5 °C (range: 2.2 °C–6.2 °C). The largest absolute increases are in winter (maximum +7 °C). Changes in mean annual discharge range from −8.5 to +69.9%. Ten GCM groups and the group ensemble mean project increases. Earlier snowmelt is dominant so the annual flood peaks in May compared with June for the baseline. Increased discharge of the Lena and other Eurasian rivers to the Arctic Ocean has the potential to impact Atlantic Meridional Overturning Circulation (AMOC). Enhanced fluxes for four groups are capable of weakening the AMOC. Changes for other groups may contribute to weakening when combined with other sources of freshwater and warmer temperatures.