The objective of this investigation is to assess the impacts of climate change on the hydrology of the Lech Valley (1,000 km2), a sub-catchment of the Danube River basin located in the northern Alps. An ensemble of nine climate projections is used to simulate the climate of a mid-21st-century scenario period (2040–2069) and an end-21st-century scenario period (2070–2099). The delta change approach overcame the gap between regional climate models (RCMs) and the hydrological model. An observed 30-year time series (1971–2000) of precipitation and temperature was perturbed according to mean monthly changes between the RCM runs. The hydrological simulations have been employed with the semi-distributed model HQsim in an off-line mode. The climate scenarios show an increase in monthly temperatures and accompanying significant changes in the seasonal precipitation patterns, including an increase in the precipitation during winter and spring and a considerable decrease in the precipitation during summer. The resulting effects on the runoff indicate large, seasonal varying changes. A decrease in monthly runoff during summer and increases in winter minimize the inter-annual disparities between low runoff in winter and high runoff in spring and summer. The overall agreement of RCM runs suggests confidence in the projections.