Hálslón reservoir is the main reservoir of the Kárahnjúkar hydropower project in the eastern highlands of Iceland. Studies for the environmental impact assessment for the hydropower project showed that sediment will fill the reservoir in about 500 years based on the present sediment transport rate. The main source of the sediment is the Brúarjökull outlet glacier which is a part of the Vatnajökull ice cap. Recent studies of the influence of climate warming on glaciers in Iceland show that they will decrease significantly and, in some cases, completely disappear during the next few hundred years. In this study, a glacier melt model for the Brúarjökull outlet glacier is constructed to predict how fast the glacier will retreat in response to accepted climate warming scenarios. The results from the glacier model are then used as input to a sediment transport mass balance model for the Hálslón reservoir, which predicts the influence of the retreat of the glacier on the sedimentation in the reservoir. The modeling shows that, instead of the reservoir being completely full of sediment in 500 years, the Hálslón reservoir will at that time still have about 50–60% of its original volume as the sediment yield will decrease as a result of the decreasing glacier size.