A labyrinth weir is a cost-effective structure for flood discharge. In a cold climate, sustained low temperature might lead to formation of an ice cover in the reservoir. If it remains in position during flood release, the resulting change in discharge capacity is an issue of concern for dam safety. CFD modeling is performed to examine its effects on flow patterns and discharge capacity. Simulations and comparisons are made for four cases - without ice (reference case, layout A), with ice cover in the whole reservoir but not within the weir (layout B), with ice cover in the inlet keys but not in the reservoir (layout C) and with a full ice cover on the water surface (layout D). The ice cover is bonded to concrete, remaining in place during flood discharges. The weir configuration is based on a laboratory model, with flow data available for layout A. CFD results are presented in terms of flow patterns, crest flow distribution, reduction in discharge, etc. The CFD results show that the ice cover in the reservoir (layout B) leads to an insignificant reduction in discharge capacity, the flow reduction is ~5%. For layout C, however, the residual capacity is (35-70)% of layout A; for layout D, its capacity is ~8% lower than in layout C. At low reservoir levels, the discharge reduction is large. Despite the flow reduction, layouts C and D still release somewhat more water than the broad-crested weir of equivalent dimensions. To enhance the discharge, it is sufficient to remove the ice cover in the inlet keys, and the remaining reservoir ice cover has negligible effects. The study is based on a labyrinth weir, the conclusion should even apply to a fuse gate and a piano key weir in similar situations.