The performance of stormwater ponds, operated under winter conditions, was modelled using the commercial software Mike21 and MOUSE. Direct and indirect effects of changing temperature were investigated. The most important effect of winter conditions is the changed hydrology, characterised by long periods with no runoff followed by snowmelt events with large runoff volumes during several days. This gives lower removal efficiencies than during a period with the same precipitation but without winter conditions. For the concentration of dissolved oxygen, wind is an important factor. Consequently the most important effect of an ice cover on the pond is that it prevents the oxygenation effects of the wind. The direct temperature effects on the removal processes are negligible compared to the indirect effects in changed hydrology and forming of ice cover.