Snow dams have been observed in many stream channels in the Arctic and the sub-Arctic, but there is no theoretical analysis of their decay processes. These processes include snowmelt, seepage erosion, down-cutting, snow-slope failure and flotation of the snow dam. Snowmelt can be determined by the energy balance. Seepage of water will cause serious erosion at the point where the stream or pond level in front of the dam intersects the dam face. Erosion by water overflowing the dam is due to down-cutting, under-cutting and thermal erosion. Down-cutting increases with discharge but decreases with the shear strength of the snow. Where a hydraulic jump occurs downstream of the dam, under-cutting can accelerate dam decay. Thermal erosion, depending mainly on water temperature, may be less significant than the previous two processes. Slope failure occurs when the driving forces exceed the resisting forces, and these are affected by snow property and snow load. When the stream or pond level downstream of the dam rises rapidly, the dam is prone to float. As peak flow often occurs during the break-up period, results of the theoretical study of the mechanisms of snow-dam decay will improve flood forecasting for Arctic streams.