How do glaciers contribute to the hydrologic cycle?

When glaciers are growing, the water that stays on the glacier as new ice does not become runoff that year. When glaciers are shrinking, the water that melts from the glacier is added to the runoff. Otherwise, the glaciers tend to release water at a fairly constant rate when it is warm enough to melt the glacier, much like the similarly constant and seasonal releases from lakes and aquifers into rivers and springs.