Annual aerial photographs of Balmhorngletscher, available since 1973, have furnished data on large ice avalanches originating in one particular location. The avalanches characteristically occur during an active phase of fast sliding. The analysis of a photogrammetric 1:2000 bedrock map, of longitudinal ice surface profiles from photographs taken at various times, and velocity measurements by thedolite has shown that the fast sliding and the release of the avalanches in an area of about 250 by 100 m is quite independent of ice thickness. It is suggested that besides the slope of the bed, rock roughness, and topographical features, the particular structure of the rock surface in the fashion of titles on a roof is a factor which influences the process of fast sliding and avalanche formation. It is suggested that meltwater gains access to cavities at the rock steps, from where it is dragged along the ice-rock interface, getting slowly worked into the basal ice layer. Many weeks after the beginning of the melt season the evolving acceleration becomes apparent, characteristically at the beginneing of September. Once the surface meltwater gains direct access to the bed through fresh crevasses, the glacier accelerates progressivey as water enters cracks, faults, and microfissures. Water pressure effects may become important as the cavities grow. While the gradual development of the active phase permits to forecase the growing avalanche risk, the prediction of the event in time is a long way off, if avalanche risk, the prediction of the eventpractical at all. ¿ American Geophysical Union 1987