Alluvial fan construction within the interior valleys of southern British Columbia was dependent upon temporary conditions resulting from deglaciation. Glacial drift was reworked by streams and mudflows to form fans whose composition is dependent upon the nature of the drift supply and the hydrologic character of the parent basin.Stratigraphic evidence suggests that fan building commenced soon after valley floors became ice-free, continued during post-glacial aggradation by major rivers and for some time after wards. Most recently, fans were built upon degradational river terraces. Mazama volcanic ash within fans indicates that their construction continued until after 6600 years B.P. After deposition ceased many fans were dissected either as local base-levels were lowered under the control of degrading major rivers or by fan-head trenching initiated as the debris supply declined. Where fan building persisted during degradation, multi-level fans were constructed.