A change to a warmer, drier climate beginning as early as 1900 was responsible for triggering a dramatic, rapid retreat of Collier Glacier, Oregon, between 1924 and 1940. Although there was a dramatic decrease in precipitation contemporaneous with this observed "step-function" response of mass loss, it is unclear if climate was the primary cause. Two topographic factors resulting from the blockage of Collier Glacier's trough by a prominent volcanic cone (Collier Cone) may have further influenced this step-function response to climate change. Specifically, (1) an increase in equilibrium line altitude (ELA) sensitivity resulting from an abnormally low ice surface gradient in the lower trough may have served only to augment climatic forcing, and (2) the presence of a subglacial lake dammed by lower trough topography may have reduced bed resistance and increased ice-velocity in the lower trough, thus maintaining glacier length as the glacier thinned. Mass balance data collected from 1989-1994 indicate that the general form of the annual mass balance (be) curve did not change appreciably over this time; instead this curve translated back and forth along the net mass balance axis from balance year to balance year, as the ELA shifted up and down. There was considerable mass loss relative to previous years during the 199 1-1992 and 1993-1994 measurement years, and a considerable increase in mass during the 1992-1993 measurement year. Between 1989 and 1993 net mass balance averaged approximately -0.4'm H20 per year. Average summer temperature appears to be more important than annual winter accumulation in controlling annual net mass balance during the study period. Characteristic secondary fluctuations of the b curve in the ablation zone were also observed during the study period. These fluctuations are apparently reflecting the distribution of net ice-surface shortwave radiation, although wind direction may also play a role. Analysis of oblique photographs (1910-1960), and topographic mapping via photogrammetric methods (1967, 1973 and 1982) and land survey (1993-1994) has revealed the continued development (i.e. steepening) of a prominent icefall. Successive years of decreased net mass accumulation may have slowed the flux of ice from above this icefall, which has in turn caused a large reservoir of ice below the icefall to deflate. The presence of this reservoir may have bolstered ice flux to the ablation zone since 1973 as terminus location has remained relatively unchanged since this time. However, if future climatic conditions continue to be relatively warm and dry, the reservoir will continue to deflate, until this ice surplus is exhausted. This may result in an abrupt reduction of ice flux to the terminus, which could in turn cause a rapid terminus retreat towards the icefall.