xv, 137 p. : ill., maps. A print copy of this title is available through the UO Libraries under the call number: KNIGHT GB565.O7 H84 2008 / This study examines the role of floods in shaping the geomorphology of the multichannel, gravel-bed upper Umatilla River, northeastern Oregon, USA. Three parts are presented: (1) the development and application of an error-sensitive aerial photo-based planform channel-change detection and measurement methodology, (2) an examination of the occurrence, variability, and landform impacts of channel widening, straightening, and lateral movement during two mid-to-Iate 20th century flood periods, (3) an investigation of the effects of these floods on channel complexity, a proxy of habitat quality and indicator of ecological health in multi-channel rivers. Floods in 1964-5 (17- to 37-year recurrence interval) scoured, widened, and straightened the active channel in conjunction with large lateral movements, bar accretion, and capture of marginal vegetated areas by lateral scour. Following the flood, lateral movements were smaller, the channel narrowed, and bars, scoured areas, and vegetation lapsed from the channel. A similar flood in 1975 also scoured, widened, and straightened the channel; however, lateral channel movement and changes in channellandforrns were less in 1975 due to latent adjustment of the channel to the first flood. Migratory straightening, meander cutoffs, and avulsions dominated lateral movements during flood periods, whereas episodes of migratory (lateral) extension and (downstream) translation of meanders dominated lateral movement between flood periods. Channel changes were spatially variable and generally greater in reaches with wide floodplains. Floods reduced the overall complexity of the river channel, although the magnitude of change was highly variable and some areas increased in complexity in response to flooding. By contrast, channel complexity increased in the period between floods, particularly in laterally confined areas where complexity loss was high during the first flood period. Two key processes appear to most affect channel complexity: (a) lateral scour and avulsions, which capture vegetation into the channel, and (2) migrations of the main channel, which reflect bar accretion and dissection. Results of this study are broadly congruent with theories (and their corollaries) emphasizing adjustment of channel dimensions, increased rates of change, and reduced complexity in response to flood disturbance, but only partially consistent with theories emphasizing large geomorphic changes in structurally confined settings. This dissertation includes both previously published and co-authored material. / Committee in Charge: Dr. Patricia F. McDowell, Chair; Dr. W. Andrew Marcus; Dr. Patrick J. Bartlein; Dr. Joshua J. Roering