THE STREAM POWER MODEL AND TERRACE GENESIS IN THE CENTRAL ROCKY MOUNTAINS

Workers have traditionally explained the formation of fluvial terraces in Rocky Mountain basins by invoking fluctuations in a river's sediment load that are dictated by changes in climatic conditions. In short, these models have suggested that incision rates were lowered during the colder glacial phases of the Pleistocene when river systems were over-whelmed with sediment supplied to them by alpine glaciation and periglacial activity. Incision rates then increased during the Holocene when warmer conditions resulted in the reduced input of sediment from glacial and periglacial sources. This study chose to test this model in fluvial terraces that formed along Wyoming's Laramie Range. The fluvial sediments found within the terrace fills were dated with optical dating techniques, which indicated that the rivers incised from 2-10 meters between ~ 22-16 ka BP, but only ~ 1-2 meters during the Holocene. These results, the opposite of those expected by the traditional sediment supply models, indicate that fluctuations in sediment availability that occurred over glacial-interglacial climatic cycles cannot explain terrace formation in this setting.

As an alternative, this study suggests that a stream power model better explains terrace formation along the Laramie Range. This model stems from the fact that the clast sizes found in the terrace fills are much coarser than those found in the more recent meanderbelt deposits. This is interpreted to suggest that flood magnitudes were comparatively higher during the Pleistocene relative to flooding events during the Holocene. Other studies from the region have reached similar conclusions and attributed this phenomenon to several factors including the presence of alpine glaciers, higher soil moisture levels, and a higher incidence of frozen ground phenomenon. The stream power model suggests that the higher discharge resulted in an increase in both lateral and vertical erosion rates, and ultimately the creation of fluvial terraces during the colder conditions of the Pleistocene. In contrast, during the warmer conditions of the Holocene, the decreased stream power resulted in meandering stream systems that were not effective at eroding either laterally or vertically through the surrounding Pleistocene terrace fills.