Island dynamics and their role in regulating sediment flux in the Middle Snake River, Idaho
Thomas, Dai B., author
Wohl, Ellen E., advisor
Anthony, Deborah J., committee member
Rathburn, Sara L., committee member
Bledsoe, Brian P., committee member
This study was conducted to provide an improved understanding of the dynamics of river islands and to investigate the role of islands in regulating sediment flux within the fluvial system. The study showed that the islands in entrenched geomorphic subreaches of the MSR form, erode, and reform in locations controlled by lateral constrictions. The geometry of the islands adjust on a decadal or even longer time scale in response to a disturbance or changes in water and sediment supply, and thus, the islands form part of a temporal and spatial continuum of bedforms. The formation of the islands regulates sediment flux through the reach. The study reach of the Middle Snake River (MSR) in Idaho contains over 300 islands within approximately 200 km between Swan Falls Dam and Brownlie Reservoir. The hydrology of the study reach has been significantly altered by upstream dams on the mainstem and dams on tributaries within the study reach. Data used in the study include: (1) historical aerial photos (1938/1939) and topographic maps (c1894-1906), (2) topographic and bathymetric survey data collected in 1997 through 1999, (3) flow measurements from 1911 to present, (4) bed material samples, (5) morphostratigraphic mapping of 194 islands and (6) stratigraphic soil profile data collected on 95 islands. The soil profile data included soil stratigraphy, soil samples (used for sediment gradations and pollen analyses), pedological descriptions, historical artifacts and charcoal fragments (used for carbon dating). A previously developed 1-dimensional hydraulic model of the study reach was used to evaluate the hydraulic conditions along MSR and to calculate the overtopping discharges of the islands. Comparison of historical aerial and topographic data with 2012 aerial photography showed evidence of the growth and erosion of islands and reworking of island chains to form new configurations, illustrating the dynamic nature of the islands over the last approximately 100 years. The historical document review also showed that the location of almost all larger islands and island groups are controlled by lateral constrictions such as tributary fans. Soil profile data, pollen analyses, historical artifacts and radiocarbon dating of soil charcoal were used to determine the approximate age of islands and to evaluate the erosional and depositional activity of the islands. The soil profile data showed an extreme range in age at some islands where the gravel platform of the islands is old (circa 7,000 years), but the overlying sediments are young (on the order of hundreds of years). Two-dimensional sediment-transport models were developed to evaluate the baseline conditions and simulate island development. Baseline conditions modeling showed the gravel- to cobble-sized material forming the core of the islands is not mobilized under the current hydrology. The islands formed in response to more recent floods from silt-sand sized sediment supply, which explains the relatively young soils overlying older gravel cores. Modeling results showed that: (1) the island geometry adjusts to a disturbance or a change in sediment supply, (2) the formation of islands regulates sediment flux, and (3) the islands form, erode, and reform in the same general locations, which supports the study hypotheses that islands form part of a temporal and spatial continuum of bedforms.