Graduate Thesis Or Dissertation

Geology, flooding & human activities : establishing a hierarchy of influence for controls on historic channel change, Willamette River, Oregon

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  • Channel evolution and influences of changing floodplain characteristics, heterogenous bank materials, and altered flow regimes were examined along the Willamette River, a large alluvial river in northwestern Oregon. The Willamette River is composed of a series of geomorphically diverse reaches, which have each evolved uniquely in the century following Euro-American settlement. The river was divided into three large (30-50 km) alluvial reaches according to physiographic characteristics. The historically anastomosing and relatively steep McKenzie Reach (uppermost study reach), extends between the confluences of the McKenzie and Long Tom Rivers. The Willamette along the lower-gradient Santiam Reach (between the confluences of the Santiam and Yamhill Rivers is primarily contained within a single channel and has experienced lower rates of erosion than upper reaches. The Long Tom Reach (extending between the confluences of the Long Tom and Santiam Rivers), acts as a transition between the upper and lower Willamette, as it is here that the channel adopts a single-thread planform and becomes more stable. To assess the role of bank materials on bank-erosion rates, a method for detecting relative differences in erodibility between bank materials along large floodplains was developed. Coupling historic patterns of channel change with a simple model of bank erodibility enabled tracking of relative changes in bank erodibility among time intervals and bank materials. The analysis was applied to the McKenzie Reach for three time periods: 1850-1895, 1895-1932, and 1972-1995, and relative differences in bank erodibility were calculated for Holocene alluvium, partially cemented Pleistocene gravels, and revetments constructed in the 20th century. This simple model of bank erodibility reveals that, for all three periods, banks composed of Holocene alluvium were at least 2-5 times more erodible than banks composed of Pleistocene gravels. Revetment installed in the twentieth century was highly resistant to erosion and was at least 10 times less erodible than Pleistocene gravels. To examine larger-scale controls of geology, flooding, and human intervention on channel stability, rates and styles of historic channel change were determined for the McKenzie, Long Tom and Santiam Reaches and were linked with events or factors that may have triggered the observed patterns of channel change. Effects of anthropogenic activities on channel change were assessed by reviewing historic documents describing settlement patterns, riparian deforestation, channel improvements, and other actions. The role of flooding was assessed by compiling gauge records, anecdotal accounts of flooding, and by comparing stream-power distributions of large historic floods against smaller, post-dam floods with a 2-D flood model. Analyses of these larger-scale controls revealed that between 1850 and 1895, a period marked by the 3 largest floods of record, all reaches experienced numerous avulsions, increases in channel width, and decreases in centerline length. During the interval 1895-1932, a period with frequent, moderate-sized floods, migration rates increased by 50-300%, sinuosity increased and channel width decreased. The interval 1932-1995 was initially marked by rapid migration, but channel stabilization and dam building slowed erosion rates, causing the Long Tom and Santiam Reaches to display similar migration rates as those recorded for 1850-1895. Along the upper Willamette (McKenzie Reach), channel change during 1972 to 1995 was primarily limited to lateral migration along areas unrestricted by revetments and occurred at rates similar to 1850-1895 levels. Channel width decreased along all reaches during the 20th century. It is hypothesized that flooding may have been the primary factor responsible for the large-scale straightening and widening that occurred during 1850-1895. Actions taken to reduce streamside wood and side-channels along the McKenzie and Long Tom Reaches may have also contributed to widening. Along some areas of the floodplain, where the largely straightened and widened 1895 channel flowed through Holocene alluvium, the channel developed small bends that subsequently migrated rapidly downstream, and triggered rapid migration of adjacent bends. This concurrence of events and conditions suggests that accelerated erosion during the period 1895-1932 results from a combination of a "primed" planform, highly erodible bank materials, and a highly erosive flow regime with many moderate-sized floods. Migration rates 1895-1932 may have also increased as a result of land clearing and snag removal, as increasing numbers of settlers occupied floodplain lands in this interval. Anthropogenic activities have no clear effect on planform or erosion rates until the 193 D's, when widespread bank stabilization and dam construction resulted in diminished migration rates, fewer avulsions, and channel narrowing. By the late century, 30-45% of each reach was stabilized with revetments, while naturally resistant bank materials bordered an additional 13-30% of the channel length. Results indicate that revetments, naturally resistant bank materials, and flow regulation restrict migration and channel movement along the modern Willamette River. Efforts aiming to increase lateral migration on the Willamette River might consider removing revetment from bends bordered by Holocene alluvium along higher-gradient areas of the floodplain. However, such efforts may not create the suite of floodplain dynamics displayed by the historic Willamette, as much of the rapid migration, side channel maintenance and avulsions were related to flooding, channel change along adjacent bends, and large wood; all of which are largely absent from the modern floodplain.
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