Valley fill and channel incision in Meyer's Canyon, northcentral Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ft848t46f

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  • Meyer's Canyon, a tributary of Bridge Creek in the John Day Basin, is a deeply incised valley fill in northcentral Oregon. The current channel is incised to the Cretaceous and Tertiary bedrock. To determine the precedence of the current incision and the variation and timing of depositional sequences, the sediments exposed by incision were examined for clues. The incision evaluated in this study occurs along the length of the lower valley fill, approximately 2300 meters, with a maximum depth of about 22 meters near the medial section of the valley. The incision occurred near the beginning of the 20th century and widened from 1951 to 1979, after which tributary headward cutting only is occurring at one location. Colluvial aprons and aggradation within and at the margins of alluvial fans indicate depositional processes again dominate. Fill sediments date from the early Holocene. Volume of the fill prior to incision was estimated to be about 10.8 mcm (million cubic meters), of which 1.2 mcm (11%) was removed by the incision. Fill sediments are contributed by coalescing alluvial fans and alluvial plain sedimentation. The Upper Drainage and Permian Tributary could potentially donate 67% of the Lower Valley fill sediments though these portions of the drainage were not studied. Early sedimentation is dominated by coarse-grained fluvial transport, followed by numerous thick fine-grained sequences, topped by debris flow/mud drape couplets where proximal fan processes dominate. Sediment size decreases and sorting increases toward the fan margins. Valley plain deposition is currently and was, within the Holocene, enhanced and influenced by thick vegetation due to perennial groundwater saturation. Aggradation throughout the Lower Valley fill has dominated over the course of the Holocene, with only one previous episode of incision coincident with the Mt. Mazama eruption, about 6900 yrs BP. Rates of accumulation have changed over the course of the Holocene. Volume rate of accumulation was 140 m3/yr prior to the Mazama eruption and 210 m3/yr following the eruption at a proximal fan location. Within the alluvial fans and plains, sediment characteristics change with distance from source of sediment. At more distal fan and alluvial plain locations, an average volume accumulation rate of 260 m3/yr was estimated prior to the Mazama eruption, and 130 m3/yr following the eruption. These rates indicate that input at the proximal locations has been increasing in the late Holocene and that aggradation may again be dominating Meyer's Canyon sedimentation. Recurrence intervals of debris flows (proximal locations) or events capable of transporting matrix-supported gravels (distal and alluvial plain locations) show an average recurrence interval of 600 yrs pre-Mazama and 1500 yrs post-Z4azama. At proximal locations, the shortest interval is after about 1200 yrs before present (BP) when debris flows occurred about every 500 years. Shorter intervals also generally occurred in all pre-Mazama locations when coarse-sediment input was rapid, probably from the Pleistocene-Holocene climate shift from cool/wet to warmer/drier. Following the Z4azama eruption, the medial section of the Lower Valley fill had rapid input of coarse debris, while proximal fan locations had massive fine-grained input. This is interpreted as a complex response, i.e., rapid runoff reworked previously deposited sediments at proximal locations and sediments were deposited at more distal locations. Fine-grained sediment accumulation followed this period until about 1200 yrs BP. The strongest evidence for a causal mechanism for incision is a complex response at the previously saturated wet-meadow, medial portion of the Lower Valley fill due to loss of riparian vegetation which maintained an oversteepend alluvial slope. The previously saturated portion of the Lower Valley fill shows an increasing transportation slope over time. This slope was probably maintained by the hydrophytic vegetation, but loss of that vegetation due to Euroamerican influence could have led to a geomorphic threshold being crossed on the oversteepened slope and channel incision ensued. The incision is widest at this point and, if width is used as a surrogate for length of time of exposure, it is likely that incision began here.
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