Stephen T. Lancaster

Sediment storage and evacuation in headwater valleys at the transition between debris-flow and fluvial processes

Sediment from landscape disturbance often enters temporary storage in valleys and evacuates over longer times, which in steeplands are poorly delimited. We hypothesize that, across process transitions (e.g., debris fl ow versus fl uvial transport), distributions of sediment transit times also change. We use fi eld surveys and extensive radiocarbon dating to assess the distribution of transit (residence) times through the proxy measurement of ages of bank deposits in two mainstem reaches of a 2.23 km 2 watershed in the Oregon Coast Range. In the downstream reach, debris fans impound fl uvial deposits; debris-fl ow, fi ne fl uvial, and coarse fl uvial deposits compose nearly equal parts of the valley fi ll; and fl uvial erosion evacuates deposits . Transit times have a sample mean of 1.22 ◊ 10 3 14 C yr and an exponential distribution, indicating uniform probability of evacuation from storage. In the upstream reach, valleyspanning debris jams impound debris-fl ow deposits composing >95% of the valley fi ll, which is routinely scoured by debris fl ows. Transit times have a sample mean of 4.43 ◊ 10 2 14 C yr and, if >100 14 C yr, a power-law distribution, indicating preferential evacuation of younger deposits and retention of older deposits. In both reaches, most sediment has short transit times (<600 14 C yr), but signifi cant volumes remain for millennia. Less than 20% of basin-wide denudation passes through these reservoirs, but the latter are still signifi cant buffers between hillslope disturbance and downstream aquatic habitat, especially for coarse sediment.

Geomorphic significance of postglacial bedrock scarps on normal-fault footwalls

The existence of well-preserved Holocene bedrock fault scarps along active normal faults in the Mediterranean region and elsewhere suggests a dramatic reduction in rates of rock weathering and erosion that correlates with the transition from glacial to interglacial climate. We test and quantify this interpretation using a case study in the Italian Central Apennines. Holocene rates are derived from measurements of weathering-pit depth along the Magnola scarp, where previous cosmogenic Cl-36 analyses constrain exposure history. To estimate the average hillslope erosion rate over similar to 10(5) years, we introduce a simple geometric model of normal-fault footwall slope evolution. The model predicts that the gradient of a weathering-limited footwall hillslope is set by fault dip angle and the ratio of slip rate to erosion rate; if either slip or erosion rate is known, the other can be derived. Applying this model to the Magnola fault yields an estimated average weathering rate on the order of 0.2-0.4 mm/yr, more than 10 times higher than either the Holocene scarp weathering rate or modern regional limestone weathering rates. A numerical model of footwall growth and erosion, in which erosion rate tracks the oxygen-isotope curve, reproduces the main features of hillslope and scarp morphology and suggests that the hillslope erosion rate has varied by about a factor of 30 over the past one to two glacial cycles. We conclude that preservation of carbonate fault scarps reflects strong climatic control on rock breakdown by frost cracking.

A simple algorithm for the mapping of TIN data onto a static grid: Applied to the stratigraphic simulation of river meander deposits

Abstract Triangulated irregular networks (TIN) in landscape evolution models have the advantage of representing geologic processes that involve a horizontal component, such as faulting and river meandering, due to their adaptive remeshing capability of moving, adding and deleting nodes. However, the moving node feature is difficult to integrate with the accumulation of a three-dimensional (3D) subsurface stratigraphy, because it requires 3D subsurface interpolation, which results in stratigraphic data loss due to heterogeneity of the subsurface and averaging effects. We present a simple algorithm that maps any changes in the configuration of TIN landscape nodes onto a static grid, facilitating the creation of a fixed stratigraphic record of TIN surface change. The algorithm provides a practical solution not only for the stratigraphic problem, but also for other problems that involve linking of models that use TIN and raster discretization schemes. An example application is presented using the river meandering module incorporated in the CHILD landscape evolution model. Examples are shown of cross-sections, and voxel distributions and geo-archaeological depth–age maps. These illustrate the type of insights that can be obtained from process-based modeling of subsurface fluvial architecture, and highlight potential applications of stratigraphic simulation.

Sediment reservoirs at mountain stream confluences: Dynamics and effects of tributaries dominated by debris-flow and fluvial processes

Radiocarbon age estimates (N = 68) from bank, terrace riser, and in-channel materials sampled from random locations near two channel confluences, a debris-flow–dominated tributary to Cedar Creek, and a fluvially dominated tributary to Golden Ridge Creek in the Oregon Coast Range, are proxies for sediment transit times through tributary and mainstem sediment reservoirs separated from one another by incised bedrock risers. Geomorphic, volumetric, stratigraphic, and sedimentologic data aided reservoir characterizations. Inferred transit-time distributions for tributary deposits are right-skewed and heavy-tailed, indicating preferential evacuation of younger deposits. The debris-flow fan is much larger than fluvial terraces on the other tributary, but mean transit times (±σ) in both reservoirs are similar: 1370 ± 2240 yr and 1660 ± 2130 yr for fan and terrace deposits, respectively. Whereas tributary deposits are much larger than mainstem deposits at both sites, mainstem deposits adjacent to the fan have a relatively short mean transit time of 442 ± 491 yr, but mean transit time in mainstem deposits adjacent to the fluvial terrace is much greater: 3870 ± 6720 yr. Reservoir flux estimates indicate that most (>60%) of the debris-flow fan tributary9s sediment yield enters fan storage, but only a small part (3%) of the fluvial tributary9s yield enters storage at the confluence. Debris flows from the debris-flow fan tributary apparently promote both greater storage of mainstem sediments and more rapid unbiased evacuation of mainstem deposits, whereas old mainstem deposits adjacent to the fluvial tributary have a much greater probability of preservation.

Periglacial debris-flow initiation and susceptibility and glacier recession from imagery, airborne LiDAR, and ground-based mapping

Climate changes in the Pacific Northwest, USA, may cause both retreat of alpine glaciers and increases in the frequency and magnitude of storms delivering rainfall at high elevations absent significant snowpack, and both of these changes may affect the frequency and severity of destructive debris flows initiating on the region9s composite volcanoes. A better understanding of debris-flow susceptibility on these volcanoes’ slopes is therefore warranted. Field mapping and remote sensing data, including airborne light detection and ranging (LiDAR), were used to locate and characterize initiation sites of six debris flows that occurred during an “atmospheric river” event (warm wet storm) on Mount Rainier, Washington, in November 2006, and data from prior studies identified six more debris flows that occurred in 2001–2005. These 12 debris flows had initiation sources at the heads of 17 gullies distributed over seven distinct initiation zones near the termini of glaciers, and all debris-flow initiation sites were located within areas exposed by glacier retreat in the past century. Gully locations were identified by their steep walls and heads on a 1-m digital elevation model (DEM) from LiDAR data collected in 2007–2008. Gullies in which debris flows initiated were differentiated from numerous non-initiating gullies primarily by the greater upslope contributing areas of the former. Initiation mechanisms were inferred from pre- and post-2006 gully width measurements from aerial photos and the LiDAR DEM, respectively, field observations of gully banks, and elevation changes calculated from repeated LiDAR, and these data indicate that debris flows were initiated by distributed sources, including bank mass failures, related to erosion by overland flow of water. Using gully-head initiation sites for debris flows that occurred during 2001–2006, a data model was developed to explore the viability of the method for characterization of debris-flow initiation susceptibilities on Mount Rainier. The initiation sites were found to occupy a restricted part of the four-dimensional space defined by mean and standard deviation of simulated glacial meltwater flow, slope angle, and minimum distance to an area of recent (1994–2008) glacier retreat. The model identifies the heads of most gullies, including all sites of known debris-flow initiation, as high-susceptibility areas, but does not appear to differentiate between areas of varying gully-head density or between debris-flow and no-debris-flow gullies. The model and field data, despite limitations, do provide insight into debris-flow processes, as well as feasible methods for mapping and assessment of debris-flow susceptibilities on periglacial areas of the Cascade Range.