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Geomorphology
Jeffrey D. Niemann and Eric Kirby

Though quantitative models of erosion and transport continue to be developed and evaluated, researchers are increasingly turning their attention to spatial and temporal variations in the state and behavior of geomorphic systems. This trend marks a significant departure from the focus on largely homogeneous, steady-state models prominent in the literature of the past five years and represents an emerging avenue of inquiry that is likely to drive the field over the next five years.

The problem of fluvial erosion into bedrock remained a hot topic in 2002, and several important improvements were made to quantitative models of bedrock incision. In particular, the role of sediment flux in modulating bedrock incision and its representation in models has been an outstanding issue. Recent laboratory results support theoretical claims that coarse bedload sediment has two opposing roles in bedrock erosion (Sklar and Dietrich, Geology, v. 29, p. 1087). At low concentrations of sediment, an increase in sediment provides more tools with which the water can abrade the bedrock. At high concentrations, increased sediment loads actually protect the channel bed as particles form a carpet of mobile sediment.

In a complementary pair of papers, Whipple and Tucker (Journal of Geophysical Research, v. 107, art. no. 2039) and Tucker and Whipple (Journal of Geophysical Research, v. 107, art. no. 2179) considered the topographic implications of simplified representations of sediment flux in bedrock erosion. Their results suggest that steady-state channel characteristics, in particular, the observed power-law relationship between river slope and the drained area, may be insensitive to the detailed nature of the erosion process. They further suggest that future testing of quantitative fluvial erosion models should focus on transient responses of basins to climatic and tectonic variations. Finally, consideration of randomly generated surfaces (Schorghofer and Rothman, Geophysical Research Letters, v. 29, art. no. 1633) showed that a power-law relation between slope and area can also occur where the slope does not depend on basin area in any causal way. Thus, the degree to which the morphology of fluvial channels reflects the operative processes remains an important and vibrant research avenue.

Important advances in the modeling of hillslope erosion were also made in 2002. Soil creep has typically been modeled as a linear function of surface slope, which leads to parabolic hillslope profiles. However, recent laboratory simulations (Roering et al., Geology, v. 29, p. 143) of disturbance-induced creep demonstrate that the transport rate depends nonlinearly on slope. Interestingly, the power spectrum of the transport rates is well described by 1/f noise, which is commonly associated with fractals and self-organized criticality. Natural examples of nonlinear creep on hillslopes have been attributed to disturbances driven by biotic processes, such as tree throw and animal burrowing (Roering et al., Geology, v. 30, p. 1115).

In addition to such advancements in our understanding of geomorphic transport processes, a number of studies emphasized the importance of spatial and temporal variability in those processes. For example, Roe and co-authors (Geology, v. 30, p. 143) developed a quantitative parameterization of orographic lifting and explored its influence on channel profiles. This process is responsible for substantial spatial variations of precipitation associated with mountain ranges, and the feedback between topography and precipitation may influence the concavity of stream profiles. Similarly, a model simulating the stochastic variability of precipitation intensity in time was also developed using a cellular automata approach (Crave and Davy, Computers and Geosciences, v. 27, p. 815). Within this model, the form of the precipitation distribution has important impacts on the roughness and drainage density of simulated topography. A stochastic distribution of storm events, coupled with a threshold shear stress for bedrock erosion, can also strongly alter the relationship between channel gradient and rock uplift rate (Snyder et al., Journal of Geophysical Research, v. 108, art. no. 2117). This effect may explain why channels experiencing dramatically different uplift rates exhibit relatively subdued differences in gradient in coastal channels in California. Finally, seasonal variations in discharge were shown to significantly influence the rates and spatial distribution of bedrock incision along a channel cross-section in Taiwan (Hartshorn et al., Science, v. 297, p. 2036). Observations indicate that vertical incision is driven by relatively frequent floods, while rare events associated with super-typhoons play a larger role in widening the channel.

Expanding to longer time scales, variability in precipitation and discharge during the Holocene-climatic variability-has been argued to be responsible for periodic storage and excavation of large amounts of sediment in Himalayan channels (Pratt et al., Geology, v. 30, p. 911). Elevation-invariant cosmogenic ages of fluvially carved bedrock surfaces in the Marsyandi suggest that sediment delivery appears to be strongly modulated by variations in precipitation. Enhanced monsoonal precipitation appears to increase landsliding and temporarily overwhelm the transport capacity of the fluvial system.

Finally, climatic variability and associated variations in sediment flux may control the generation and preservation of fluvial terraces. A quantitative model that simulates both vertical incision and lateral valley widening by bedrock channels was developed (Hancock and Anderson, GSA Bulletin, v. 114, p. 1131). According to this simple model, an increase in the ratio of water discharge to sediment flux promotes valley incision while a lower ratio promotes valley widening. Furthermore, strath terraces do not necessarily coincide with periods of steady climatic forcing. In a field study of Holocene terraces in the Olympic Mountains, Wegmann and Pazzaglia (GSA Bulletin, v. 114, p. 731) present data that appear to support some of these predictions. Vertical incision seems to occur over brief periods of time while lateral movement of channels and valley widening may occur over protracted periods.

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Niemann is an assistant professor in the Department of Civil and Environmental Engineering at Pennsylvania State University. E-mail: jniemann@engr.psu.edu. Kirby is an assistant professor in the Department of Geosciences at Pennsylvania State University. E-mail: ekirby@geosc.psu.edu.

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