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Highlights
Geomorphology
Noah P. Snyder

Highlights in geomorphology include significant advances in three different realms: tectonic-climatic-erosional interactions, linkages between geomorphic systems, and evidence for past fluvial processes on Mars.

The year saw much progress but little consensus in research on feedbacks among tectonic, climatic and erosional processes in active orogenic belts. It began with two experimental studies (Dmitri Lague and colleagues, Journal of Geophysical Research, v. 108, art. no. 2008; Stéphane Bonnet and Alain Crave, Geology, v. 31, p. 123) and closed with a series of field-based reports in Nature (v. 426) by first authors D.W. Burbank, Simon Dadson and Peter Reiners. In between, numerous researchers made important contributions using theoretical approaches.

The Lague and Bonnet groups did small-scale laboratory experiments on the role of precipitation rates in setting the relief structure of mountain ranges. Lague focused on the development of drainages under conditions of constant precipitation and uplift rates, finding that an erosion threshold was necessary to explain the resulting relief. Bonnet and Crave investigated the response of the system to changes in precipitation rates and found that increased precipitation yielded decreased mean elevation. These studies made progress by combining measurements of surficial features and processes with exhumation-rate calculations using thermochronometers. Some researchers (such as Burbank and coworkers, Nature, v. 426, p. 652) view tectonics as the most important driver of rock uplift and erosion rates. Others (for example, Reiners and colleagues, Nature, v. 426, p. 645; Wobus and others, Geology, v. 31, p. 861) emphasized the role of precipitation in driving these rates. In a Nature "News and Views" article (v. 426, p. 612) accompanying these papers, Peter Molnar stressed the need for developing physically based erosion models to provide a theoretical context to evaluate feedback hypotheses.

In 2003, numerous workers continued to develop and constrain erosional models. Field-based tests of fluvial incision theories were presented in the Journal of Geophysical Research (v. 108) by Jonathan Tomkin and others (art. no. 2308) and by Peter van der Beek and Bishop (art. no. 2309). Stock and Dietrich (Water Resources Research, v. 39, art. no. 1089) presented field-based evidence supporting the development of a new theory of erosion by debris flows. Gerald Roe and collaborators (Journal of Geophysical Research, v. 108, art. no. 2315) developed a model to capture the influence of enhanced erosion due to orographic precipitation on the relief structure of mountain ranges.

Innovative approaches to investigate quantitative linkages among geomorphic systems were a common research thread. Rolf Aalto and co-workers, for example, applied lead-210 dating to Amazonian foreland floodplain sediments and linked flood-event deposits with La Niña precipitation events in the Andes upstream (Nature, v. 425, p. 493). In coastal Central California, Leslie Anne Perg and collaborators (Geology, v. 31, p. 299-302) demonstrated that cosmogenic radionuclides can track and quantify the contributions to longshore littoral transport of sand derived from rivers and backwearing of seacliffs. Dadson and colleagues (Nature, v. 426, p. 648) measured erosion rates over three different timescales in Taiwan and found that the highest recent rates correlate with areas of historic seismicity, weak lithology and frequent typhoons. These studies illustrate the advances being made by joining geomorphic and hydrologic techniques with geochemical and geophysical methods to look at coupled systems of sediment production, transport and deposition.

Geomorphic interpretations of digital images and datasets collected last year by instruments orbiting Mars provided much process-based evidence for fluvial features on the planet. These findings require martian paleoclimate scenarios consistent with flowing rivers fed by precipitation. Papers by Michael Malin and Kenneth Edgett (Science, v. 302, p. 1931) and Jeffrey Moore and colleagues (Geophysical Research Letters, v. 30, art. no. 2292) presented the first direct observations of features around Holden crater indicating the development of fluvial erosional and deposition features on Mars' surface. Malin and Edgett used orbiter topography and imagery to identify a partially exhumed lacustrine delta. Moore and coworkers placed the delta and its drainage basin in the context of flow processes and event magnitude and frequency. Using high-resolution Martian topography, Brian Hynek and Roger Phillips (Geology, v. 31, p. 757) demonstrated more extensive integrated drainage patterns than previous estimates. J. Taylor Perron and collaborators (Geophysical Research Letters, v. 30, art. no. 1747) interpreted rocky debris slopes imaged by the Mars Orbiter Camera as actively evolving by a process of ice-driven creep.

Finally, 2003 marked a milestone in geomorphology literature with the creation of the Earth Surface edition of the Journal of Geophysical Research. Editor Robert Anderson (v. 108, art. no. 6000) emphasized that the new journal is the natural outgrowth of the current vigorous interest in geomorphology throughout the earth-science community, providing a venue for publication of interdisciplinary research on earth-surface processes by the American Geophysical Union. The first issue was published in November with articles covering topics from glacial chemistry to submarine erosion.

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Snyder leaves a postdoctoral research position at the U.S. Geological Survey in Santa Cruz in summer 2004 to join Boston College's Department of Geology and Geophysics as assistant professor. E-mail: noah.snyder@bc.edu.

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