We seek a quantitative understanding of the processes that create landscapes. In addition to landscapes on Earth, we study the surfaces of other planets and moons, including Mars and Titan. Our approach combines theory and modeling, field and remote sensing observations, data from planetary missions, and laboratory experiments. Nearly all of our projects are aligned with one of three main themes: landscape patterns, climate and landscape evolution, and planetary surfaces.

Landscape Patterns

Landscapes on Earth and other planets contain strikingly regular patterns at scales ranging from a few centimeters to hundreds of kilometers. By studying how these topographic patterns emerge from the processes that shape a landscape, we can learn how landscapes record the major factors that shape a planet's surface over time, including bedrock geology, tectonics, climate, and life. Here is an overview of recent projects focused on specific landscape patterns:

Patterns in river networks

River networks are among the most widespread and recognizable erosional landforms on Earth. In addition to transporting water and sediment across the continents, drainage networks develop characteristic scales and patterns that reflect the main factors that shape landscapes. Our aim is to understand the origin and evolution of these patterns. Taylor Perron, graduate student Paul Richardson, postdoc Ken Ferrier, and visiting student Mathieu Lapôtre recently showed that the familiar branching structure of tributaries at the uppermost reaches of river networks arises from two coupled instabilities in an eroding landscape, and that the size of the smallest river basins with tributaries (which varies from one landscape to the next) is a signature of the erodibility of the underlying rock and the ability of the land surface to produce runoff.

River networks also create striking patterns at finer spatial scales, as they interact with hillslopes. Valleys in many landscapes are evenly spaced, like the teeth on a comb. We showed that evenly spaced valley arise through a feedback in which neighboring valleys compete for water as they erode the landscape, and that the valley spacing is a signature of the relative strengths of river incision and soil creep.

Rivers are also commonly used to reconstruct tectonic patterns in space and time. In a collaboration with Leigh Royden (MIT), we explored the application of analytical solutions for the transient evolution of river elevation profiles to the reconstruction of a landscape's tectonic history. Our analysis shows that a river's elevation profile does not necessarily preserve a complete record of its uplift history, and provides a means of quantifying how much of that information may have been lost. In a separate paper, we show how a procedure for river profile analysis based on our analytical approach improves on conventional techniques.

Recent papers:

Royden and Perron (2013), Solutions of the stream power equation and application to the evolution of river longitudinal profiles. J. Geophys. Res., in press, doi:10.1029/2012JF002493. PDF

Perron and Royden (2013). An integral approach to bedrock river profile analysis. Earth Surface Processes and Landforms, 38, 570-576, doi:10.1002/esp.3302. PDF

Perron et al. (2012), The root of branching river networks, Nature, 492, 100-103, doi:10.1038/nature11672. [link]

Perron and Fagherazzi (2012). The legacy of initial conditions in landscape evolution. Earth Surface Processes and Landforms, 37, 52-63, doi:10.1002/esp.2205. PDF

Perron et al. (2009). Formation of evenly spaced ridges and valleys. Nature, 460, 502-505. PDF / News & Views by K.X. Whipple PDF

Perron et al. (2008). Controls on the spacing of first-order valleys. J. Geophys. Res., 113, F04016, doi:10.1029/2007JF000977. PDF

Perron et al. (2008). Spectral signatures of characteristic spatial scales and nonfractal structure in landscapes. J. Geophys. Res., 113, F04003, doi:10.1029/2007JF000866. PDF

Patterns in bedforms

Bedforms created by wave-generated oscillatory flows are a widespread and visually striking signature of the interaction of complex flows, sediment transport, and bed topography. Postdoc Justin Kao, graduate student Kim Huppert, and undergraduate Abby Koss are collaborating with Paul Myrow (Colorado College) to compare field-scale experiments in a laboratory wave tank with numerical experiments to understand how irregular patterns (defects) in rippled beds accommodate the adjustment of bedforms to changes in the driving flow. We have found that ripples develop characteristic defects that record whether their spacing is widening or narrowing, and that these defects are commonly observed in the rock record.

We are also working to develop a new model of bedform evolution that accounts for how upstream and downstream bed topography can influence the flow stress and sediment transport at a given point. Such effects are critically important in systems in which bedform patterns are controlled primarily by length scales in the flow, such as the length of the excursion in an oscillatory flow.

Recent papers:

Kao and Perron (in review). Fast approximation of bed shear stress in oscillatory flow over bedforms. J. Geophys. Res.

Lamb, M.P., W.W. Fischer, T.D. Raub, J.T. Perron and P.M. Myrow (2012), Origin of giant wave ripples in Snowball Earth cap carbonates. Geology, 40, 827-830, doi:10.1130/G33093.1. PDF

Climate and Landscape Evolution

Despite the obvious importance of climate in shaping Earth's surface, a quantitative understanding of how climate governs the long-term development of landscapes has been elusive. We are studying several natural experiments in which landscapes have evolved under varying climatic conditions while other factors have remained relatively constant.

Rainfall and erosion across ocean islands

Volcanic ocean islands are an ideal natural laboratory for studying climate's effects on landscapes: the bedrock is relatively homogeneous, the entire island has experienced approximately the same tectonic history, the initial surface can sometimes be dated, and the interaction of tradewinds with high topography creates dramatic gradients in rainfall. Postdoc Ken Ferrier and graduate student Kim Huppert are exploring how erosional processes and landscape evolution have responded to one of Earth's steepest rainfall gradients, on the Hawaiian island of Kaua'i. In one project, a collaboration with Sujoy Mukhopadhyay (Harvard) and Matt Rosener and Jonathan Stock (USGS) we identified an empirical correlation between rainfall rates and long-term erosion rates, and compared these rates with shorter-term sediment yields to help assess possible threats to coral reefs. In a second project, we focused on bedrock river incision, which drives the development of many landscapes, and showed that the efficiency of river incision depends strongly on rainfall rate, providing rare evidence of a long-suspected climatic influence. This result helps to quantify the influence of climate on one of the main processes that shapes landscapes, and also supports theoretical arguments that rainfall gradients can, through their influence on river incision, alter the form of entire mountain ranges.

We have also been collaborating with several other groups on problems involving the interaction of climate and landscapes on ocean islands, ranging from the response of valley incision after flank collapse to the long-term hydrologic evolution of islands.

Recent papers:

Ferrier et al. (2013). Evidence for climatic control of bedrock river incision. Nature, 496, 206–209, doi:10.1038/nature11982. [link]

Ferrier et al. (2013), Covariation of climate and long-term erosion rates across a steep rainfall gradient on the Hawaiian island of Kaua'i. GSA Bulletin, doi:10.1130/B30726.1. PDF

Jefferson et al. (in review). Controls on the hydrological landscape evolution of shield volcanoes and volcanic ocean islands. The Galapagos as a Natural Laboratory for the Earth Sciences, AGU Geophysical Monograph.

Lee et al. (2012). Reduction of tropical land region precipitation variability via transpiration, Geophysical Research Letters, 39, L19704, doi:10.1029/2012GL053417. PDF

Lamb et al. (2007). Formation of amphitheater-headed valleys by waterfall erosion after large-scale slumping on Hawai'i, GSA Bulletin, 119, 805-822. PDF

Sea level cycles and coral reefs

The effects of climate variability are not limited to landscapes above sea level. Coral reefs form dramatic landscapes, particularly around ocean islands. Charles Darwin proposed a general model in which reefs progress through a well-defined sequence as an island subsides, beginning as a narrow fringing reef near the coast, then forming a barrier reef surrounding a lagoon, and finally developing into an atoll. But Darwin didn't consider the glacial sea level cycles that have been occurring for the past few million years. How does sea level shape coral reef landscapes? Graduate student Michael Toomey, in collaboration with Andrew Ashton (WHOI), has shown that reefs around many islands (such as the Hawaiian Islands) do not follow Darwin's proposed sequence, and display a wider variety of forms. We developed a simple model for reef growth that illustrates how the rates of reef accretion and island vertical motion control a reef's profile, and we found that glacial sea level variations are essential for reproducing the observed variety of reef forms. Pleistocene coral reefs appear to bear a strong imprint of glaciations.

Recent papers:

Toomey et al. (2013). Profiles of ocean island coral reefs controlled by sea-level history and carbonate accumulation rates, Geology, doi:10.1130/G34109.1. [link]

Microclimate and asymmetric topography

Graduate student Paul Richardson is studying how microclimates create asymmetric hillslopes. This small-scale natural experiment takes advantage of reliable controls on a landscape's geologic and tectonic history, and demonstrates how Earth's surface energy balance can influence long-term erosion.

Planetary Surfaces

One of the most exciting developments in geomorphology is the acquisition of imagery and topography for planetary surfaces. We use spacecraft observations, experiments, and simple models based on terrestrial theory to constrain the rates and histories of processes that shape planetary landscapes, especially processes driven by water or other liquids.

Rainfall and rivers on Titan

Titan, Saturn's largest moon, may be the only other solar system body with active rivers on its surface. In a recent analysis of the morphology of valley networks near the Huygens probe landing site, we found evidence that the valleys were incised by surface runoff, and calculated that the minimum methane rainfall rates required to form these features was similar to storms on Earth. Currently, Graduate students Ben Black and Yodit Tewelde, in collaboration with Devon Burr (U. Tennessee), are mapping drainage networks on Titan and using landscape evolution models to constrain the extent to which fluvial erosion has shaped the moon's surface.

Recent papers:

Burr et al. (2013), Fluvial features on Titan: Insights from morphology and modeling. GSA Bulletin, 125, 299-321, doi:10.1130/B30612.1. PDF

Aharonson et al. (2013). Titan's Surface Geology. In Titan: Surface, Atmosphere and Magnetosphere, edited by I. Mueller-Wodarg, C. Griffith, E. Lellouch, and T. Cravens, Cambridge University Press, in press. PDF

Black et al. (2012), Estimating erosional exhumation on Titan from drainage network morphology. J. Geophys. Res., 117, E08006, doi:10.1029/2012JE004085. PDF

Perron et al. (2006). Valley formation and methane precipitation rates on Titan. J. Geophys. Res., 111, E11001, doi:10.1029/2005JE002602. PDF

Paleoclimate records in the Martian polar caps

Layered deposits of ice and dust in Mars' polar caps are perhaps the most compelling climate records on the planet. Through a recent collaboration with Peter Huybers (Harvard), we discovered that the finest-scale beds in the polar layered deposits are often periodic, with a characteristic thickness of 1 to 2 meters. Graduate student Mike Sori, in collaboration with Huybers and Oded Aharonson (Caltech), is now exploring how different formation mechanisms might record changes in solar radiation due to long-term variations in Mars' orbit, and whether we can identify these changes by analyzing images of the polar stratigraphy.

Recent papers:

Sori et al. (in review). A procedure for testing the significance of orbital tuning of the Martian polar layered deposits. Icarus.

Limaye et al. (2012). Detailed stratigraphy and bed thickness of the Mars north and south polar layered deposits, J. Geophys. Res., 117, E06009, doi:10.1029/2011JE003961. PDF

Perron and Huybers (2009). Is there an orbital signal in the polar layered deposits on Mars? Geology, 37, 155-158, doi:10.1130/G25143A.1. PDF

Kite et al. (2009). True polar wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars. EPSL, 280, 254-267. PDF

Ancient oceans and floods on Mars

Several lines of evidence suggest that an ocean might once have filled the northern lowlands of Mars, but topographic profiles along the margins of the lowlands do not follow surfaces of equal gravitational potential (i.e., sea level), as the shorelines of a standing body of water should. In a recent collaboration with Jerry Mitrovica (Harvard), Michael Manga and Mark Richards (Berkeley), Isamu Matsuyama (U. Arizona), and Amy Daradich (U. Toronto), we showed that these long-wavelength topographic trends can be explained by deformation that occurred in response to true polar wander (TPW), a reorientation of the planet with respect to its rotation axis.

Mars has also experienced huge floods that may have been the largest ever to occur in the solar system, but poorly constrained flow depths and velocities make it difficult to estimate the discharge (flux) of these ancient flows. Graduate student Hendrik Lenferink is conducting laboratory experiments to constrain the discharge of the floods based on the paths they followed as they emptied into the northern lowlands.

Recent papers:

Daradich et al. (2008). Equilibrium rotational stability and figure of Mars. Icarus, 194, 463–475. PDF

Perron et al. (2007). Evidence for an ancient martian ocean in the topography of deformed shorelines. Nature, 447, 840-843. PDF / News & Views by Maria Zuber. PDF

Matsuyama et al. (2006). Rotational stability of dynamic planets with elastic lithospheres. J. Geophys. Res., 111, E02003, doi:10.1029/2005JE002447. PDF