Landscapes in the lab

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Science  03 Jul 2015:
Vol. 349, Issue 6243, pp. 32-33
DOI: 10.1126/science.aac5794

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Rivers dissect much of Earth's surface into conspicuous networks of valleys and hillslopes. Viewed from an airplane, these erosional networks might appear fractal; a close look at a part presents a view indistinguishable from the whole (1). But meter-scale topographic measurements reveal that the extent of landscape dissection by down-carving streams is finite and limited by the size of smooth, undissected hillslopes that separate adjacent valleys (1, 2). Within a drainage basin, hillslopes commonly have a characteristic size (see the figure), yet across different landscapes this scale can vary from meters to a kilometer (1, 3). What imposes this scale on a landscape, and why does it change from one landscape to another? On page 51 of this issue, Sweeney et al. (4) demonstrate using controlled laboratory experiments that landscape scale is set by a competition between river incision that cuts valley networks and diffusive hill-slope processes that fill them in. Their experiments highlight that the extent of valley incision is an emergent, dynamic landscape characteristic that depends on a delicate balance of forces shaping the landscape.