Credit: AAAS

Not all canyons are created equal. Steep canyons with amphitheatre-shaped heads are thought to result from the activity of groundwater, on the basis of studies of such features that developed in sand, for example in the Florida Panhandle. In such settings, groundwater emerges in springs and can destabilize slopes. Growth of the canyon is achieved not by surface flow, such as in a river, but by the retreat of canyon heads by periodic toppling of material that could have been softened by groundwater.

But when amphitheatre-headed canyons are carved in hard rock, the possibility that surface water was involved cannot be discounted. Michael Lamb of the University of California at Berkeley and his colleagues focused on the Box Canyon in Idaho, USA (Science 320, 1067–1070; 2008). The location is ideal to investigate the ways in which water shapes the Earth's surface, because the canyon seemed a perfect example of groundwater-aided carving: a spring originates at its head and provides almost all of the water that flows in this canyon and there is no surface drainage upstream of the canyon head.

But the researchers found heaps of boulders near the canyon head, which are usually associated with pools of water that assemble under waterfalls, and they discovered scour marks made by surface water extending upstream from the rim of the canyon. In addition, hydraulic calculations suggest that the water flow in the present stream is grossly insufficient to carve a canyon of this size. Something far more powerful would be needed and groundwater-related erosion is not a viable candidate to provide that force.

The most likely process for the formation of Box Canyon is an immense flood. Such a surge would have had to persist for between about a month and six months, eroding the canyon rapidly headward and transporting the resulting debris out of the canyon. Exposure ages of the rocks suggest that the canyon formed tens of thousands of years ago. The flood is unlikely to have resulted from heavy rainfall because precipitation in this area was probably quite low at the time, just as it is now.

Similarly shaped canyons have been found in volcanic terrains on Mars. Landforms like Box Canyon that go back to brief periods of catastrophic flooding may be more suitable terrestrial analogues for these martian canyons than amphitheatre-headed canyons formed by groundwater activity in sand.