How Does a River Steal Another River? Erosion Rearranging the Map

Rivers erode headwards, meaning the upper end of a valley cuts backwards into the high ground as material is removed at the steepest part of the channel, and the rate depends on how steep the river is and how resistant the rock is. Where two river systems drain opposite sides of a divide, the one with the steeper gradient or the weaker rock cuts back faster, and eventually its headwaters reach across the divide into the valley of the other. When that happens the upper part of the second river is diverted into the first, because water follows the steeper path, and the capture is permanent since the new channel then deepens rapidly with the added flow. The divide has effectively moved, and the drainage map is redrawn.

A capture leaves marks that can be identified long afterwards:

• •An elbow of capture, a sharp and otherwise inexplicable bend where the diverted river turns into its new course
• •A wind gap, a dry valley through the divide where water used to flow and no longer does
• •A beheaded stream, the remnant of the lower river left with a valley far too large for the water now in it
• •A misfit stream more generally, where channel size and valley size do not match
• •Gravel of the wrong rock type downstream of the capture point, carried from the old catchment
• •Fish and other aquatic species shared between river systems that are no longer connected

A capture was documented in progress in 2016, which is exceptionally rare since the events usually take far longer than anyone is observing. A glacier in the Yukon retreated enough that its meltwater, which had drained northward to the Bering Sea, found a new route and was diverted southward to the Pacific over a period of a few days. The abandoned river dropped dramatically, a lake level fell, and the chemistry and sediment load of both systems changed measurably. The researchers who described it argued that the trigger was glacier retreat attributable to warming, which makes it a documented geomorphic consequence of climate change, and the speed of the event was the striking part, since the textbook treatment of the process implies geological timescales.

Rivers also lose flow rather than gaining it, and the mechanisms are worth setting beside capture. A river crossing permeable rock can lose water into the ground entirely, disappearing at a swallow hole and continuing underground, which is normal in limestone country and produces dry valleys above active cave systems. Deposition can block a channel and force the water elsewhere, which is how a river shifts across a floodplain and how deltas rebuild themselves. Glaciers and their deposits have blocked and diverted major rivers repeatedly, with several present-day courses in northern Europe and North America dating from ice retreat rather than from anything older. Uplift can tilt a landscape and reverse a gradient, reversing the flow of a river over geological time. And human engineering does all of these deliberately, with diversions, canals and dams rearranging drainage on a scale comparable to the natural processes.

Captures have consequences that outlast the event. The aquatic species of the two systems are mixed, which is why fish distributions frequently make no sense in terms of present-day connections and are used as evidence for former ones, and molecular work on such populations can date past captures independently of the geology. Sediment supply downstream changes abruptly, which affects channel form, floodplains and deltas far away. Water resources shift between regions, which has practical consequences where the systems cross political boundaries. And the pattern of capture over long periods has reorganised continental drainage repeatedly, with several of the world's largest rivers having reversed direction or been assembled from formerly separate systems, which is reconstructed from the sediment record in the basins they feed.

Headward erosion lets one river cut across a divide into another's valley, diverting the flow permanently because water follows the steeper path. A sharp bend, a dry wind gap and a valley far too large for its stream are the evidence. A capture was documented over a few days in 2016 when a retreating glacier's meltwater changed ocean basins.