meander

Although humans more often name the land in terms of themselves (finger lake, headwall, a neck of land, etc.), sometimes the reverse takes place and a human behavior takes its name from a feature of the Earth. The river Menderes, which rises in what is today western Turkey and was known among the ancient Greeks as the Maiandros of Phrygia, flows to the Aegean with such seeming reluctance that it continually doubles back on itself, wreathing its floodplain in loop after serpentine loop of wandering channel. The name of the river, descended to modern English from Greek, gives us meander, which is our best verb for expressing randomness in thought and movement. The coinage is not without its irony, however, for the meandering of a river is only superficially random. If one thinks of a river or stream as energy moving through terrain, one can begin to see the ordered sinuosity of a meandering channel as the expression of how that energy is spent by the river and absorbed by the land. A river does two kinds of work: it transports its volume of water downhill, and it also transports some amount of earthen freight consisting of silt suspended in water (suspended load) and rocks and gravel tumbling and skipping along the riverbed (bed load). If by virtue of velocity or volume a river has more energy than it needs to accomplish its work, it will spend that extra energy to reshape its course. Sometimes a river will incise its channel, ultimately lowering its bed, which is a way of diminishing the gradient at which it moves downhill; this in turn diminishes its velocity and the energy it has to spend. In other instances a river will extend its channel laterally by carving a series of curves or meanders. Here the river diminishes its gradient by lengthening the distance over which it descends a given amount of elevation. Meander formation is especially characteristic of rivers that flow through soft material and lack the erosive tools (i.e., bed load) to incise their channels. The Mississippi, twining from Cairo, Illinois, to New Orleans, is a classic meanderer, while the canyon-carving Colorado River is the Mississippi’s incisive opposite. (The famous goosenecks of the San Juan River in southeastern Utah are an anomalous hybrid of both processes. They consist of meanders incised into canyons hundreds of feet deep, the formation of which no one fully understands.) The physics of meandering are consistent across all scales of flowing water, from great streams like the Mississippi to rivulets of meltwater on the surface of a glacier. Notably, the distance from the apex of one meander to the next, known as the wavelength of the meander, tends to be seven to fifteen times the width of the channel. Also noteworthy is the fact that meandering is not restricted to terrestrial waters and distractible humans. Ocean currents like the Gulf Stream meander, as do the jet stream and other currents of air.

William deBuys