The Settling Rate

Authored by Thomas Steinwinder

In his book Rivergator, John Ruskey describes the “Wild Miles” on the Mississippi River. “These are the places where the landscape is filled with giant islands bounded by endless muddy banks and sandbars, where the river is overseen by big skies and where the sun sets uninterrupted by buildings or wires and where the big river predominates with wild creative beauty.” As a Mississippian and fellow water lover, this account resonates deeply, like the drone notes in the delta blues. However, it is the very next sentence that strikes the chord, a full thumbed strum in open tuning. “Each high water results in shifting sand dunes and remade sandbars.” This sentence moves beyond describing the Mississippi; it begins to hint at the process, the foundational forces that drive every interaction we have with rivers.

The rate at which a particle settles is dependent upon a grand battle between gravity, drag, and suspension. Gravity accelerates the particle toward the center of the Earth at 32 feet per second squared – about the same acceleration as a Formula One racecar from 0 to 60 miles per hour. However, the moment the particle begins to move downward, a drag force is applied in the opposite direction. The density of the particle, size, shape, and viscosity of the surrounding water all affect magnitude of drag, slowing the particle like a base jumper tossing a parachute. Now enters a third combatant, suspension velocity: the speed of flow that overcomes gravity and keeps the sediment suspended. One pulling down, one pushing up, and one moving laterally.

These three engage in a contest that shapes the most beautiful of the Earth’s fluvial features: boulder strewn mountain streams, cobbled braids of the highlands, the white sandy deposits on the inside arc of a meander, the two-toned progression of sandbars, the beached banks, the coffee-colored fertile flood plains, and ultimately the marsh where river, bayou, and sea are indistinguishable.

Roughly 500 million tons of sediment are carried down the Lower Mississippi to the Gulf of Mexico each year. This is enough to add 300 feet of coastline to Louisiana. The mountains of Montana and the farms of Mississippi alike all migrate down river in an epic journey to a final state of rest. The entire continent is flattening as the high points erode and low points fill in. Rivers are the mechanism for this grand balancing.

Standing beside the great rivers, we cannot avoid or deny the connection. We know the epic journey well: the deep pull toward quiescence, the internal revolt against calm, and how the velocity of our surroundings keeps us in suspension. We look out to the mid channel and see the drift wood racing downstream, undirected and uncontrolled. We have an uncomfortable empathy for the debris. It reminds us of our own chaos. We move our gaze to the shallows and appreciate the relative calm, the increased clarity. Our shoulders begin to drop, clenched jaw relaxes, breathing slows, and that one string is finally brought into tune with the rest – resonance. We begin to fall out of suspension and gravity overcomes our aimless attempts at drag. We look down at our own feet, at the billion grains of sand that surround us on the bank and realize that finally, we too have now settled.

Photo Credit: David Hanson, www.davidhanson3.com