The Mechanism of Movement
Saltation begins when the velocity of a moving fluid (wind or water) reaches the fluid threshold, the minimum speed required to overcome the inertia of a resting particle. Once this threshold is met, the fluid exerts two main forces on the sediment grain: a drag force that pushes it forward and a lift force that pulls it upward from the surface. This lift is strong enough to briefly launch the grain, but not powerful enough to keep it suspended indefinitely in the flow.
After being launched, the particle follows a characteristic parabolic trajectory, pulled back toward the surface by gravity. The particle is accelerated horizontally by the fluid during its flight, causing it to travel a distance significantly longer than the height of the jump. This height is usually only a few centimeters above the ground in wind. This trajectory is often described as ballistic, meaning the path is primarily governed by the initial launch energy, fluid drag, and gravitational pull.
The most distinctive feature of saltation occurs upon landing, where the momentum of the falling particle is transferred to the surface bed. This impact, often called a splash, dislodges other resting particles in a chain reaction, which is the primary mechanism for sustaining the movement. The impact can either cause the original particle to rebound and begin a new jump, or it can eject new grains into the air. Ejecting new grains effectively lowers the energy required for further transport, a condition known as the impact threshold.
Environments Where Saltation Occurs
Saltation is a pervasive process across Earth’s surface, manifesting in distinct ways depending on whether the transporting fluid is air or water. The difference in fluid density and viscosity creates two categories: aeolian and fluvial saltation. Aeolian saltation, driven by wind, is most prominent in arid and semi-arid environments like deserts, along sandy beaches, and in areas with moving snow.
In aeolian saltation, which typically involves medium-sized sand grains between 0.1 and 0.5 millimeters in diameter, the large density difference between the quartz particle and the air allows for high-velocity trajectories. These grains can be launched several centimeters high and travel many meters downwind before landing. Aeolian saltation often accounts for 50 to 90 percent of all sediment movement in a given wind event.
Fluvial or aqueous saltation occurs in flowing water, such as riverbeds and streams, where the fluid is much denser than air. This increased density and viscosity mean the buoyancy and drag forces on the particle are far greater, causing the saltating motion to be less pronounced and the trajectories to be much shorter. Particles in water are lifted only momentarily and remain close to the riverbed, moving in short, rhythmic hops that contribute to the bedload transport of gravel and sand.
Distinguishing Saltation from Other Transport
Saltation exists as an intermediate process between the two other main modes of sediment transport: suspension and traction. The distinction among these three processes is based primarily on the size of the particle and the duration it remains out of contact with the surface. Saltation involves particles that are too large to be held indefinitely in the fluid but are light enough to be lifted briefly by the fluid’s force.
Suspension involves very fine particles, typically silt and clay less than 0.1 millimeters in diameter. These particles are light enough that turbulent eddies within the flow keep them fully mixed within the fluid column. They can remain airborne or in the water for extended periods, traveling hundreds of kilometers before settling.
In contrast, traction, also called surface creep, moves the largest particles, often exceeding 0.5 millimeters in diameter, which are too heavy to be lifted by the fluid at all. These heavy grains move by rolling or sliding along the surface, remaining in continuous contact with the ground. Their movement is often initiated or aided by the impact of saltating grains splashing into them, pushing them forward in short increments.
How Saltation Shapes the Earth
The bouncing action of saltation is a powerful geomorphic agent, shaping landscapes across the globe. The repeated transport and accumulation of sand by aeolian saltation is the primary mechanism behind the formation and migration of large landforms such as sand dunes in desert environments. As grains are moved up the gentle, windward slope of a dune and drop over the crest, the dune structure itself is shifted downwind.
Saltation also causes significant erosion through a process known as abrasion or sandblasting. The high-velocity impact of millions of saltating grains wears down objects in their path, polishing or pitting exposed rock surfaces, power lines, and human-built infrastructure near the ground. Furthermore, the impacts from saltating particles are responsible for dislodging the much finer dust-sized grains from the surface, which are then carried high into the atmosphere by wind in suspension, contributing to the formation of widespread dust storms.