The flow of water in rivers and streams constantly moves a significant amount of material, a geological process known as sediment transport. This process is responsible for shaping river channels, creating floodplains, and delivering material to lakes and oceans. The total material a river carries is called its load. The heaviest component of this load is the bed load, which consists of larger, denser particles. Bed load is the sediment that travels along the very bottom of the river channel, maintaining contact with the streambed as it moves downstream.
Defining Bed Load and Its Movement
Bed load is the coarse material that is too heavy to be fully lifted and carried within the water column. This sediment typically includes larger particles like sand, gravel, pebbles, and sometimes even cobbles and boulders, depending on the river’s energy level. The movement of this material occurs in two primary ways: traction and saltation.
Traction describes the process where the largest and densest particles are rolled, pushed, or slid along the riverbed by the force of the flowing water. These massive grains maintain continuous contact with the channel floor as they are slowly driven forward. This action requires a substantial amount of energy from the water.
Saltation involves medium-sized particles, such as fine gravel and sand grains, being lifted temporarily into the flow. They drop back to the bed in a series of short, interrupted leaps. The particle’s momentum causes it to strike the bed upon landing, often dislodging other grains and launching them into the flow, continuing the process. While these particles leave the streambed, their excursions are short-lived, which is why they are still categorized as part of the bed load.
Factors Influencing Bed Load Transport
Bed load transport is directly controlled by the physical characteristics of both the water flow and the sediment itself. Water velocity is the primary driver, as the force exerted by the moving water on the channel floor, known as shear stress, must be strong enough to overcome the particle’s weight and the friction holding it in place. There is a specific threshold known as the “critical velocity,” which is the minimum water speed required to initiate the movement of a particular particle size.
Once this critical velocity is reached, even a slight increase in flow can cause a significant, rapid increase in the rate of bed load movement. The characteristics of the sediment particles, including size, shape, and density, are also influential. Larger, heavier, and more angular particles require a much higher velocity to be set into motion compared to smaller, lighter, or more rounded grains. For instance, an increase in river discharge during a flood event can mobilize massive boulders that remain stationary during normal flow conditions.
The Three Modes of Sediment Transport
The three main modes of sediment transport are bed load, suspended load, and dissolved load, each defined by the particle size and its position in the water column.
The suspended load consists of the lightest, finest particles, such as silt and clay, which are carried continuously aloft within the moving water. These particles are kept in suspension by the water’s turbulence and do not settle out until the flow velocity significantly decreases. This often gives the river a murky or muddy appearance. This load is typically moved high up in the water column, far above the streambed.
The dissolved load is composed of chemical ions and soluble minerals, such as calcium and sodium, which have been dissolved from the surrounding rock and soil. This load moves along with the water molecules and is a significant part of the total material transported by a river, contributing to the water’s chemical composition. The bed load stands apart as the coarsest material that actively interacts with the channel floor.