Stream load refers to the entire collection of material a flowing body of water, such as a stream or river, transports from one location to another. This material includes everything from microscopic dissolved minerals to large rocks and boulders. The movement of this load is a fundamental geological process, acting as a natural conveyor belt that shapes the Earth’s surface through continuous cycles of erosion, transport, and deposition. The amount and nature of the material being carried reflect the stream’s energy and its ability to interact with the channel and surrounding landscape.
The Three Types of Stream Load
The total stream load is categorized into three distinct components based on the physical state and location of the material within the water column. The Dissolved Load consists of invisible ions and chemical compounds that are products of chemical weathering, where water reacts with bedrock and soil. These solutes, often including bicarbonate, calcium, sodium, and chloride ions, are carried completely within the water, moving at the same velocity as the flow.
The Suspended Load is composed of fine-grained particles, primarily silt and clay, which are small and light enough to be held up by the water’s turbulence. These particles give the water its cloudy or muddy appearance, especially during periods of high flow. This component often constitutes the largest portion of the total load transported by major river systems.
The Bed Load encompasses the coarser, heavier materials that are too large to remain suspended. This component includes sand, gravel, pebbles, and occasionally large cobbles or boulders. These particles are in constant or intermittent contact with the stream bed, moving much slower than the water or the suspended material above them.
Mechanisms of Sediment Transport
The solid components of the stream load (the suspended and bed loads) are moved downstream through specific physical mechanisms. Suspension is the process that keeps fine materials like clay and silt aloft within the water column. This occurs when the upward forces created by water turbulence exceed the downward pull of gravity on the particles.
Larger, medium-sized particles, such as sand, move primarily through a mechanism called Saltation. The grains are lifted briefly by the current, travel a short distance in a hopping or bouncing motion, and then fall back to the streambed, often dislodging other particles upon impact. This rhythmic, skipping movement is characteristic of material too heavy for continuous suspension.
The largest and heaviest particles, including gravel and small cobbles, are transported by Traction, involving a continuous rolling or sliding motion along the channel floor. These mechanisms—suspension, saltation, and traction—collectively determine how the stream physically moves the sediment, with each requiring a different minimum level of flow energy to overcome the particle’s weight and friction.
Factors Controlling Load Capacity
The amount and size of material a stream can carry are governed by two related measurements: capacity and competence. Stream capacity refers to the maximum total quantity of sediment a stream can transport. Competence, in contrast, is a measure of the largest individual particle size a stream is able to move.
The most significant factor controlling both capacity and competence is the water’s velocity, or speed of flow. An increase in velocity provides substantially more energy to overcome the friction and weight of sediment particles. For example, a slight increase in water speed results in a disproportionately large rise in competence, as the maximum mass a stream can move varies by approximately the sixth power of its velocity.
Another primary control is the stream’s discharge, which is the total volume of water passing a point per unit time. Greater discharge provides a larger body of water to generate turbulence and to move the load, directly impacting capacity. The gradient, or steepness of the channel slope, also influences velocity; a steeper slope accelerates the water, thereby increasing both the stream’s competence and its total capacity.