Stream load is a geologic term referring to the solid matter carried by a stream (Strahler and Strahler, 2006). Erosion and bed shear stress continually remove mineral material from the bed and banks of the stream channel, adding this material to the regular flow of water. The amount of solid load that a stream can carry, or stream capacity, is measured in metric tons per day, passing a given location. Stream capacity is dependent upon the stream’s velocity, the amount of water flow, and the gradation (because streams that occur on steeper slopes tend to have greater flow and velocity) (Strahler and Strahler, 2006).
There are two main sources of stream erosion: hydraulic action and abrasion. All of the materials added to normal stream flow through these processes increase the overall stream load (Strahler and Strahler, 2006).
Hydraulic action describes the erosion caused by the dragging of water over the stream bed and bank. This dragging, coupled with the impact of small parties, easily loosens and erodes smaller alluvial matter, such as gravel, sand, silt and clay (Mangelsdorf, 1990). One powerful example of hydraulic action is bank caving, which normally occurs when a stream loosens sediment and undercuts a bank. Consequently, large masses of sediment slump and collapse into the stream, adding significantly to the stream’s load (Strahler and Strahler, 2006). The severity of hydraulic action increases with stream velocity and current stream load.
Abrasion occurs when larger rock particles roll and strike against bedrock walls, chipping and splintering particles and pieces of rock (Strahler and Strahler). As these cobbles and boulders roll across the stream bed, they continue to crush and grind the bedrock, producing an assortment of eroded rock sizes (Ritter, 2006). Again, the severity of this type of erosion is dependent upon stream velocity and stream load (i.e. the presence of larger rock particles)..
Mineral materials of many different shapes and particle sizes erode and contribute to overall stream load. Differences in the size of those materials determine how they will be transported down stream. Stream load is broken into three types: dissolved load, suspended load, and bed load (Ritter, 2006).
Dissolved matter is invisible, and is transported in the form of chemical ions. All streams carry some type of dissolved load. This type of load can result from mineral alteration from chemical erosion, or may even be the result of groundwater seepage into the stream. Materials comprising the dissolved load have the smallest particle size of the three load types (Strahler and Strahler, 2006).