Stream load is a geologic term referring to the solid matter carried by a stream

Stream

A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...

 (Strahler and Strahler, 2006). Erosion

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 continually removes mineral material from the bed

Stream bed

A stream bed is the channel bottom of a stream, river or creek; the physical confine of the normal water flow. The lateral confines or channel margins, during all but flood stage, are known as the stream banks or river banks. In fact, a flood occurs when a stream overflows its banks and flows onto...

 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

Velocity

In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

, 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).

Types of stream erosion

There are two main sources of stream erosion: hydraulic action and abrasion

Abrasion (geology)

Abrasion is the mechanical scraping of a rock surface by friction between rocks and moving particles during their transport by wind, glacier, waves, gravity, running water or erosion. After friction, the moving particles dislodge loose and weak debris from the side of the rock...

. All of the materials added to normal stream flow through these processes increase the overall stream load (Strahler and Strahler, 2006).

Hydraulic Action

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 particles, easily loosens and erodes smaller alluvial

Alluvium

Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...

 matter, such as gravel

Gravel

Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

, sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, silt

Silt

Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

 and clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 (Mangelsdorf, 1990). One powerful example of hydraulic action is bank caving, which normally occurs when a stream loosens sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 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

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).

Types of stream load

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 load

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).

Suspended load

Suspended load is composed of fine sediment particles suspended and transported through the stream. These materials are too large to be dissolved, but too small to lie on the bed of the stream (Mangelsdorf, 1990). Stream flow keeps these suspended materials, such as clay and silt, from settling on the stream bed. Suspended load is the result of material eroded by hydraulic action at the stream surface bordering the channel as well as erosion of the channel itself. Suspended load accounts for the largest majority of stream load (Strahler and Strahler, 2006).

Bed load

Bed load rolls slowly along the floor of the stream. These include the largest and heaviest materials in the stream, ranging from sand and gravel to cobbles and boulders. There are two main ways to transport bed load: traction and saltation

Saltation (geology)

In geology, saltation is a specific type of particle transport by fluids such as wind or water. It occurs when loose material is removed from a bed and carried by the fluid, before being transported back to the surface...

. Traction describes the “scooting and rolling” of particles along the bed (Ritter, 2006). In stream load transport, saltation is a bounce-like movement, occurring when large particles are suspended in the stream for a short distance after which they fall to the bed, dislodging particles from the bed. The dislodged particles move downstream a short distance where they fall to the bed, again loosening bed load particles upon impact (Ritter, 2006).

Flood and stream load

Floods create a scenario in which stream flow and velocity are unusually high due to the drastic addition of water to a stream. These heightened characteristics increase both the potential of stream erosion and heavier stream load (Knighton, 1998). Flooded streams are often responsible for heavy sediment transportation and deposition downstream. Stream capacity is greatly increased during a flood (Knighton, 1998). During a flood, increased suspended load may be visible, giving the stream a muddy color.