Streams are Earth’s most important erosional agents. In addition to having the ability to deepen and widen their channels, streams also have the capacity to transport enormous quantities of sediment that are delivered by sheet flow, mass movement, and groundwater. Eventually much of this material is deposited to create a variety of landforms.

Stream Erosion

A stream’s ability to accumulate and transport soil and weathered rock is aided by the work of the weathering, erosion, and mass movement processes. When stream flow is confined in a channel, the erosional power of a stream is related to its slope and discharge. When the flow of water is sufficiently strong, it can dislodge particles from the channel and lift them into the moving water. In this manner, the force of running water swiftly erodes poorly consolidated materials on the bed and sides of a stream channel. On occasion, the banks of the channel may be undercut, dumping even more loose debris into the water to be carried downstream.

In addition to eroding unconsolidated materials, the force of water in a stream can also cut a channel into the landscape. A stream’s ability to erode is greatly enhanced by the particles it carries. These particles can be any size, from large boulders in very fast-flowing waters to sand and gravel-size particles in somewhat slower flow. Just as the particles of grit on sandpaper can wear away a piece of wood, so, too, can the sand and gravel carried by a stream abrade a bedrock channel. Moreover, pebbles caught in swirling eddies can act like “drills” and bore circular potholes into the channel floor (Figure 1).

Chemical weathering also impacts the erosion of river channels. Streams flowing over soluble bedrock, such as limestone or sandstone, are susceptible to a form of chemical erosion called corrosion. In this process, bedrock is gradually dissolved as stream water carrying corrosive solutions comes into contact with the bedrock and breaks apart its mineral matter.

Transportation of Sediment

All streams, regardless of size, transport some weathered rock material (Figure 2). Streams also sort the solid sediment they transport because finer, lighter material is carried more readily than larger, heavier particles. Streams transport their load of sediment in three ways: (1) in solution (dissolved load), (2) in suspension (suspended load), and (3) sliding, rolling, or bouncing along the bottom (bed load).

Dissolved Load

Most of the dissolved load is brought to a stream by groundwater and is dispersed throughout the flow. When water percolates through the ground, it acquires soluble soil compounds. Then it seeps through cracks and pores in bedrock, dissolving additional mineral matter. Eventually much of this mineral-rich water finds its way into streams.

The velocity of streamflow has no effect on a stream’s ability to carry its dissolved load; material in the solution goes wherever the stream goes. Precipitation of the dissolved mineral matter occurs when the chemistry of the water changes, when organisms create hard parts, or when the water enters an inland “sea,” located in an arid climate where the rate of evaporation is high.

Suspended Load

Most streams carry the largest part of their load in suspension. Indeed, the muddy appearance created by suspended sediment is the most obvious portion of a stream’s load (Figure 3). Usually only fine silt- and clay-size particles can be carried this way, but during a flood, larger particles are transported as well. During a flood, the total quantity of material carried in suspension increases dramatically, as people whose homes have been flooded can attest.

The type and amount of material carried in suspension are controlled by two factors: the stream’s flow velocity and the settling velocity of each sediment grain. Settling velocity is defined as the speed at which a particle falls through a still fluid. The larger the particle, the more rapidly it settles toward the streambed (Figure 4). In addition to size, the shape and specific gravity of particles also influence settling velocity. Flat grains sink through water more slowly than do spherical grains, and dense particles fall toward the bottom more rapidly than do less dense particles. As long as flow velocity exceeds settling velocity, sediment remains suspended and is transported downstream. Deposition occurs when flow velocity falls below the settling velocity of a particle.

Bed Load

A portion of a stream’s load of solid material consists of sediment that is too large to be carried in suspension. These coarser particles move along the bottom (bed) of the stream and constitute the bed load. In terms of the erosional work accomplished by a downcutting stream, the grinding action of the bed load is of great importance.

The particles that make up the bed load move by rolling, sliding, and saltation. Sediment moving by saltation (saltare = to leap) appears to jump or skip along the stream bed. This occurs as particles are propelled upward by collisions or lifted by the current and then carried downstream a short distance until gravity pulls them back to the bed of the stream. Particles that are too large or heavy to move by saltation either roll or slide along the bottom, depending on their shapes.

Compared with the movement of suspended load, the movement of bed load through a stream network tends to be less rapid and more localized. A study conducted on a glacially fed river in Norway determined that suspended sediments took only a day to exit the drainage basin, while the bed load required several decades to travel the same distance. Depending on the discharge and slope of the channel, coarse gravels may only be moved during times of high flow, while boulders are moved only during exceptional floods. Once large particles are set in motion, they are usually carried short distances. Along some stretches of a stream, bed load cannot be carried at all until it is broken into smaller particles.

Capacity and Competence

A stream’s ability to carry solid particles is described using two criteria: capacity and competence. Capacity is the maximum load of solid particles a stream can transport per unit of time. The greater the discharge, the greater the stream’s capacity for hauling sediment. Consequently, large rivers with high flow velocities have large capacities.

Competence is a measure of a stream’s ability to transport particles based on size rather than quantity. Flow velocity is the key: Swift streams have greater competencies than slow streams, regardless of channel size. A stream’s competence increases proportionately to the square of its velocity. Thus, if the velocity of a stream doubles, the impact force of the water increases four times; if the velocity triples, the force increases nine times, and so forth. Hence, large boulders that are often visible during low water and seem immovable can, in fact, be transported during exceptional floods because of the stream’s increased competence.

By now, it should be clear why the greatest erosion and transportation of sediment occur during floods. The increase in discharge results in greater capacity, and the increased velocity produces greater competency. Rising velocity makes the water more turbulent, and larger particles are set in motion. In just a few days, or perhaps a few hours, a stream at flood stage can erode and transport more sediment than it does during many months of normal flow.

Deposition of Sediment

Deposition occurs whenever a stream slows, causing a reduction in competence. Put another way, particles are deposited when flow velocity is less than the settling velocity; as a stream’s flow velocity decreases, sediment begins to settle, largest particles first. In this manner, stream transport provides a mechanism by which solid particles of various sizes are separated. This process, called sorting, explains why particles of similar size are deposited together.

The general term for sediment deposited by streams is alluvium. Many different depositional features are composed of alluvium. Some occur within stream channels, some occur on the valley floor adjacent to a channel, and some are found at the mouth of a stream. We will consider the nature of these features later in the chapter.

• Streams erode when turbulent water lifts loose particles from the streambed. The focused “drilling” of the stream armed with swirling particles also creates potholes in solid rock.

• Streams transport their load of sediment dissolved in water, in suspension, and along the bottom (bed) of the channel.

• A stream’s ability to transport solid particles is described using two criteria: Capacity refers to how much sediment a stream is transporting, and competence refers to the particle sizes the stream is capable of moving.

• Streams deposit sediment when velocity slows and competence is reduced. This results in sorting, the process by which like-size particles are deposited together.

• alluvium: Unconsolidated sediment deposited by a stream.

• bed load: Sediment that is carried by a stream along the bottom of its channel.

• capacity: The total amount of sediment a stream is able to transport.

• competence: A measure of the largest particle a stream can transport; a factor that is dependent on velocity.

• dissolved load: The portion of a stream’s load that is carried in solution.

• potholes: Circular depressions in bedrock stream channels created by the abrasive action of particles swirling in fast-moving eddies.

• saltation: Transportation of sediment along a stream bed through a series of leaps or bounces.

• settling velocity: The speed at which a particle falls through a still fluid. The size, shape, and specific gravity of particles influence settling velocity.

• sorting: The process by which solid particles of various sizes are separated by moving water or wind. Also, the degree of similarity in particle size in sediment or sedimentary rock.

• suspended load: The sediment carried within the body of flowing water.