Stream Capacity and Competence

Streams channel water downhill under the influence of gravity. Stream capacity is a measure of the total sediment (material other than water) a stream can carry. Stream competence reflects the ability of a stream to transport a particular size of particle (e.g., boulder, pebble, etc). With regard to calculation of stream capacity and competence, streams broadly include all channelized movement of water, including large movements of water in rivers.

Under normal circumstances, the major factor affecting stream capacity and stream competence is channel slope. Channel slope (also termed stream gradient) is measured as the difference in stream elevation divided by the linear distance between the two measuring points. The velocity of the flow of water is directly affected by channel slope, the greater the slope the greater the flow velocity. In turn, an increased velocity of water flow increases stream competence. The near level delta at the lower end of the Mississippi River is a result of low stream velocities and competence. In contrast, the Colorado River that courses down through the Grand Canyon (where the river drops approximately 10 ft per mile [3 m/1.6 km]) has a high stream velocity that results in a high stream capacity and competence.

Channelization of water is another critical component affecting stream capacity and stream competence. If a stream narrows, the velocity increases. An overflow or broadening of a stream channel results in decreased stream velocities, capacity, and competence.

The amount of material (other than water) transported by a stream is described as the stream load. Stream load is directly proportional to stream velocity and stream gradient and relates the amount of material transported past a point during a specified time interval. The greater the velocity, the greater the sum of the mass that can be transported by a stream (stream load). Components of stream load contributing to stream mass include the suspended load, dissolved load, and bed load. Broad, slow moving streams are highly depositional (low stream capacity) while high velocity streams have are capable of moving large rocks (high stream competence).

Alluvial fans form as stream streams channeling mountain runoff reach flatter (low, slope, low gradient) land at the base of the mountain. The stream loses capacity and a significant portion of the load can then settle out to for the alluvial fan.

The ultimate site of deposition of particular types and sizes of particles is a function of stream capacity and stream competence (along with settling velocity of particles). These factors combine to allow the formation of articulated sedimentary deposits of gypsum, limestone, clay, shale, siltstone, sandstone, and larger rock conglomerates. No matter how low the stream capacity, the solution load usually retains ions in solution until the water evaporates or the temperature of the water cools to allow precipitation.

In confined channels, stream competence can vary with seasonal runoff. A stream with low volume may only be able to transport ions, clays, and silt in its solution and suspension loads and transport sand as part of its saltation load. As stream flow increases the stream competence and during seasonal flooding stream may gain the competence to move pebbles, cobbles and boulders.