Student Question

# What are the differences between stream capacity and stream competency? How does it relate to "suspended load"?

Gravity is the influence that drives the water channeled downhill in streams. A stream is defined as all channelized movement of water, including large movements of water in rivers, such as the Yangtze River and the Mississippi River. Channel, channelized and channelizing are defined as the eroded pathway that the stream of water follows and the fact of these waters following eroded pathways.

Stream capacity is the total quantity of sediment that a stream can carry, move, transport. Streams of water carry sediment. Each stream has a capacity for what can be carried by the volume and velocity (force) of water being channelized.

Stream competence is the size of sedimentary particle that can be carried, moved, transported in a channelized stream of water. Each stream has a competency for what size of sedimentary particle it can move by its force (volume and velocity). This competency may change as stream volume and flow of velocity change due to seasonal flooding and other factors of topography.

The difference between stream capacity and stream competence is that capacity measures quantity of sediment while competence measures size of particles comprising sediment. Some sedimentary particle sizes from large to small (or coarse to fine) are boulder, rock, pebble, sand, silt, clay.

Under normal circumstances, since channelized streams are influenced by the force of gravity, the major factor affecting stream capacity (quantity of sediment) and stream competence (sediment particle size) is channel slope. Channel slope derives the measurement of stream gradient. While each stream has its own slope, various streams may share the same gradient. Channel slope is the measure of the angle, from a horizontal zero degree slope, of the degree of slope of a given stream of water. The channel slope is measured using two points some distance from each other. The difference in the elevation at the two points divided by the linear distance between the points equals the channel slope and yields the stream gradient: e.g., (1) points A and B are at elevations of 300 feet and 100 feet, with a distance between of 20 yards, so the stream gradient is a 10 degree angle; (2) points C and D are at elevations of 360 ft and 140 ft, with a distance between of 20 yards, so the stream gradient is a 10 degree angle; the gradients are the same though the slopes of elevation of the channels are different.

The stream flow velocity (i.e., speed of a quantity of material past a given point during a specific time interval) of a stream of water is directly related to the channel slope of the stream. The greater the slope, the higher the flow velocity; the lesser the slope, the lower the flow velocity. Examples are the greater slope of the Colorado River flowing through the Grand Canyon and the lesser, almost flat, slope of the Mississippi River delta flowing into the Gulf of Mexico. The greater Colorado River slope results in high velocity and concordantly high stream capacity and competence. The lesser, nearly flat, Mississippi delta slope results in low velocity and concordantly low stream capacity and competence [stream capacity: quantity of sediment; stream competence: size of sediment particle].

Since water streams flow in channels under the influence of gravity, channelization is also a critical component affecting stream capacity (quantity) and stream competence (size), which are both directly related to stream flow velocity. Conversely, channelization and capacity/competence/velocity are indirectly related. As channelization increases, the others decrease: a broad channel has low velocity, lower capacity and lessened competence. Conversely, a narrow channel has high velocity, higher capacity and greater competence. Channelization also affects the stream load, which is related to stream discharge. Stream load is carried on stream discharge and is defined as "the sum of the mass that can be transported by a stream" (Gale Cengage and DiVenere). Stream load is directly related to both stream velocity and stream gradient (derived from channel slope). Stream load (carried on stream discharge) is measured as the quantity/volume of stream material carried/transported across a given point in a specified interval of time [this definition is similar to that for velocity, which is speed of quantity/volume of material past a given point in a specified interval of time]. There are a number of kinds of stream loads with varying components that contribute to stream mass, including:

• suspended load (fine clay and silt particles)
• dissolved load (dissolved particle ions)
• bed load (coarse sand and gravel)
• saltation load (temporarily kicked up sand and gravel that cannot remain in suspension but for brief periods since they exceed stream competency) (DiVenere)

Since stream load is directly related to velocity/capacity/competence, the higher the velocity, the greater the sum of the mass of sedimentary material that can be transported by the stream load. Conversely, the lower the velocity, the lesser the sum of sedimentary material that can be transported by the stream load, which is carried by the stream discharge. Stream load correlates with stream channel width. The broader the channelization, then the more depositional the stream load. The narrower the channelization, then the more suspended the stream load. Broad channels have low velocity and high frequency of allowing sedimentary deposits. Narrow channels have high velocity and lower frequency of allowing sedimentary deposits. Broad channelization creates sedimentary aggrading (build-up), while narrow channelization allows the continued movement, or transportation, of larger sizes of sediment.

Alluvial fans result from depositation with the broadening channelization of mountain streams when they descend to the mountain base. Alluvial fans form when mountain streams channeling runoff reach low gradient (low channel slope) flattening of the land. Alluvial fans may flow into a larger body of water, like a lake or an ocean, but not all do.

alluvial: from Latin alluvium meaning "washed against" or "in water"

When the stream gradient (derived from the measurement of the channel slope) is a low gradient, flow velocity decreases, capacity (amount carriable) decreases and competency (size of particle carriable) decreases. As a result, a significant proportion of the stream load can convert to depositional load and can settle out of the steam flow to form the sedimentary alluvial fan. A function of stream capacity, stream competence and the settling velocity of particles (i.e., the speed at which various sizes particles are apt to leave the stream flow and become deposits) determines the ultimate site of deposition of types and sizes of sedimentary particles in the alluvial fan. Deposits in an alluvial fan may be very fine in size at the edges while coarse and pebbly at the mouth, or opening from the mountain channel, of the alluvial fan. This is due to the broadening channelization and the settling velocity of particles. The function of capacity, competence and settling velocity allow formation of articulated formations that are composed of deposits of "gypsum, limestone, clay, shale, siltstone, sandstone, and larger rock conglomerates" (Gale Cengage).

articulation: the deposit of particles in accordance with the process of sedimentation in a "statistical order connections and successions" (Deleuze).

Even with a low gradient and low capacity (and low competence and velocity), the solution load (dissolved ions of sedimentary particles) retains ions in solution (ions are not usually deposited with sedimentary particles) until the water evaporates, leaving ions behind, or the temperature of the water stream cools sufficiently to allow precipitation of the ions into the sedimentary deposits.

Stream competence can vary proportionally to the volume of runoff, which varies seasonally with greater runoff volume in flood times and lesser runoff volume in other than seasonal flood times. Channelized streams in confined channels with low volume may be able to carry, or transport, only ions, clays, and silt in its solution load and suspension load (silt: fine particles between sand and clay in size). Confined channels may also be able to transport sand short distances as part of its saltation load (saltation load: particles kicked up by the force of the stream flow and carried in the suspension load temporarily, since the particle size exceeds the stream competency). As seasonal flooding increases, stream volume and stream flow velocity in confined channel streams increase, which concordantly increases both capacity and competence. Increased capacity means the stream volume carries greater quantity of sediment. Increased competence means stream load carries greater sedimentary particle sizes. Because of increased competence, the stream may gain enough competence to carry particles as large as "pebbles, cobbles and boulders" (Gale Cengage). If the stream is not confined, then channelization broadens and volume and velocity do not increase resulting in no increase of capacity or competence.

Sources:

Kenn Oberrecht. "Sediment Transport and Deposition," Oregon Department of State Lands

Dr. Vic DiVenere. "Stream Flow and Sediment Transport," Columbia University.

Deleuze and Politics. Eds. Ian Buchanan, Nicholas Thoburn.

"Stream Capacity and Competence." World of Earth Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. Vol. 2. Gale Cengage, 2003.

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