Streams II: Classifications

Terms: influent, effluent, perennial, intermittent, ephemeral, loosing and gaining streams, channel habit or pattern, straight, braided, anabranching (anastomosing) meandering, point bar, thalweg, longitudinal bar, cutbank, cutoff, pool, riffle, oxbow lake, natural levee, backswamp, suspended-load channel, mixed-load channel, bedload channel, helical flow, hydraulic geometry, graded stream, steady state equilibrium, stream power, dynamic equilibrium, aggradation, degradation, transport limited, supply limited, youth, mature, old age, William Morris Davis, consequent, antecedent, subsequent, insequent, superimposed. How well do you know your terms? Crossword: Interactive Web version / PDF

Classic Fluvial Classification Systems

Terms derived from these classifications are used frequently in geology, so take time to become familiar with them.  All of these classifications are based on one or more concepts or characteristics of a stream that reflect process and behavior.  The Rosgen method (Rosgen and Silvey, 1996) presented at the end of this section, incorporates those attributes measurable in the field to the classification of streams.  Because of it systematic approach and usefulness when comparing different stream types, most governmental and environmental agencies use the Rosgen method of stream classification.

1. *Constancy of flow
2. *Contribution to or from the groundwater table
3. *Downstream gains or losses in flow
4. Channel morphology
5. Channel composition
6. Equilibrium conditions
7. Depositional or erosional regime
8. Genetic classification
   

Constancy of flow

Perennial streams flow all year around because their channels are in constant contact with the groundwater table. Most well developed streams in New England are perennial.

Intermittent (seasonal) streams flow continuously only during certain seasons when the groundwater table is high.  Fingertip tributaries are often intermittent.

Ephemeral streams, such as arroyos in the western U.S., never intersect the water table and therefore flow only when it rains. Gullies and rills are ephemeral channels as are canyons in arid regions where the ground water table is very deep. Back to fcs

Contribution to or from the groundwater table

Influent streams contribute water to the groundwater table.  They are a type of loosing stream (see below) because discharge decreases downstream as water percolates into the bed.

Effluent streams receive water from the groundwater table. Perennial streams are effluent all  year around whereas intermittent streams are influent or effluent on a seasonal basis. Back to fcs

Downstream changes in flow

Loosing stream: A stream that experiences a downstream decrease in discharge.

The tendency is for streams to increase discharge downstream as their drainage networks expand.   However, there are a number of condition that will cause streamflow to decrease, such as climate, water usage, and depth of water table.  The Niger and Nile rivers progress from humid to arid regions where there is less recharge and greater evaporation, therefore their overall discharge decreases downstream.  The low discharge of the Colorado River as it enters Mexico results from withdrawal of water for metropolitan areas. Arroyos and distributaries on arid alluvial fans loose water through percolation into the bed. 

Niger River NASA visible earth, URL http://visibleearth.nasa.gov/view_rec.php?id=2243

Nile River  NASA URL: http://earth.jsc.nasa.gov/debrief/ISS002/nileFiles/ISS002-701-10.htm

Colorado River NASA Earth Observatory, URL: http://eol.jsc.nasa.gov/EarthObservatory/ColoradoRiverDeltaBajaCalifornia.htm

gaining stream: A stream experiencing increased discharge downstream. Most effluent streams are gaining streams. Back to fcs

Channel Composition

Alluvial channels can be classified by the type of load composing their channel:

1. Suspended-load channel: <3% of particle load is bedload
2. Mixed-load channel: 3-11% is bedload
3. Bed-load channel: >11% is bedload Back to fcs

Alluvial Channel Patterns (straight, meandering, braided, anabranching)

transports the eroded sediment downstream where it's deposited in the low velocity zone on the inside bank of the next meander. Such accumulations are called pointbar deposits. Meander inflections, or crossovers, often contain shallow riffles, particularly in gravel streams.  A meander bend can become isolated when opposing cutbanks on a broadly looping meander intersect.  The intersection is called a meander cuttoff and the isolated meander segment an oxbow lake. A chute cuttoffs occurs when a new channel is carved through the neck by a flood.  Avulsion is the periodic switching or relocation of a channel.

Why do streams meander?

This is a two part question, neither of which has a universally accepted answer.

1. What mechanisms are responsible for meander formation?

A popular hypothesis is that sediment failure into a channel will deflect the flow thereby initiating an oscillatory pattern that will generate meander formation downstream.  This likely occurs but doesn't explain meandering channels in rock or the surface of glaciers.  Another hypotheses suggests that meandering is generated by eddies or vortices form in respond to bed irregularities.

2. Why do stream meander?

Meandering helps a stream balance energy.   Meander formation reduces channel gradient (Δe/L), which governs the rate at which PE is transferred to KE (stream power). When a stream has more energy than it can dissipate through turbulence and sediment transport it will carve meanders to reduce its gradient and stream power.

Figure 9. Various types of cutoffs.  Meander cutoffs and the formation of oxbow lakes are largely restricted to suspended load channels with broad looping meanders.

Braided channel: woven channel

Braided streams contains numerous longitudinal bars which force the channel to bifurcate into numerous ever-changing threads. The bars are covered during high flows and emerge during low flows forming a braided plain.   Braided channels typically have high bedload, variable discharge, and poorly vegetated, easily eroded banks.  Glacial outwash streams are braided because of their high sediment load and seasonally variable discharge.  A meandering stream can become locally braided in reaction to a sudden influx of sediment from a bank or tributary.

Properties of meandering vs. braided streams Characteristics Meandering (fig. 8) Braided (fig. 9) *Discharge (Q) stable-moderately variable highly variable *Load suspended > bedload high bedload *Bank erodibility low to moderate high *Bank composition clay, till, or silt sand, gravel *Bank vegetation good poor channel gradient moderate-to-low moderate-to-high Width/Depth low high Table 3. Varying characteristic between meandering and braided channels. An * marks causative characteristics.	Figure 10. Meandering Yukon River, Yukon flats northeastern Alaska. Note the oxbow lakes and successive point bar deposits. From Oil and Gas Assessment of Yukon Flats, East-Central Alaska, 2004, U.S. Geological Survey Fact Sheet 2004-3121 2004
		Figure 11. Braided channel of the Platte River in Central Nebraska. Note the longitudinal bars in the channel. from Zelt and Frankforter Water-Quality Assessment of the Central Nebraska Basins?\Entering a New Decade, U.S. Geological Survey Fact Sheet 013-03

Anabranching (Anastomosing or multichannelled):

These streams appear superficially similar to braided streams except the bars or islands are not formed by contemporaneous deposition but by erosion. Anabranching streams have more than one channel separated by stable vegetated islands that are rarely covered during floods.  

• Burk and Hamilton Rivers Queensland Australia (NASA GFS)
• Bramaputra River (NASA GFS) Back to fcs

Braided and Anabranching Channels

Figure 11. Braided streams contain mobile bars that are covered during flooding and emerge during low flow.	Figure 12. Anabranching (Anastomosing) rivers have more than one permanent channels that flow around stable islands.

Equilibrium conditions

Graded Stream (steady state; balanced, regime)

The characteristics of a stream's channel reflects the balance between the available energy (discharge and gradient) and the work required to move water and sediment through the channel. For example, if the stream is out of balance then it will make the following adjustments.

1. channel scouring: Results = lowers gradient, increases depth and/or width and decreases velocity
2. deposition: Results = increases gradient and increases velocity, thereby facilitating bedload transport

The table below outlines the variables that can and can't be adjusted by a stream.

Independent Variables (Streams cannot adjust these)	Semidependent variables (Variables that interact)	Dependent (dependent on all others)
Discharge entering the channel Sediment load entering the channel Ultimate base level	channel width channel depth bed roughness (bedforms) grain size of load (what it picks up) channel pattern	slope (adjusted through erosion, deposition, or changing sinuosity)
Stream Power per unit area of the bed = pgQ/w = pgdvs

An alluvial stream which regulates its various parameters (depth, width, slope, velocity, etc.) to obtain the most efficient conditions for flow and sediment transport is known as a graded stream. A graded stream therefore is capable of maintaining a steady-state conditions over time, primarily over time intervals perceived by humans.  Below are two views of equilibrium.  The view on the left is of graded stream or reach as perceived on a yearly basis.  The stream erodes and deposits about a mean value and appears to be in a steady-state condition.  However, over the long interval of geologic time most streams are progressively eroding the landscape.  Dynamic equilibrium involves fluctuations about a moving average.  At present streams are still recovering from changes in sea level, sediment influx, and climate change following glaciation.

Views of Equilibrium

Figure 13.  Perception of equilibrium depend on the time scale considered.  Over the short period streams and channel reaches appear to be in a steady-state equilibrium (left), but in fact are changing over millions of years (right).

Characteristics of the profile of a graded (steady state) stream or reach:

1. Slope of the longitudinal profile is concave upward, increasing exponentially upstream.  Because sediment up stream is coarse and flow is less, a steeper slopes and greater kinetic energy are required to transport it. (Potential energy is transferred to kinetic energy at a faster rate along a steep gradient).
2. No falls or basins exist within the profile.
3. No net erosion or deposition occurs along its channel. (Input = Output)
4. The stream is capable of handling all sediment introduced to it from its tributaries. (Input = Output)

Non-graded stream

Energy is not evenly distributed along the profile of a non-graded stream as evidenced by the presence of falls and basins. Falls result in a concentration of energy, which promotes erosion, while basins result in a decrease in energy, which promotes deposition.

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Aggradational and Degradational streams (non-graded, non-regime state)

Changes in basel level will cause a stream to either deposit (aggrade) or erode (degrade) it's channel.  An increase in base level, such as that caused by subsidence, damming, or rising sea level will reduce the gradient and cause a stream to deposit sediment.  A lowering of base level will cause incision, or entrenching (e.g. the Goosenecks of the San Juan).  One of the best examples of a non-regime stream is the Colorado River.  Uplift of the Colorado Plateau, along with other factors, initiated incision 5-3 ma. More recently, construction of of the Grand Canyon and Hoover dams have created local base levels that not only promote deposition upstream, but erosion downstream.  Discharge at the mouth has been so reduced that sediment no longer reaches the delta plain.  Ultimately, If total stream power is greater than that required to transport the sediment provided it (supply limited), then the stream will erode. If stream power is less (transport limited) than that required the stream will aggrade.

Total power available for erosion is decreased by:

1. decreasing the gradient through erosion
2. decreasing gradient by increasing base level (e.g. rising sea level, damming, subsidence, etc)
3. increasing sediment discharge from tributaries (transporting sediment expends energy)
4. decreasing discharge (climatic changes, withdrawal or diversion of water by man, percolation of water through stream bed, etc).

Questions to ponder: Why does a dam typically results in deposition above it and erosion below it? Why is the Nile River delta eroding?

Figure 14. Generalized cross-sections and profiles of non-regime (degradational and aggradational) and regime (equilibrium) states.

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Genetic classification

• Consequent stream: developed as a consequence of regional tilting. Consequent streams follow the regional slope.
• Subsequent stream: A secondary, structurally controlled drainage developed independently of the primary consequent drainage.
• Superimposed stream: Stream pattern developed on an overlying rock sequence (e.g. horizontal strata) that is superimposed and maintained on an underlying, structurally different series of rocks (e.g. folded strata).
• Captured stream or drainage: A stream or drainage system that is diverted and incorporated into an adjoining basin.  Captured streams typically have tributaries that are barbed.
• Obsequent stream: A stream that locally flows opposite to the regional slope.
• Antecedent stream: A stream that was established prior to the growth of the structure across which it cuts. The stream was able to maintain its course as the structure was uplifted.
• Insequent stream: A stream developed entirely through the processes of headward erosion. 
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Fluvial stages of development

William Morris Davis (1850-1934), physical geographer and Harvard Professor, developed several theories of landscape evolution including a fluvial cycle where he envisioned the progressive evolution of streams from youthful to old age. Each stage was defined by particular morphologic elements.

Figure 14. Davisian evolution of a stream.

Youthful (initial): Narrow v-shaped valley, no floodplain, steep gradient Mature (intermediate): broad valley with flood plain, meandering stream, lower gradient Old Age (terminal): river meanders over a broad plain with oxbow lakes, stream gradient of very low Rejuvenation: change in base level renews youthful conditions

Systematic Classifications and Field Measurements

Rosgen method of stream classification is a three tiered system that sorts streams into major categories within the setting of the landscape from the fingertip tributary to the main valley trunk.  Measureable attributes such as channel pattern sinuosity, slope entrenchment, and width/depth ratio are all used to determine stream type.

Read:

Fundamentals of the Rosgen Stream Classification System- Module assembled by the EPA based on Rosgen, D.L. and H.L. Silvey (1996).

Guide to field techniques: - The whys and hows to stream-related field studies

Harrelson, Cheryl C; Rawlins, C. L.; Potyondy, John P., 1994, Stream channel reference sites: an illustrated guide to field technique: Gen. Tech. Rep. RM-245. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 61 p.

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 Lindley Hanson/Department of Geological Sciences/Salem State College/Geomorphology/GeoIndex/QkRef