|Geoindicators | GEOIN | Publications | Events | Links|
|Applying geoindicators | Checklist | Image gallery | Scientific Contributors | Contact Us|
|home | back | all documents||mail to friend | printer friendly version|
Alluvial streams (rivers) are dynamic landforms subject to rapid change in channel shape and flow pattern.
Water and sediment discharges are the principal determinants of the dimensions of a stream channel (width,
depth, and meander wavelength and gradient). Physical characteristics of stream channels, such as width/depth
ratio and sinuosity, and types of pattern (braided, meandering, straight) are significantly affected by changes
in flow rate and sediment discharge, and by the type of sediment load in terms of the ratio of suspended to bed
load [see stream sediment storage and load]. Changes in stream morphology
within a few years indicate changes in
water and/or sediment discharge. For example, increases in width indicate an increase in discharge and/or an
increase in coarse sediment load, and decreases indicate the opposite. Width/depth ratio tends to increase with
bank erosion and with higher coarse loads. Short-term changes may be in response to a specific flood event, whilst
longer-term changes over a sequence of events may reflect fundamental alterations in discharge and/or sediment
load. The pattern and degree of development of active bars are good indicators of sediment load. Channels in
arid and semi-arid areas are highly variable in form, whereas those with perennial flow exhibit a much closer
relationship between dimensions and discharge. Higher energy channels and those in less consolidated materials
are the most dynamic and sensitive. Because changes in alluvial channel cross-section, especially width, may
indicate change in streamflow characteristics, known discharges can be expressed as a simple power relation with
channel width (Q = aWb, where Q = discharge, a = a coefficient, W = channel width, and b is an exponent). At
ungauged stream sites, therefore, measurements of channel morphology can lead to indirect estimates of discharge,
which, if varying with time, may indicate changes in mean discharge or in the occurrence of floods at specific
Steep mountain streams, tumbling over large cobbles and boulders, may remain stable for years or decades until a very large storm delivers sufficient water to cause a channel scouring flood to occur. Such channels are not true alluvial channels, and the relations among the hydraulic and sediment factors are quite different from the quasi equilibrium conditions existing in the alluvial rivers of drylands and flood-plains that are the main subject of this geoindicator.
SIGNIFICANCE: Stream morphology is sensitive to changes in magnitudes of water and sediment discharges. In the absence of hydrologic and streamflow records, an understanding of stream morphology can help delineate environmental changes of many kinds. Channel instability can affect floodplain and valley land uses directly through erosion and deposition or by change in the frequency of flooding. Flood related changes in stream channel morphology and position can be very rapid, particularly in arid and semi-arid areas, and place significant limits on land use (e.g. on channel islands or along banks undergoing erosion). Diversity in channel morphology and associated sediments, which form the substrate and habitats for ecological assemblages, is a key component and necessary condition for biodiversity. The natural environment is best maintained by allowing stream channels to function normally through erosion, deposition and morphological change.
HUMAN OR NATURAL CAUSE: Changes in stream morphology reflect changes in the hydrological balance and runoff and/or in the sediment supply. These may be due to land use changes, such as land clearance, farming and forest harvesting or climate changes or a combination of both. Reservoir construction and river stabilisation measures may have significant effects. Assumptions that most major changes in stream morphology in historical and decadal timescales are due to human causes have now been replaced by evidence that climatic variations even at this timescale and in all environments can be at least as important, if not a more significant causal factor. Distinguishing climatic from human causes remains a challenge in many situations. Active tectonics can cause changes in stream morphology that are usually recognizable. Stream channels that have been directly modified and engineered will try to regain their natural morphology unless the modifications are continually maintained.
ENVIRONMENT WHERE APPLICABLE: Streams flowing through alluvium or other unconsolidated material in terraces and flood plains.
TYPES OF MONITORING SITES: Characteristic stream reaches and cross-sections.
SPATIAL SCALE: mesoscale to regional
METHOD OF MEASUREMENT: Repeated ground surveys using GPS, total station or theodolite, or aerial surveys using digital or analytical photogrammetry, image analysis. Cross and gradient profiles by repeat surveying. Water and flood levels by survey, mapping and streamflow gauges. Flow and sediment load by automated and manual measurements. Simple monitoring of form, especially channel position, channel pattern and incidence of bars by repeat ground photography. Change in sediment features and vegetation cover by sampling of bars or monitoring by fixed quadrants and photography. Contemporary monitoring can be complemented by historical documentary evidence, especially of channel form and position from maps and aerial photographs [see stream sediment storage and load].
FREQUENCY OF MEASUREMENT: Depends on observed rate of change, but no less than once every 5 years. Annual measurement of highly dynamic channels is preferable. Changes should be measured after major flood events.
LIMITATIONS OF DATA AND MONITORING: It is difficult to assess morphological change without historical records. Measurements need to be continued over several years. Floods or channel movement may destroy observation sites.
APPLICATIONS TO PAST AND FUTURE: Relict channels can indicate previous channel position (horizontal and vertical) and dimensions. Monitoring of channel morphology extends understanding of types and rates of responses to environmental changes. Effects of both individual events and of phases of discharge or sediment input can then be predicted. Longer-term adjustments may also be evident in progressive aggradation or incision related to tectonics, drainage changes or base-level changes. Adjustments are propagated through channel systems and may take a long time to reach all parts. Data may possibly be useful for predictions of up to several decades
POSSIBLE THRESHOLDS: Erosive floods can alter stream-channel morphology nearly instantaneously; adjustment to a disturbance may be initially rapid but slower after 5 to 10 years. Severe bank erosion during floods can lead to major channel switching: the Yellow River in China has large-scale abandoned channels separated by 600 km. Meander amplitude can increase to intersection and cutoff. Stream variability involving changes between straight, meandering, and braided forms can be sharp and reflect changes in sediment load or valley gradients as a result of active tectonics, tributary influences or progressive aggradation or incision.
Gurnell, A. & Petts, G. (Eds.) 1995. Changing River Channels, John Wiley & Sons, Chichester.
Miller, A. J. & Gupta, A. (Eds) 1999. Varieties of Fluvial Form. John Wiley & Sons, 538 pp.
Rapp, C.F. and T.B. Abbe 2003. A framework for delineating channel migration zones. Washington State, Dept. Transportation. (www.ecy.wa.gov/biblio/0306027)
Rosgen, D. L. 1996. Applied River Morphology. Wildland Hydrology Books, Pagosa Springs, USA.
Schumm, S.A. & B.R.Winkley (eds) 1994. The variability of large alluvial rivers. New York: American Society of Civil Engineers Press.
OTHER SOURCES OF INFORMATION: Hydrology/water and river agencies, departments of highways (at bridge crossings), geological surveys, historical archives.
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Condition of riverine ecosystems; stability of islands and channels, and jurisdictional boundaries defined by rivers.
OVERALL ASSESSMENT: Monitoring stream channel morphology can be useful when no data are available on sediment load, flow rates and other hydrologic parameters.
|© Geoindicators Initiative GEOIN|