| Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Streams > What is a Stream? > Stream channel morphology |
Brief Description: Alluvial streams (rivers) are dynamic landforms subject to rapid change in channel shape and flow pattern. Water and sediment discharges determine the dimensions of a stream channel (width, depth, and meander wavelength and gradient). Dimensionless characteristics of stream channels and types of pattern (braided, meandering, straight) and sinuosity 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. Dramatic changes in stream bank erosion within a short time period indicate changes in sediment discharge. Because changes in alluvial channel cross-section, especially width, may indicate change in the discharge characteristics of the stream, 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 recurrence intervals. Significance: Channel dimensions reflect magnitude 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. Changes in stream pattern, which can be very rapid in arid and semi-arid areas, place significant limits on land use, such as on islands in braided streams and meander plains, or along banks undergoing erosion. Human or Natural Cause: Significant changes in stream dimensions, discharge and pattern may reflect human influences such as water diversion and increased sediment loads resulting from land clearance, farming, or forest harvesting. Such variations are also responsive to climatic fluctuations and tectonics. Environment Where Applicable: Alluvial streams occupying bottomlands comprised of terraces and flood plains. Types of Monitoring Sites: Characteristic stream reaches Spatial Scale: mesoscale / regional Method of Measurement: Repeated ground and/or aerial surveys of channel patterns and cross- sections, using streamflow gauges, channel cross-section monuments, and other automated and manual loggers. Frequency of Measurement: Depends on observed rate of change, but no less than once every 5 years. Limitations of Data and Monitoring: It is difficult to gauge stream change without historical records. Floods may destroy observation sites. Applications to Past and Future: Data can be used for predictions of up to about 10 years. Possible Thresholds: Meander amplitude can increase until cutoff is inevitable. Stream variability involving changes between straight, meandering, and braided forms can reflect changes on valley gradients as a result of active tectonics and tributary influences. |
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