Steep mountain streams composed of gravel and boulders can take on many forms. For example, a streambed may have alternating sequences of pools and bars formed by sediment deposits, or it may flow in a channel with a relatively featureless planar bed. Here Palucis and Lamb explore why these streambed types tend to occur over a specific range of slopes and whether slope alone determines the shape of a stream channel.
Previous observations of natural streams show that the steepness of a stream channel is linked to its shape. In general, as channel slope increases, channel beds tend to transition from having alternate bars to a plane bed to steps and pools. At the steepest slopes in the channel network, water tumbles over large, individual boulders in cascade-type streams.
Field observations, experiments, and theoretical analyses have also shown that other factors, such as channel width to depth ratios, the grain size of streambed sediments relative to the channel width, and the occurrence of debris flows, influence channel shape. However, the link between these factors and channel slope has remained unclear.
In this new study, the authors investigated this link by combining field data from 373 mountain streambeds with theoretical equations describing sediment transport and water flow. The team found that the factors that control channel shape, such as the channel width to depth ratio, also change with slope. This variability explains why a channel’s slope can be a good predictor of its shape.
However, although the analysis predicted that certain slope ranges correspond to unique channel forms, it also found that some slopes correspond to multiple forms. This finding suggests that for a stream whose steepness favors more than one channel form, the most stable form will arise from competition between mechanisms that vary in strength depending on local conditions.
For example, the range of slopes that can result in alternate bars overlaps with the slope range for step-pool formation. If a stream with a slope in this overlapping range is confined to a narrow channel width (such as by a canyon or closely spaced hillslopes) or it experiences a high influx of sediment from landslides, steps and pools may be more likely to emerge than alternating bars.
Scientific understanding of factors controlling stream shape will continue to evolve as more field data become available. For this study, the authors demonstrated that slope alone cannot always predict a certain streambed shape and local conditions should be considered. Accounting for local conditions may be especially important when predicting streambed shape in unique conditions, such as after a disturbance (a fire, for example), in an artificial channel (such as a laboratory flume), or on another planet. (Geophysical Research Letters, https://doi.org/10.1002/2017GL074198, 2017)
—Sarah Stanley, Freelance Writer