Channel Forming Discharges and Scaling Relationships in Small Streams

One of the major challenges in predicting or mitigating the impacts of disturbance on hydrologic systems is to link changes in hydrology to changes in sediment delivery and transport. Because of the complexity of modelling, usually only one system is considered in isolation, with any potential changes in the corresponding system inferred. For instance, a study of a small watershed might consider changes to peak flows or to sediment delivery to the channel, but not alteration in channel pattern caused by those changes. Linking our understanding of expected changes in hydrology and sediment transport is therefore important for improving land use management. In order to improve this understanding, the development of models and concepts linking hydrologic change to geomorphic change, and vice versa, is necessary. Channel and reach parameters (such as width, depth, slope, and channel pattern) reflect the adjustment of the stream channel to inputs of water, wood and sediment from upstream and upslope. Therefore, channel parameters can be used as indicators which synthesize the hydrologic and geomorphic processes occurring in a watershed (Goodwin et al, 1998). Two parameters which are particularly relevant are the bankfull discharge and the effective discharge. Bankfull discharge (Wolman and Leopold, 1957) is defined as the discharge at which the stream channel is full to the top of its banks, but not flooding over the bank. Effective discharge (Wolman and Miller, 1960) is defined as the discharge that, averaged over time, transports the most sediment. Estimating the frequency, magnitude, and duration of bankfull and effective discharge in a single stream reach provides an indication of the stream channel's stability and the frequency with which geomorphically effective events occur in the watershed upstream. Determining the bankfull and effective discharge for multiple streams across a region enables regionalization, consideration of scaling relationships, and evaluation of the relative importance of individual factors, such as stream channel gradient, or availability of large woody debris, which contribute to the stream channel stability. Factors governing the frequency, magnitude, and duration of bankfull and effective discharge for large, alluvial rivers are generally well described in the scientific literature. By contrast, for small streams, the situation is not well understood, and some of the results published to date have been contradictory. Small streams and headwater basins cover the majority of the landscape in mountainous terrain. Evaluating regional and scaling relationships for bankfull and effective discharge in small streams is therefore important for issues of forest management and response to climate change