KIRK BRYAN AWARD: MOUNTAIN RIVER FORM AND PROCESS

High-gradient rivers in mountainous regions share many characteristics with lower gradient alluvial rivers, but also typically have irregular longitudinal profiles, spatially heterogeneous process domains and channel morphology, large and highly variable hydraulic resistance, and nonlinear sediment transport. We presently have testable, quantitative predictions for various characteristics of mountain rivers, including spatial distribution of discharge, stream power, downstream hydraulic geometry, substrate type, instream wood, and channel morphology. It remains challenging, however, to quantify the relative importance of progressive spatial patterns (i.e., downstream trends) versus local influences. The spatial distribution of channel morphologic types and the dynamics of instream wood illustrate progress and continuing challenges to quantitatively predicting mountain river form and process. Channel morphology correlates strongly with reach-scale gradient, which is partly independent and commonly displays abrupt longitudinal discontinuities along mountain rivers. Because of this correlation, channel morphology can be predicted with reasonable accuracy using topographic maps or DEMs. Channel morphology also correlates with grain size, substrate mobility, total frictional resistance and partitioning of resistance, aquatic habitat, and responsiveness of the channel to disturbance, thus facilitating prediction of the spatial distribution of each of these characteristics within a mountain stream network based on gradient. Volume of wood stored in the channel, expressed as volume/channel area, tends to decrease downstream and wood mobility increases downstream. The proportion of wood clustered into jams peaks at mid-basin. Sources of wood recruitment and loss vary in relative importance longitudinally and at the reach scale in response to varying valley geometry and forest characteristics. Instream wood also displays nonlinear reactions with channel morphology, such as when wood volume exceeds a threshold and alters channel morphology by altering bedform geometry or creating a forced alluvial reach or anastomosing planform. Patterns such as these emerging from the recent literature need to be calibrated for diverse field settings.