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[1] Debris flows have typically been viewed as two-phase mixtures of sediment and water, but in forested mountain landscapes, wood can represent a sizable fraction of total flow volume. The effects of this third phase on flow behavior are poorly understood. To evaluate whether wood can have a significant effect on debris flow runout in small mountainous watersheds, we used a landscape-scale model combining empirical, stochastic, and physical submodels of storms, fires, forest growth, tree fall, wood decay, soil production and diffusion, landslide initiation, debris flow runout, and fluvial sediment transport. We examined changes in the cumulative distribution function of debris flow runout lengths in a small (2 km 2 ) watershed in the Oregon Coast Range due to presence or absence of two hypothesized effects of wood: (1) velocity reduction due to entrainment of wood in the runout path and (2) velocity reduction due to changes in flow direction angle. The model was calibrated such that the distribution for simulations including both effects was similar to that measured in the study basin, and amounts of wood in the simulation and the field, both fallen in small valleys and incorporated by debris flows, were comparable. Removal of either effect, or both, significantly shifted runout length distributions to longer lengths. Simulations and field observations indicate that with wood, fluvial transport is a significant source of sediment output, few debris flows reach the outlet, and debris flow deposits are widely distributed throughout the network. Simulations indicate that without wood, basin sediment yield greatly increases, that yield is dominated by longer-runout debris flows, and that debris flow deposits are concentrated in the low-gradient reach near the outlet.

Young basalt terrains offer an exceptional opportunity to study landscape and hydrologic evolution through time, since the age of the landscape itself can be determined by dating lava fl ows. These constructional terrains are also highly permeable, allowing one to examine timescales and process of geomorphic evolution as they relate to the partitioning of hydrologic fl owpaths between surface and sub-surface fl ow. The western slopes of the Cascade Range in Oregon, USA are composed of a thick sequence of lava fl ows ranging from Holocene to Oligocene in age, and the landscape receives abundant precipitation of between 2000 and 3500 mm per year. On Holocene and late Pleistocene lava landscapes, groundwater systems transmit most of the recharge to large springs (≥0·85 m 3 s −1) with very steady hydrographs. In watersheds >1 million years old, springs are absent, and well-developed drainage networks fed by shallow subsurface stormfl ow produce fl ashy hydrographs. Drainage density slowly increases with time in this basalt landscape, requiring a million years to double in density. Progressive hillslope steepening and fl uvial incision also occur on this timescale. Springs and groundwater-fed streams transport little sediment and hence are largely ineffective in incising river valleys, so fl uvial landscape dissection appears to occur only after springs are replaced by shallow subsurface stormfl ow as the dominant streamfl ow generation mechanism. It is proposed that landscape evolution in basalt terrains is constrained by the time required for permeability to be reduced suffi ciently for surface fl ow to replace groundwater fl ow.

Abstract:We develop a new method for analysis of meandering channels based on planform sinuosity. This analysis objectively identifies three channel-reach lengths based on sinuosity measured at those lengths: the length of typical, simple bends; the length of long, often compound bends; and the length of several bends in sequence that often evolve from compound bends to form multibend loops. These lengths, when normalized by channel width, tend to fall into distinct and clustered ranges for different natural channels. Mean sinuosity at these lengths also falls into distinct ranges. That range is largest for the third and greatest length, indicating that, for some streams, multibend loops are important for planform sinuosity, whereas for other streams, multibend loops are less important. The role of multibend loops is seldom addressed in the literature, and they are not well predicted by previous modelling efforts. Also neglected by previous modelling efforts is bank-flow interaction and its role in meander evolution. We introduce a simple river meandering model based on topographic steering that has more in common with cellular approaches to channel braiding and landscape evolution modelling than to rigorous, physics-based analyses of river meandering. The model is sufficient to produce reasonable meandering channel evolution and predicts compound bend and multibend loop formation similar to that observed in nature, in both mechanism and importance for planform sinuosity. In the model, the tendency to form compound bends is sensitive to the relative magnitudes of two lengths governing meander evolution: (i) the distance between the bend cross-over and the zone of maximum bank shear stress, and (ii) the bank shear stress dissipation length related to bank roughness. In our simple model, the two lengths are independent. This sensitivity implies that the tendency for natural channels to form compound bends may be greater when the banks are smoother.