HEADWATERS TO BIG RIVERS: NETWORKS, DISTURBANCE, AND PHYSICAL HETEROGENEITY

Debris flows and gully erosion episodically flush sediment and wood stored in headwater channels, often building debris fans at low- to high order stream confluences and altering morphology of larger channels in humid and semi-arid environments. The tendency has been to treat debris flows and gully erosion as a threat to riverine ecosystems in managed landscapes and as an unknown in wilderness watersheds. Recent studies are indicating that episodic erosion acts as agents of habitat creation and morphological heterogeneity from headwaters to big rivers. Consequently, network geometry that organizes the distribution of channels in a basin, interacting with watershed disturbances, are important players in emerging paradigms in riverine ecology that focus on physical heterogeneity as sources of biological productivity and diversity. Influences of tributary confluences on morphology of mainstem channels include changes in gradient, substrate sizes, channel and floodplain widths, hydraulic geometry, and in sediment and wood storage. We reviewed 14 studies documenting confluence effects at 168 junctions spanning 6 orders of magnitude in drainage area. In both humid and semi-arid environments, large tributaries are required to create confluence effects in large rivers. A simple scaling relationship of tributary to mainstem drainage area reveals the control that basin shape exerts on network-wide effects of confluences. Oval shape basins favor confluence effects while rectilinear basins do not, and variations in network geometry cause deviation from this general tendency. The increasing separation of tributaries of increasing size downstream in many networks points to a geometric control on the changing spatial scale of riverine heterogeneity linked to confluences downstream in networks, with headwater streams driving the finest scale of heterogeneity in small to moderate mainstem channels. In addition, storms, fire, and floods trigger accelerated sediment supply that form and maintain debris flow, flash flood, and alluvial fans, and hence their morphological effects in rivers. The frequency and magnitude of fan forming events also scale with size of watersheds, indicating a spatio-temporal influence on the distribution of fan ages and therefore on the ability of fans to alter channel morphology.