Influence of coarse lag formation on the mechanics of sediment pulse dispersion in a mountain stream, Squire Creek, North Cascades, Washington, United States

Abstract

[1] Mountain channels closely coupled to landslide-prone hillslopes often exhibit bed surface grain sizes coarser than transportable by annual high flows. Coarse particles within poorly sorted sediment delivered to channels by mass-wasting processes may not be readily transported as bed load and can consequently form lag deposits that influence the morphology, hydraulic roughness, and sediment storage within mountain channel networks. A tracer study and comparison of supply and bed grain size distributions from a valley-spanning landslide in the North Cascades of Washington state were used to derive relations between shear stress and the probability of particle entrainment and erosion from the sediment pulse. Rapid bed surface armoring formed a relatively immobile lag deposit within 2 years. Covering of 20% of the bed by lag boulders with <5% probability of entrainment was sufficient to retard vertical incision and force considerable channel widening during a flood with an 8-to 152-year recurrence discharge on locally gauged streams. Our results imply that numerical models of sediment pulse evolution that do not explicitly incorporate the influence of lag formation may substantially overestimate long-term dispersion rates. The grain size distribution and lithology of a sediment input relative to the flow competence of the receiving channel are important factors influencing the rates and mechanisms of sediment pulse dispersion and the sediment capacitance provided by coarse-grained sediment pulses in mountain drainage basins.

Enhanced sediment delivery in a changing climate in semi-arid mountain basins: Implications for water resource management and aquatic habitat in the northern Rocky Mountains

“…Nevertheless, debris flows can produce, rapid, dramatic change, causing: (1) extensive channel reorganization along their runout path (Cenderelli and Kite, 1998;Dunham et al, 2007); (2) deposition of massive deposits of sediment and wood at their terminus, frequently expressed as a tributary fan that temporarily blocks or diverts the receiving channel (Benda et al, , 2004aLewicki et al, 2006); and (3) a downstream wave of sediment and wood that alters channel morphology, substrate size, and bed stability (Sutherland et al, 2002;Cui and Parker, 2005;Brummer and Montgomery, 2006;Ferguson et al, 2006;Lisle, 2008;Lewicki et al, 2006). Channel aggradation above the debris fan and along the path of the downstream wave of sediment increases flood risk and can destabilize channel morphology.…”

Mountains and Montane Channels

“…the sizes transported are characteristically smaller than the full range of sizes present in the channel (Lauffer and Sommer, 1982;Adenlof and Wohl, 1994;Lenzi, D'Agostino and Billi, 1999;Whiting et al, 1999;Zimmermann and Church, 2001;Church and Hassan, 2002;Brummer and Montgomery, 2006);…”

Variable hillslope‐channel coupling and channel characteristics of forested mountain streams in glaciated landscapes

“…In such settings, fluvial systems characterized by simple concave-up longitudinal profiles typically exhibit hillslope-channel sediment connectivity (i.e. An idealized progression of channel types in these systems would include strongly coupled debris flow-dominated channels at the headwaters followed by transitional (partially coupled) channels where debris-flow disturbance (mainly deposition) coexists with fluvial reworking, and finally (uncoupled) fluvially-dominated channels further downstream (Montgomery and Buffington, 1997;Church, 2002;Gomi et al, 2002;Brummer and Montgomery, 2003). An idealized progression of channel types in these systems would include strongly coupled debris flow-dominated channels at the headwaters followed by transitional (partially coupled) channels where debris-flow disturbance (mainly deposition) coexists with fluvial reworking, and finally (uncoupled) fluvially-dominated channels further downstream (Montgomery and Buffington, 1997;Church, 2002;Gomi et al, 2002;Brummer and Montgomery, 2003).…”