Purpose Wildfires represent one of the major natural disturbances within forested landscapes and have potential implications for the quality and function of downstream aquatic ecosystems. This study aimed to determine if a wildfire in a mountainous, forested watershed in British Columbia, Canada, caused a change in the dominant sediment source in the immediate 1-2 years following the wildfire, and if the sediment sources changed over the medium term (3-7 years) as the landscape recovered. Materials and methods Source materials (surface soil, subsurface soil and channel bank material) and fluvial (suspended and channel bed) sediment samples were collected over the period 2004 to 2010 from a watershed burnt by a wildfire in 2003, and from an adjacent watershed that was not impacted (FRNs) caesium-137 ( 137 Cs) and unsupported lead-210 ( 210 Pb un ). An unmixing model was used to calculate the relative source contributions of the fluvial sediment samples. Results and discussion 137 Cs and 210 Pb un were concentrated in the upper layers of surface soils in both watersheds and were statistically different to concentrations in subsurface and channel bank material. In the burnt watershed, FRN concentrations were greatest in the ash layer. Sediment sources as determined by the unmixing model were 100% subsurface/channel bank material in the unburnt watershed, while in the burnt watershed 8.5±2.5% was derived from surface soils. In both watersheds, there were no major changes in the relative contributions from surface soil and from subsurface/channel bank material over the period 2004 to 2010. Thus, while the wildfire did cause a change in sediment sources, it was fairly subtle and did not conform to the effects following wildfire described for other studies in contrasting environments, which typically document a major increase in hillslope contributions relative to channel bank sources. Conclusions There was a limited response in terms of finegrained sediment sources (and also sediment fluxes) in the burnt watershed. The reason for this muted response to a severe wildfire is likely to be the lack of precipitation, especially winter precipitation and the associated snowmelt, in the first year following the wildfire. Thus while the landscape was primed for erosion and sediment transport, the lack of a driving force meant that there was a limited immediate postfire sediment response.Keywords Caesium-137 . Wildfire . Disturbance response . Fallout radionuclides . Sediment sources . Unsupported lead-210

Bank strength due to vegetation dominates the geometry of small stream channels, but has virtually no effect on the geometry of larger ones. The dependence of bank strength on channel scale affects the form of downstream hydraulic geometry relations and the meandering-braiding threshold. It is also associated with a lateral migration threshold discharge, below which channels do not migrate appreciably across their floodplains. A rational regime model is used to explore these scale effects: it parameterizes vegetation-related bank strength using a dimensionless effective cohesion, C r *. The scale effects are explored primarily using an alluvial state space defined by the dimensionless formative discharge, Q*, and channel slope, S, which is analogous to the Q-S diagrams originally used to explore meandering-braiding thresholds. The analyses show that the effect of vegetation on both downstream hydraulic geometry and the meandering-braiding threshold is strongest for the smallest streams in a watershed, but that the effect disappears for Q* > 10 6 . The analysis of the migration threshold suggests that the critical discharge ranges from about 5 m 3 /s to 50 m 3 /s, depending on the characteristic rooting depth for the vegetation. The analysis also suggests that, where fires frequently affect riparian forests, channels may alternate between laterally stable gravel plane-bed channels and laterally active riffle-pool channels. These channels likely do not exhibit the classic dynamic equilibrium associated with alluvial streams, but instead exhibit a cyclical morphologic evolution, oscillating between laterally stable and laterally unstable endmembers with a frequency determined by the forest fire recurrence interval. Figure 4. (A) Dimensionless hydraulic geometry data from Andrews (1984) are shown along with the power function for W* used to derive Equation 10. (B) Q*-S curves predicted by the CCV model and the CRB equations are compared with the data from Andrews (1984). , μ ′ = 1). (C) Data from sparsely vegetated meandering channels is compared with the limit braiding threshold (C r * = 0, μ ′ = 1). (D) Data from more densely vegetated single-thread channels are compared against the range of thresholds predicted by the CCV model (shown and labeled in A) assuming a range of reasonable bank strength parameters.

In August 2003, the McLure forest fi re burned 62% of the drainage basin of Fishtrap Creek. Streamfl ow has been measured there since the early 1970s, and suspended sediment concentration and channel morphology have been monitored since the fi re. Although the short post-fi re period (four years) limits our ability to draw fi rm conclusions about streamfl ow changes, there has been no obvious increase in peak fl ows since the fi re. However, the total runoff during the freshet period does appear to have increased and the onset of snowmelt appears to occur about two weeks earlier than it did prior to the fi re. Suspended sediment records from Fishtrap Creek and from an unburnt reference stream nearby are similar, suggesting that the burnt areas have remained relatively stable and that the sediment supply to Fishtrap Creek has not been dramatically altered. In contrast, the stream channel morphology has changed, widening by over 100% of the original width in some places and transforming from a laterally stable plane-bed morphology to a laterally active riffl e-pool morphology. The timing and magnitude of the observed morphologic changes are consistent with the predicted decline in bank strength due to root decay, implying that the observed changes are associated with an internal instability associated with changes to the stream boundaries, rather than with the more typically reported externally driven instabilities caused by changes in streamfl ow or sediment supply. This delayed response in the absence of large changes in streamfl ow or sediment supply, while 'unusual' in that it has not been documented in the previous literature, may be a common mode of response, particularly in wat'ersheds with nival fl ow regimes.

No abstract

No abstract