The role of large woody debris in modulating the dispersal of a post-fire sediment pulse

Geomorphology 246 (2015) 351–358

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The role of large woody debris in modulating the dispersal of a post-fire sediment pulse Lauren E. Short a, Emmanuel J. Gabet a,⁎, Daniel F. Hoffman b a b

Department of Geology, San Jose State University, San Jose, CA 95192, USA Atkins Global, Missoula, MT 59802, USA

a r t i c l e

i n f o

Article history: Received 14 November 2014 Received in revised form 15 June 2015 Accepted 22 June 2015 Available online 29 June 2015 Keywords: Sediment pulse Debris flow Sediment wave Large woody debris

a b s t r a c t In 2001, a series of post-fire debris flows brought ~30,000 m3 of sediment, deposited as fans, to the narrow valley floor of Sleeping Child Creek in western Montana (USA). In 2005, pebble-counts and surveys of the channel in proximity to six of the debris flow fans documented a regular sequence of fine-grained aggradation upstream of the fans, incision through the fans, and coarse-grained aggradation downstream of the fans. These measurements were repeated in 2012. We found that the delivery of large woody debris (LWD) over the intervening 7 years has been a dominant factor in the disposition of the debris-flow material. The amount of LWD in the study reach has increased by as much as 50% in the areas with a high burn severity, leading to the formation of large logjams that interrupt the flow of sediment along the streambed. Nearly all of the surveyed reaches have aggraded since 2005, including those that had initially begun incising through the debris flow deposits, and the streambed has become generally finer. We hypothesize that, over the next few decades, debris flow sediment not colonized and anchored by riparian vegetation will trickle out of the affected reaches as the logjams slowly degrade. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In mountainous terrain, sediment is often delivered to rivers in large instantaneous pulses, typically as debris flows and landslides, that overwhelm their transport capacity (e.g., Madej and Ozaki, 1996; Sutherland et al., 2002; Benda et al., 2003). The geomorphic effects of these pulses include floodplain and channel aggradation (e.g., James, 1991), transient increases in channel width (e.g., Beschta, 1984; Madej and Ozaki, 1996), and the creation of low terraces (Miller and Benda, 2000). Within the channel itself, the introduction of this sediment can decrease bed relief (e.g., Cui and Parker, 2003; Bartley and Rutherford, 2005) and lead to a fining of the bed material (e.g., Roberts and Church, 1986; Madej et al., 2009). With respect to fluvial sediment transport, the addition of finegrained sediment can help mobilize coarse bed material by reducing the critical shear stress (Wilcock, 1998; Cui and Parker, 2003) while, upstream of the sediment slug, sediment flux may decrease because of a reduction in slope (Sutherland et al., 2002). Although the recovery from some of these changes may be relatively rapid, other impacts may affect channels for decades to centuries. For example, channel aggradation may persist for over 30 years after a sediment pulse (Madej and Ozaki, 1996), and large boulders introduced to the fluvial network by landslides and debris flows may become semipermanent features (Benda, 1990). Over the longer term, if the delivery of sediment ⁎ Corresponding author. E-mail address: [email protected] (E.J. Gabet).

http://dx.doi.org/10.1016/j.geomorph.2015.06.031 0169-555X/© 2015 Elsevier B.V. All rights reserved.

outpaces the long-term transport capacity of the fluvial network owing, for instance, to a change in climate, the overall nature of the landscape may be altered as the valleys fill (Meyer and Pierce, 2003). Finally, in addition to their effect on the trajectory of geomorphic evolution, sediment pulses to rivers may have more immediate impacts on local fish populations. For example, the addition of coarse material to streams provides building material for salmonid spawning nests but too much fine material increases egg mortality (e.g., Newcombe and Jensen, 1996, and references therein). Two end-member models describe how sediment pulses are processed by the fluvial system. According to the first, the introduced sediment translates downstream as a coherent wave (Gilbert, 1917; Griffiths, 1979; Meade, 1985; Kasai et al., 2004; Bartley and Rutherford, 2005). As the peak of the wave approaches a reach, the channel widens and its bed aggrades; after its passage, the channel narrows and incises down toward its original elevation (e.g., Madej and Ozaki, 1996). Lisle et al. (2001), however, found little evidence for pure translational propagation of sediment waves, and other studies have documented dispersion-dominated behavior in the flushing of sediment pulses (e.g., Roberts and Church, 1986; Knighton, 1989; Lisle et al., 1997). Moreover, recent observations of sediment releases from decommissioned dams in Oregon (Sandy River) and Washington (Elwha River) have failed to support any detectable translation of sediment (T. Lisle, USFS, pers. comm., 2015). Determining the mechanics of sediment redistribution via dispersion under specific conditions is important for our understanding of landscape evolution, and long-term field studies are especially

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critical because landscape adjustment is generally slow (Madej and Ozaki, 2009). In this contribution, we returned to the site of a previous investigation that documented the initial channel response to a series of large postfire debris flows. We performed the same set of measurements as the previous study with the goal of developing a general model for how channels change and adjust to the delivery of large amounts of sediment and wood after fires. In 2000, high temperatures and an unusually dry summer created ideal wildfire conditions in Montana (USA). The Sleeping Child Creek watershed (Fig. 1) was burned by a series of small wildfires that eventually merged into a large fire complex (Parrett et al., 2004). About 600 ha of the Sleeping Child Creek watershed (3.5% of the total catchment area) were burned with 65% of the area classified as a high severity burn (Hyde et al., 2007). In July 2001, thunderstorms brought intense rainfall to the area, triggering a series of debris flows. The debris flows, with volumes ranging from 500 to 3400 m3 (Parrett et al., 2004; Hoffman and Gabet, 2007; Gabet and Bookter, 2008), originated from zero- and first-order drainages along Sleeping Child Creek within watersheds with high to moderate burn classifications (Hyde, 2003; Parrett et al., 2004) The debris flows came to rest at the valley bottom, forming fans composed of coarse sand, pebbles, cobbles, boulders, ash, and large woody debris (Hoffman and Gabet, 2007). Because the valley bottom is narrow, the fans spread across the entire width of the valley floor and locally buried the bed of Sleeping Child Creek. Five years after the fire, Hoffman and Gabet (2007) documented the creek's response to these large, spatially confined, and instantaneous inputs of sediment. The study found a recurring pattern of

morphological changes along the river. The channel reach above each debris flow fan was single-thread and aggrading, with gentle slopes and fine bed sediments. Where Sleeping Child Creek passed through the fans, it had incised, creating high-gradient entrenched reaches with steep banks; in addition, the silt, sand, and small gravel had been winnowed from the debris flow deposits, leaving behind a coarse bed of large gravel and boulders. The reaches below the fans were braided with gravel beds, and pockets of sand had been deposited in lowvelocity shelters along the margins of the channel and behind large woody debris (LWD). Hoffman and Gabet (2007) concluded that the rate of recovery toward an equilibrium state — defined as a return to sediment transport continuity (Pitlick, 1993) — would depend on the river's ability to continue incising through the debris flow material and the rate at which the accommodation space behind the fans would fill with sediment. Seven years after Hoffman and Gabet studied Sleeping Child Creek's response to the debris flows, we repeated their topographic surveys, pebble counts, and wood surveys to document the continuing evolution of the channel. 2. Materials and methods 2.1. Study site Sleeping Child Creek, a tributary of the Bitterroot River, is located in the Sapphire Mountains of west-central Montana (Fig. 1). The region is in the continental climate transition zone between the Pacific Northwest and the Rocky Mountains (Hyde et al., 2007). The summers are

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Fig. 1. Study site location in Montana (46°06′42.3″ N, 114°00′34″ W). The studied reach of Sleeping Child Creek is between Two Bear Creek and Divide Creek.

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moderately warm with intense short-duration storms, and the winters are mild with snowfall; the mean annual precipitation, measured at the nearest station 17 km from Sleeping Child Creek, is 70 cm (Hyde, 2003). The watershed of Sleeping Child Creek is underlain by Proterozoic gneiss, granite, and schist (Vuke et al., 2007). The soils are geologically young and have a gravelly and sandy texture (Hyde, 2003). The 169-km2 watershed has average slopes of ~25°, a mean elevation of 1900 m, and is vegetated by a mixed conifer forest of Douglas fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta), Ponderosa pine (Pinus ponderosa), and sub-alpine fir (Abies lasiocarpa) (Hyde et al., 2007). The study reach of Sleeping Child Creek is a mixed alluvial–bedrock channel with an estimated bankfull discharge of 12.3 m3/s (Hoffman and Gabet, 2007); bankfull channel widths range from 3 to 20 m and depths range from 1 to 2 m. The absence of cut stumps and forest roads in the study area suggests that it was never logged; in addition, the basin has not burned in a major fire since 1889 (Hyde et al., 2007). 2.2. Field methods To document the morphological changes in Sleeping Child Creek since 2005, we repeated, in 2012, the surveys performed by Hoffman and Gabet (2007). At the same six fans as the previous study (Fig. 2), we surveyed a reach-scale longitudinal profile and three channel cross sections with a level, stadia rod, and measuring tape. Cross section locations monumented in 2005 with rebar and cairns were reoccupied in 2012. At each site, one cross section was surveyed 10–30 m upstream of the fan, another was surveyed through the middle of the fan, and the last was surveyed 10–30 m downstream of the fan; these will be referred to as the ‘up-fan’ reach, the ‘mid-fan’ reach, and the ‘down-fan’ reach, respectively (Fig. 3). Short longitudinal profiles of Sleeping Child Creek at each debris flow fan began 0–10 m upstream of the up-fan cross section and ended 0–10 m beyond the down-fan cross section. A longitudinal profile of the entire study reach was constructed from the 2011 1:24,000 USGS Bald Top Mountain and Deer Mountain topographic maps. The grain-size distribution of the surface bed material at each cross section was measured with a pebble count (n = 200) (Wolman, 1954). As in Hoffman and Gabet (2007), the pebble counts were performed at the 18 cross sections. In addition, a survey of large woody debris was taken along Sleeping Child Creek, between its junctions with Two Bear Creek and Divide Creek, using a hip chain. The study reach was divided into 10-m sections, and the number of pieces

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of large woody debris within the channel was recorded in each section. To be consistent with Hoffman and Gabet (2007), large woody debris was defined as a piece of wood in the channel with a minimum diameter of 25 cm and a length greater than the channel width. The raw data from the 2005 and 2012 surveys can be found in Hoffman (2005) and Short (2014), respectively. 2.3. Analysis The magnitude of sediment aggraded or eroded from each cross section between 2005 and 2012 was estimated by calculating the difference in cross sectional area using ImageJ software (Rasband, 2012). Because of log jams, the original locations of some surveys were inaccessible and the cross sections had to be moved; however, the new surveys were never farther than 2 m upstream or downstream of the 2005 surveys. In addition, when the original location could not be reoccupied, care was taken to ensure that the new location was representative of the reach. Consequently, we are unable to quantify the uncertainty in our aggradation and erosion estimates at these sites but we assume that they are small relative to the large net changes that we observed. 3. Results 3.1. Changes in channel morphology Net aggradation occurred along 11 of the 18 reaches from 2005 to 2012 (Fig. 4). At each site, the greatest amount of aggradation was typically upstream of the fans, and the least amount of change was where the channel cut through the fan (i.e., the mid-fan cross sections). The down-fan cross section of debris flow 6 (DF6), with a net loss of approximately 60 m2 of sediment, was the only location which experienced significant erosion. Note, also, that the volume of aggradation upstream of DF6 is not well-characterized by the change in cross sectional area. Sometime between 2010 and 2012, a beaver dam was built across Sleeping Child Creek at the upstream edge of the fan (see later); the backwater effect has induced sedimentation at least ~200 m upriver. The upstream reaches had the gentlest gradients in 2012, as was the case in 2005 (Fig. 5). Between the two measurement periods, the slope of 9 of the 16 reaches became steeper while 6 became gentler (note: as noted earlier, we were unable to survey two reaches because of logjams). Of the 9 reaches with increases in slope, 7 were mid-fan and down-fan reaches.

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3.2. Bed material size

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Bed material generally became finer downstream, with median grain sizes (D50) decreasing from cobbles to gravels along the study reach (Fig. 6A). At the reach scale, the finest sediment (silt to sand) was typically immediately upstream of the fans and the coarsest was where the channel cut through the fans. The three fans higher up in the watershed (DF4, DF5, DF6) had finer sediment in the upstream reaches than the three lower fans, likely because the former were larger and thus created greater channel constrictions. At all locations, the particle size distribution changed from 2005 to 2012 (2-sample Kolmogorov–Smirnov test; α = 0.05). Since 2005, the bed along the study reach had become finer, with a decrease in D 50 at all but two of the cross sections (Fig. 6B). The down-fan section of DF6 was the only site where the D50 became coarser. 3.3. Spatial distribution of large woody debris (LWD)

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The total amount of wood in the study reach increased since the previous survey. In 2005, the highest concentration of LWD was at the fan deposits where trees had been carried down or toppled by the debris flows (e.g., DF1, DF2, and DF6) (Hoffman and Gabet, 2007). In 2012, peaks in LWD were again associated with the debris flow fans; however, these were logjams (defined here as multiple interlocking pieces of wood) formed upstream of the fans by wood transported by the river but unable to pass through the constricted reaches incised through the debris flow sediment (Fig. 7). Other logjams formed downstream of the fans in wide braided reaches that appeared to be too shallow for efficient wood transport. To highlight the spatial relationships between the debris flow fans, burn severity, and wood in the stream, a 5-point (i.e., a 1000-m window) running average of LWD pieces was taken of the 2005 and 2012 data. The total amount of wood decreased downstream of DF3 in the intervening 7 years (Fig. 8). Upstream of DF3, however, the amount of LWD rose by ~50% over the same time period. A notable exception to the general increase in LWD was the reach between DF5 and DF6, a bedrock section of the river with fewer trees near the bank of the creek compared to the alluvial reaches. Where the LWD increased, it continued to cluster around debris flow fans, as it did in 2005. Because of the low gradients just upstream of the fans and the narrowing of the channel as it cut through the fan, the debris flow deposits impounded LWD transported by the river. The newly introduced wood appeared to be primarily from burned hillslopes adjacent to the channel. We noted many leaning snags (dead trees) and could hear them fall over during windy conditions. The highest LWD concentrations were measured in the part of the basin with the highest burn intensity (Hyde, 2003), which is where the greatest number of dead trees would be expected (Fig. 8). Finally, the LWD peak at DF6 (Fig. 8) was not from a log-jam but a beaver dam. Based on Google Earth™ satellite imagery, sometime between 2010 and 2012 a pre-existing log jam was fortified by beavers between the up-fan and mid-fan cross sections of DF6 (Fig. 9). This created a marshy environment that was about 50 m wide and continued 200 m upstream. The dam appeared to have been mostly composed of opportunistically scavenged wood but gnaw marks on some trunks indicated that they had been felled by the beavers.

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Fig. 4. Change in channel cross-sectional area from 2005 to 2012. Note the separate scale for debris flow fan 2 (DF2). Most of the cross sections experienced net aggradation.

Three modes of channel change occurred along Sleeping Child Creek from 2005 to 2012. The first mode was characterized by net aggradation and a decrease in D50 (Fig. 10A). This pattern was documented in 11 of the 18 cross sections: five up-fan reaches of the debris flow fans, two midfan reaches, and four down-fan reaches. The coarse bed documented in

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2005 had been buried by fine material that was often laterally extensive. The primary driver of the aggradation was the logjams that created extensive low-velocity areas. The second mode was typified by incision of the bed and a decrease in bed D50 (Fig. 10B). This pattern was documented in 6 of 18 cross sections: one up-fan reach of the debris flow fans, four mid-fan, and one down-fan. Channel incision accompanied by fining of the bed is unexpected and contrasts with observations made elsewhere (e.g., Madej et al., 2009). We propose that, immediately after the deposition of the debris flow fans, the channel began to incise down through the sediment. This initial response, however, appears to have been interrupted by the introduction of LWD that formed logjams and triggered aggradation. In-channel wood increases hydraulic friction, thereby reducing the boundary shear stress acting on the bed and allowing finer particles to remain stable on the bed surface (Buffington and Montgomery, 1999). The third mode applies only to the down-fan reach of DF6 (Fig. 10C), which experienced net incision and an increase in D50. This section was

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wide, gravelly, and braided with no obvious pockets of fine sediment trapped behind large woody debris. The response of the channel here appeared to be controlled by the beaver dam directly upstream, which created a pond and likely locally reduced the supply of fine-grained sediment. Levine and Meyer (2014) also documented a coarsening of the bed in reaches below beaver dams. 4.2. Proposed model for continuing response One of the immediate effects of the debris flows on Sleeping Child Creek was the interruption of the continuity of bedload transport (Hoffman and Gabet, 2007). The reach above each fan became a site of net aggradation as sediment was impounded behind the debris flow deposits. In contrast, the steep reaches through the fans were sources of sediment as the channel incised down through the deposits. The coarse component of this eroded material was deposited in the downfan reaches, forming a braided channel, while the fine sediment traveled downstream to be trapped behind the next debris flow fan. After the 2005 surveys, Hoffman and Gabet (2007) hypothesized that recovery at Sleeping Child Creek would take the following path. Incision through the mid-fan reaches would continue until the lag deposits became too coarse to be mobilized; at the same time, aggradation in the up-fan

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Debris flow fan Fig. 6. (A) Median particle size (D50) at each cross-section in 2012. In general, the finest sediment at each site was in the up-fan reach and the coarsest was in the mid-fan reach. (B) Change (%) in D50 since 2005 (2005 D50 data from Hoffman, 2005). The bed material became finer at nearly all of the cross sections. There was no measurable change in D50 for the upstream cross section of DF6.

Fig. 7. An example of a logjam, located at DF 5.

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Running average of pieces of large woody debris per 200 m

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reaches would continue until the accommodation space was filled. These two processes would cause the gradients of each type of reach to converge until the continuity of bedload transport was restored. Our 2012 survey, however, indicates that the geomorphic change at Sleeping Child Creek has been following a different trajectory. The introduction of LWD since 2005 has created even more impediments to the throughput of bedload than the original debris flow deposits. The logjams throughout the river have created numerous sediment traps, leading to only localized redistribution of sediment. Although, soon after the debris flows in 2001, Sleeping Child Creek began to incise down through the fans, leaving behind a coarse lag (Hoffman and Gabet, 2007), enough wood has fallen into the channel since 2005 to halt incision and promote aggradation. In some reaches, the 2005 channel bed has been buried (Fig. 9A); in others, the aggradation has not yet reached the 2005 channel bed elevation (Fig. 9B). Whereas the presence of several fresh-looking gravel bars (i.e., uncolonized by algae or other vegetation) ~1 km downstream of the lowermost debris flow fan (DF1) is evidence that, since 2001, some coarse material from the debris flow deposits has been transported downstream, the vast majority of bed material delivered by the debris

flows apparently has not moved beyond the affected stretch of river because of the numerous logjams. The fate of that impounded sediment will depend on the persistence of the logjams and the rate at which it is colonized and stabilized by riparian vegetation. The persistence of the logjams, in turn, depends on the rate of wood decay and wood recruitment (Eaton et al., 2012). As noted earlier, the source of the LWD shifted from the debris flows to the burned hillslopes, particularly in the upper portions of the basin classified as high burn areas (Fig. 7) (see also Benda et al., 2003), and even several decades after the fire, tree mortality rates may remain high (Jones and Daniels, 2008). The lower reaches surrounded by moderately burned slopes had less wood in 2012 than in 2005, indicating that, averaged over 7 years, the loss of LWD in these areas from decay and transport was happening more quickly than the delivery of ‘fresh’ wood. However, the LWD in the lower reaches was sufficient to create sediment traps that buried the coarse bed; indeed, others have found that LWD can persist in a channel for more than a decade (Jones and Daniels, 2008; King et al., 2013). Given the potential for continued addition of wood to the channel, as well as the possibility that, once introduced into the channel, it may remain there for at least a decade, riparian vegetation may have time to colonize the impounded sediment, protect it from erosion, and further delay its evacuation from the watershed. In the initial assessment of Sleeping Child Creek, not only was the delayed delivery of LWD unanticipated, but the migration of beavers into its upper reaches was also unexpected (Hoffman and Gabet, 2007). Although the watershed is within the historical range of the North American beaver (Castor canadensis) (Jenkins and Busher, 1979), beavers appear to have been absent from Sleeping Child Creek prior to their arrival between 2010 and 2012. Increases in beaver populations in post-fire watersheds have been documented elsewhere where they are presumably attracted to the regrowth of new riparian vegetation, an important food source (Lawrence, 1954). In Sleeping Child Creek, at the site colonized by the beavers (DF6), the backwater effect from a log-jam had already created a large marshy environment; the beavers fortified this pre-existing dam and made it larger (see also Allen, 1983). If other beavers migrate into the watershed and build more dams along the creek, this will further impede the transport of debris flow material out of the basin. Given that large woody debris repeatedly detains sediment along Sleeping Child Creek, the simple conceptual models of a sediment wave either translating downstream or diffusing do not appear to apply in this situation. On average, the volume of each debris flow was

Fig. 9. Google Earth™ imagery of DF6. Flow direction is north. Left image is from 27 Sep 2010 and right one is from 6 Aug 2013. In the late summer of 2010, the reach upstream of DF6 was braided with sand bars visible in the image. Three years later, a beaver pond had formed and the sand bars were submerged.

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Fig. 10. The three modes of channel change between 2005 and 2012. (A) Aggradation accompanied by fining of the bed material (e.g., mid-fan cross section of DF4). (B) Incision followed by fining of the bed material (e.g., down-fan cross section of DF4). These cross-sections were interpreted to have initially incised through the debris flow sediment but subsequently aggraded with the delivery of LWD from the hillslopes. (C) Incision accompanied by an increase in the bed material size. The down-fan cross section of the DF6 was the sole representative of this category. The 2005 sediment data and cross sections are from Hoffman (2005).

~ 3000 m3 (Gabet and Bookter, 2008); with a total of 10 debris flows along the 5-km study reach (Fig. 2), this suggests that ~ 30,000 m3 of sediment was delivered to the narrow valley floor in 2001 (assuming an average pre-fire channel width and depth of 5 and 1 m, respectively, spreading this sediment evenly along the study reach would increase the bed elevation by ~1 m). Our observations suggest that only a small portion of the debris flow deposits has been flushed downriver. Over time, in the absence of a catastrophic channel-clearing flood, each logjam will decay and liberate material not stabilized by vegetation. However, because of the repeated interruptions in channel transport capacity along the length of the creek, the punctuated release of sediment from individual log-jam failures will likely have a minimal effect on the total export of sediment from the watershed. Although LWD appears to have a dominant impact on sediment transport in Sleeping Child Creek, the role of scale is important. The width of the valley bottom at Sleeping Child is typically 10–20 m wide and, in the free-flowing reaches of the channel, bankfull widths range from 5–10 m; therefore, this is a fluvial system vulnerable to being constricted by LWD. In contrast, Madej and Ozaki (2009), working on a river with widths generally in the range of 50–100 m, found that LWD had little effect on the disposition of a sediment pulse (see also Wohl and Jaeger, 2009, and Eaton and Hassan, 2013).

A final issue to consider is how this sequence of fire, debris flows, and delivery of LWD relates to the long-term geomorphic evolution of Sleeping Child Creek. Prior to 2000, the watershed last burned during a stand-replacing fire in 1889 (Hyde et al., 2007), consistent with the 100–400 year recurrence interval for severe fires in the northern Rocky Mountains (Arno et al., 1999). Visual inspection of channel reaches not affected by the 2000 wildfire did not reveal any lasting effects from this earlier fire, such as old logjams and extensive alluviated areas. Furthermore, every large fire is unlikely to be followed by an intense thunderstorm that triggers debris flows. The fluvial environment at Sleeping Child Creek, therefore, may only be dramatically disturbed every few centuries and then allowed to recover over the course of several decades. Of course, extrapolating this particular scenario back through time fails to account for the potential role of beavers in modifying valley bottoms. Before being driven to the brink of extinction in the nineteenth century, beavers may have had profound impacts on mountain streams in the region (Naiman et al., 1988). 5. Conclusion In our study of post-fire debris flow deposits in Sleeping Child Creek, we found that the dispersion of the sediment pulses is complicated by the presence of LWD. Large pieces of wood brought down by debris

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flows have been augmented, several years later, by snags falling into the channel from adjacent hillslopes, particularly in the severely burned parts of the watershed. The LWD has formed logjams, and encouraged the construction of a beaver dam, that have impounded sediment and prevented the re-establishment of continuous bedload throughput down the length of the channel. Indeed, some reaches that had initially begun incising through the debris flow deposits began aggrading after snags fell in from the banks. The fire, therefore, has resulted in two episodes of aggradation: the first driven by the debris flow sediment and the second, years later, by the delivery of LWD. The channel will likely continue to aggrade in the highly burned areas until the decay of LWD outpaces its replenishment from nearby hillslopes, and the logjams begin to disintegrate and release their mobile sediment. Unless beavers continue to colonize Sleeping Child Creek and disrupt the flow of sediment or a catastrophic flood flushes the LWD from the channel, the debris flow material will trickle downstream over the next several decades. Acknowledgments We are grateful to H. Simon and N. Sylva for their enthusiastic help in the field. D.W. Andersen and P. Messina are thanked for their comments on an earlier draft. We appreciate the helpful comments contributed by T. Lisle and four anonymous reviewers. K. Hyde is thanked for discussions and for introducing E. Gabet to this field site. References Allen, A.W., 1983. Habitat suitability index models: beaver. FWS/OBS-82/10.30. U.S. Department of the Interior, Fish and Wildlife Service, Fort Collins, CO. Arno, S.F., Parsons, D.J., Keane, R.E., 1999. Mixed-severity fire regimes in the northern Rocky Mountains: consequences of fire exclusion and options for the future. In: Cole, D.N., McCool, S.F., Borrie, W.T., O'Loughlin, J. (Eds.), Wilderness Science in a Time of Change Conference. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula, MT, pp. 225–232. Bartley, R., Rutherford, I., 2005. Re-evaluation of the wave model as a tool for quantifying the geomorphic recovery potential of streams disturbed by sediment slugs. Geomorphology 64, 221–242. Benda, L., 1990. The influence of debris flows on channels and valley floors in the Oregon Coast Range, U.S.A. Earth Surf. Process. Landf. 15, 457–466. Benda, L.E., Veldhuisen, C., Black, J., 2003. Debris flows as agents of morphological heterogeneity at low-order confluences, Olympic Mountains, Washington. Geol. Soc. Am. Bull. 115 (9), 1110–1121. Beschta, R.L., 1984. River channel response to accelerated mass soil erosion. In: O'Loughlin, C.L. (Ed.), Symposium on Effects of Forest Land Use on Erosion and Slope Stability. East–west Center, Honolulu, pp. 155–164. Buffington, J.M., Montgomery, D.R., 1999. Effects of hydraulic roughness on surface textures of gravel-bed rivers. Water Resour. Res. 35 (11), 3507–3521. Cui, Y., Parker, G., 2003. Sediment pulses in mountain rivers: 1. Experiments. Water Resour. Res. 39 (9). http://dx.doi.org/10.1029/2002WR001803. Eaton, B.C., Hassan, M.A., 2013. Scale-dependent interactions between wood and channel dynamics: modeling jam formation and sediment storage in gravel-bed streams. J. Geophys. Res. Earth Surf. 118 (4), 2500–2508. http://dx.doi.org/10.1002/ 2013JF002917. Eaton, B.C., Hassan, M.A., Davidson, S.L., 2012. Modeling wood dynamics, jam formation, and sediment storage in a gravel bed stream. J. Geophys. Res. Earth Surf. 117. Gabet, E.J., Bookter, A., 2008. A morphometric analysis of post-fire progressively-bulked debris flows in southwest Montana, USA. Geomorphology 96 (34), 298–309. Gilbert, G.K., 1917. Hydraulic-mining debris in the Sierra Nevada. U.S. Geological Survey Professional Paper 105. Griffiths, G.A., 1979. Recent sedimentation history of the Waimakariri River, New Zealand. J. Hydrol. N. Z. 18, 6–28. Hoffman, D.F., 2005. Effects of Sediment Pulses on Channel Morphology and Sediment Transport in a Gravel-Bed River (M.S. Thesis), University of Montana, Missoula (89 pp.).

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