Dry sediment loading of headwater channels fuels post-wildfire 1 debris flows in bedrock landscapes 2

11 Landscapes following wildfire commonly have significant increases in sediment yield and debris 12 flows that pose major hazards and are difficult to predict. Ultimately, post-wildfire sediment 13 yield is governed by processes that deliver sediment from hillslopes to channels, but it is often 14 unclear the degree to which hillslope sediment delivery is driven by wet versus dry processes, 15 which limits the ability to predict debris-flow occurrence and response to climate change. Here 16 we use repeat airborne lidar topography to track sediment movement following the 2009 Station 17 Fire in southern California, USA and show that post-wildfire debris flows initiated in channels 18 filled by dry sediment transport, rather than on hillsides during rainfall as typically assumed. We 19 found widespread patterns of 1–3 m of dry sediment loading in headwater channels immediately 20 following wildfire and before rainfall, followed by sediment excavation during subsequent 21 storms. In catchments where post-wildfire dry sediment loading was absent, possibly due to 22 differences in lithology, channel scour during storms did not occur. Our results support a fire- 23 flood model in bedrock landscapes whereby debris flow occurrence depends on dry sediment 24 loading rather than hillslope-runoff erosion, shallow landslides, or burn severity, indicating that 25 sediment supply can limit debris-flow occurrence in bedrock landscapes with more frequent 26 fires. 27


INTRODUCTION 28
Sediment yields following wildfire often greatly exceed background erosion rates 29 (Moody et al., 2013), threatening life and property at the wildland-urban interface in 30 mountainous terrain (Cannon and DeGraff, 2009). Predicting the magnitude of this increase in 31 sediment yield and the consequences of wildfire for longer-term landscape evolution requires a 32 mechanistic understanding of how sediment is delivered from hillslopes to channels and the 33 degree to which post-wildfire erosion is limited by hillslope sediment supply (Roering and 34 Gerber, 2005;Lamb et al., 2011). 35 In landscapes continuously mantled in soil, post-wildfire sediment yield is governed 36 primarily by rainfall (Gartner et al., 2014). That is, predominately wet processes such as rilling 37 (Wells, 1987), shallow landsliding (Gabet, 2003), and excavation of existing channel deposits 38 (Santi et al., 2008)  ). In this model, more frequent fires predicted over the next century due to climate change 43 (Westerling and Bryant, 2008;Mann et al., 2016) should lead to increased sediment yields and 44 hazards because of the assumed inexhaustible supply of hillslope soil. However, it is unclear if 45 these ideas developed for soil-mantled hillslopes also apply to steep, bedrock-dominated 46 landscapes. 47 In landscapes where slopes are steeper than the angle of repose, sediment is transported 48 dry from hillslopes to channels immediately following wildfire by rolling and bouncing 49 downslope by gravity alone (i.e., dry ravel) due to incineration of vegetation dams that temporarily trap soil (Krammes, 1965;Florsheim et al., 1991;Lamb et al., 2011). The loading of 51 cobble and boulder-mantled headwater channels with relatively fine sediment (e.g., sand and fine 52 pre-rainfall data that are necessary for isolating the importance of dry sediment transport 73 processes. 74 Here we present repeat airborne lidar analysis of the 2009 Station Fire, which burned 650 75 km 2 in the steep topography of the western San Gabriel Mountains, CA (Fig. 1). The San Gabriel 76 Mountains have served as a natural laboratory for post-wildfire debris flow studies for decades, 77 including pioneering work that helped develop the current understanding of dry ravel processes 78 (e.g., Krammes, 1965), soil hydrophobicity and runoff erosion (e.g., Wells, 1987), and net 79 sediment export into debris basins (Lavé and Burbank, 2004). In this study, we use ideally timed 80 airborne lidar surveys to show the systematic spatial pattern of post-fire loading of headwater 81 valleys by dry ravel and subsequent excavation of channel fills during storms. 82

83
We utilized three airborne lidar surveys to constrain the timing and magnitude of 84 landscape-scale erosional response to the 2009 Station Fire (see Table DR1  watersheds along the range front between the southern strand of the San Gabriel fault zone and 120 the Sierra Madre fault zone (Fig. 1). When averaged at the scale of small watersheds (1-2 km 2 ), 121 lidar-derived calculations of net channel erosion from steep, burned watersheds are equivalent to 122 up to 4 cm of hillslope erosion (Fig. 1). 123

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Our data indicate a direct connection between the loading of headwater channels with dry 125 ravel deposits immediately following wildfire and the subsequent patterns of channel erosion due 126 to floods and debris flows (Fig. 2). The September 2009 lidar data provide a rare snapshot of 127 post-fire dry sediment loading in channels prior to rainfall, which is confirmed by topographic 128 change where pre-fire lidar exists (Fig. 2C) and is identifiable in the topography as characteristic 129 low-sloping sediment fills and debris cones (Fig. DR3). Notably, inspection of regions with 130 limited post-wildfire erosion response shows no evidence of channel fills (Fig. DR4). We 131 interpret the connection between dry ravel loading of channels post-fire and increased channel 132 erosion following rainfall to reflect a hillslope sediment supply control on post-wildfire sediment 133 yield and debris flows initiated due to dry ravel loading. 134 Although dry ravel loading of headwater channels leads to high post-wildfire sediment 135 yield in our study area, our data and prior work reveal complexities in the evolution of sediment 136 sources over time. First, there was a systematic pattern of channel erosion that exceeded dry 137 ravel deposition (Fig. 2D), indicating the scouring of pre-existing channel deposits (Santi et al., 138 2008). Notably, we observed this scour only in channels loaded with dry ravel following fire, 139 suggesting that the relatively fine-grained ravel deposits helped to initiate in-channel failure as 140 debris flows (Prancevic et al., 2014), and that these flows in turn scoured older channel fills to between catchment slope, burn severity, and post-wildfire erosion (Fig. 3). Instead, despite 159 similarities in topography (Fig. DR6), burn severity (Fig. DR3), fire history (Fig. DR7), and 160 vegetation cover (Figs. DR8 and DR9), there is a strong contrast between high post-wildfire 161 erosion along the southern range front and minimal erosional response north of the South San 162 observed pattern of post-wildfire erosion (Fig. DR6), suggesting that the difference may be 165 related to lithology. The South San Gabriel Fault Zone has juxtaposed granodiorites, tonalites, 166 and gneisses to the north with more fractured and mafic lithologies (hornblende diorite; biotite 167 monzogranite) to the south (Campbell et al., 2014). It is possible that soil production rates are 168 lower to the north, which caused a sediment-supply limitation, or that subtle differences in 169 sediment size and shape or bedrock roughness made post-fire soils more stable (DiBiase et al., 170 2017). While future work is needed to evaluate these hypotheses, our results support the idea that 171 small differences in topography, sediment properties, or lithology can lead to dramatic changes 172 in sediment yield on hillslopes that are very near the limit of sediment stability because dry ravel 173 is inherently a threshold process. 174

CONCLUSIONS 175
Overall, our data highlight key differences in the fire-flood cycle between soil-mantled 176 and bedrock landscapes that are important for understanding post-wildfire debris flow hazards 177 and longer-term landscape evolution. Rather than commonly used metrics of slope and burn 178 severity, predicting debris flow occurrence in bedrock landscapes requires constraining the 179 storage, routing, and particle sizes of dry ravel, which depends on pre-fire vegetation cover, long 180 term sediment production rates from bedrock, and hillslope-channel connectivity (Lamb et  during subsequent storms, which further amplifies sediment yield. In contrast, catchments 185 without post-fire ravel accumulation in channels did not show scour during storms. Thus, the 186 spatial pattern of dry ravel loading may largely determine post-fire sediment yield and debris flow occurrence. While dry ravel is generally associated with steep, bedrock hillslopes, 188 predicting the spatial pattern of loading remains a challenge. This challenge needs to be solved to 189 determine how landscapes will respond to a changing climate with increased fire frequency 190 because, unlike soil-mantled hillslopes, sediment yield from bedrock slopes is controlled by 191 sediment supply. Fortunately, the accumulation of thick sediment fills in channels immediately 192 following fire is readily measurable by airborne lidar and allows for direct quantification of 193 likely post-fire sediment yields and debris-flow hazards prior to rainfall. 194

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We