Fire geomorphology: Fire-related erosion helps to shape our landscapes

The frequency of large wildfires has increased on all vegetated continents (Bowman et al., 2009). Wildfires can have profound influences on erosion rates, particularly in steep mountain basins. Fire-related erosion varies in both magnitude and impact as a function of burn severity, basin characteristics, and timing and intensity of local climate conditions (Cannon et al., 2001, Meyer and Pierce, 2003). This study evaluates fire-related sedimentation from small steep tributaries of the Middle Fork Salmon River (MFSR) in central Idaho (see figure 1) to evaluate the timing, frequency, and magnitude of fire-related erosion. Rates and magnitudes of sediment delivery from hillslopes to fluvial systems influence long-term landscape evolution, aquatic habitat, reservoir capacity, and infrastructure stability (e.g. roads and development).

Short-term sediment yields have been measured using sediment traps and gauges and are orders of magnitude lower than long-term (10³-10⁴ yr) sediment yield estimates. Sediments traps are not able to measure large debris flow events. We combine directly measured recent (1997-2008) fire-related debris flow sediment yields and reconstructed fire-related debris flow frequencies to quantify the contribution of fire-related debris flows to long-term (10³-10⁴) sediment yields. Holocene debris flow frequencies are estimated from ­­­­¹⁴C-dating charcoal fragments preserved in fire-related deposits exposed in incised alluvial fans.

Recent large debris flow events have occurred in years following moderate to high severity fires that burned densely vegetated north-facing slopes in the MFSR (see figure 2). Burn severity data was obtained from the Monitoring Trends in Burn Severity Project (USDA Forest Service/U.S. Geological Survey). These large events have incised into easily erodible, geothermally altered Idaho Batholith granite bedrock and alluvial fan surfaces, deposited material adjacent to the incised channel, (see figure 3) and deposited a new fan surface extending into the larger main channel. Observations suggest that modern large debris flows in the MFSR primarily initiate from increased surface runoff, rilling (concentrated water flow in small channels) and progressive sediment bulking (e.g. Meyers and Wells, 1997) from erosion within the channel (see figure 4). These type of debris flows differ from those generated by saturation of colluvial hollows, which can occur on both unburned (e.g. Iverson 1997) or burned (e.g. Meyer et al., 2001) hillslopes.

Fire influences hydrologic processes in watersheds by 1) reducing the amount and rate of infiltration (water on the ground entering the soil), 2) altering vegetation structure, density, and root stabilization of soils and 3) changing soil and sediment properties (Shakesby and Doerr, 2006). Fires decrease surface roughness by removing vegetation and organic litter and can increase the water repellency of soils (DeBano, 2000). All of these influences increase surface runoff during years following fire. This, in turn, increases erosion of surface sediments, rilling, and debris flows in small steep drainages during years immediately following fire. Recent fire-related debris flows in the MFSR have incised into older alluvial fans and delivered large amounts of sediment into the main channel.

Tributary junction alluvial fans are fan-shaped depositional landforms that form at junctions where higher gradient, sediment filled streams dump into larger lowland areas where streams lose energy. Fans are located where there is accommodation space available to store sediment over time. Incision of alluvial fans by subsequent large debris flows expose past deposits and facilitates investigation of fire-related sedimentation over longer timescales. Alluvial fans act as sediment traps, capturing large individual events over time, preserving a record of deposition that can span long timescales.

Approximately 50%, of study site fan deposit thicknesses, were identified as fire-related. Fans in more-forested basins contain ~74% fire-related deposits compared to ~40% in less-forested basins. This difference could be attributed to increased preservation of large woody material in more-forested basins or to more severe fires in more-forested basins. Although increased vegetation density retains more sediment on hillslopes, burning this vegetation, potentially leads to larger erosional responses (e.g. Cannon et al., 2001). In lower less-forested basins, sheetflooding events were more common and compose greater fan thickness (~26%) compared to more-forested basins (~4%). Sheetflood events make up ~13% of the total investigated fan thickness. This suggests that sheetflood events are smaller in magnitude and deliver less sediment to the fluvial network than debris flows, which compose 62% of total fan thickness. The remaining fan thickness was composed of overbank flood deposits, channel deposits, buried soils, or deposits that could not be confidently identified.

What climate conditions are associated with large fire-related debris flows preserved in the stratigraphic record?

Paleoclimate records from the lower Salmon River and from the headwaters of the MFSR suggest that late Pleistocene-early Holocene (14-8.5 ka) climate was generally wetter, cooler, and more variable, mid-Holocene (8.5 – 5.5 ka) climate was generally drier, warmer, and more stable, and late Holocene (4 ka - present) climate was generally cooler, wetter, and more variable (Davis et al., 2002; Whitlock et al., 2010). Mid-Holocene alluvial fan deposits compose ~4% of total dated deposit thickness (see figure 5), suggesting decreased sediment transport during warm, arid, stable climate conditions. Deposits that date between 14 - 8.5 ka and < 5.5 ka compose ~22% and ~74% of total dated fan thickness (see figure 5) suggesting increased sediment transport during cooler, wetter and more variable climate conditions.

How much sediment do fire-related debris flows contribute to long-term (10³-10⁴ yr) sediment yields?

Measured debris flow deposit volumes indicate sediment yields between 1,000–75,000 Mg/(km^2.event) [Mg=10⁶grams=1 metric ton]. Fire-related debris flow frequency is greatest during the last 2 ka. While this may reflect better preservation of younger deposits, it may also indicate elevated fire frequency and severity. Based on 39 fire-related debris flows, dated to the last 6 ka, and assuming past similar sediment yields are similar to modern yields, fire-related debris flows have contributed ~190 Mg/(km^2.yr) (see figure 6). This suggests that fire is a major driver in long-term sediment yields, accounting for ~70% of a ~6500 yr basin-wide sediment yield for the larger Salmon River (Kirchner et al., 2001).

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