CHARACTERIZING SEDIMENT MOBILIZATION AND LANDSCAPE RESPONSE TO WILDFIRE AND EXTREME FLOODING USING SHORT-LIVED FALLOUT RADIONUCLIDES 137CS AND 210PB, COLORADO FRONT RANGE

The fallout radionuclides ¹³⁷Cs and ²¹⁰Pb adhere strongly to mineral and organic matter and are useful for studying sediment mobilization through a landscape. Rapid decay of ¹³⁷Cs and ²¹⁰Pb (half-lives of 30 and 22 years, respectively) make them useful tools to measure sediment movement over the past century. Here, we present radionuclide analyses from Fourmile Canyon, Colorado Front Range, which experienced a 25 km² wildfire in September 2010, post-fire hillslope erosion and flooding in summer 2011 and 2012, and severe flooding and erosion in September 2013. We have sampled burned and unburned hillslopes, gully and tributary deposits, alluvial fans at the toes of tributaries, overbank deposits along Fourmile Creek of layered post-fire and 2013 flood stratigraphy, active channel deposits such as sediment (sands/fines) directly adjacent to the current Fourmile Creek main channel, and suspended sediment. Depth profiles of ¹³⁷Cs and ²¹⁰Pb on hillslopes consistently show highest concentrations of both fallout radionuclides in the upper 3 cm, a sharp decline with depth, and total inventories contained within 7-9 cm. A comparison of select unburned and burned/eroded hillslopes sites reveals differences consistent with ~3-5 cm of post-fire erosion and removal of ash-rich surface material enriched in ¹³⁷Cs and ²¹⁰Pb in the three years following the 2010 wildfire. Prior to the 2013 flooding, layered alluvial fan and overbank deposits, sand samples within and adjacent to the channel, and suspended sediment displayed concentrations of radionuclides consistent with surface erosion, mobilization and deposition of near surface hillslope material. Layered overbank sand and gravel deposits, tributary sands, and near-channel sand collected after the September 2013 floods, however, had near-zero concentrations consistent with erosion, mobilization and deposition of deep (>10 cm depth) material containing no ¹³⁷Cs or ²¹⁰Pb. Overall, deposits sampled and analyzed to date highlight the utility of fallout radionuclides for source-to-sink tracking of sediment derived from shallow processes, particularly overland flow, compared with deeper processes such as the landslides, debris flows and channel erosion that occurs in association with intense rain and flooding.