225-7 Linking Upstream Debris Flows to Watershed-Scale Water Quality and Sediment Yields Over Annual to Millennial Timescales

Session: Critical Zone Science: Intersection of Processes Linked to Geomorphology, Ecology, Fire and Climate (Posters)

Poster Booth No.: 225

Presenting Author:

Jennifer Pierce

Authors:

Bergstrom, Anna¹, Pierce, Jennifer L.², Huber, Dave³, Formigli, Kyle⁴, Enterkine, Josh⁵, Chaffee, Bryn⁶, Killingstad, Gabrielle⁷

(1) Geoscience, Boise State University, Boise, ID, USA, (2) Geoscience, Boise State University, Geoscience, Boise, ID, USA, (3) Gesocience, Boise State University, Boise, ID, USA, (4) Geoscience, Boise State University, Boise, Idaho, USA, (5) Geoscience, Boise State University, Boise, Idaho, USA, (6) Geoscience, Boise State University, Boise, Idaho, USA, (7) Geoscience, Boise State University, Boise, Idaho, USA,

Abstract:

While many studies examine the influence of wildfire on hillslope erosion, and the importance of hillslope sediment inputs to the generation of fire-related debris flows, fewer efforts have examined the impact of fire-related debris flows on stream physical and chemical water quality parameters in the year following a wildfire. This study summarizes the results of a 1 year, intensive field campaign which measures the impacts of the Wapiti fire which burned ~520 km2 in central Idaho in 2024.  In the year following the fire, multiple debris flows in 1st and 2nd order streams occurred, two of which partially blocked the South Fork Payette River. Soil ammonium (NH4+) and phosphate (PO43-) concentrations, as well as total carbon (TC) (0-2 cm) are elevated at burned hillslope locations relative to unburned controls and show an increasing trend with burn severity. Stream chemistry in DF watersheds have the highest NO3- and PO43- concentrations; however, channel substrate and hillslope soil nitrate (NO3-) concentrations are not significantly elevated. Comparisons with burned tributaries and an unburned control stream indicate burned tributaries generally have higher NO3- but have only slightly elevated PO43-. Dissolved organic carbon (DOC) is comparable at all sites and is higher during spring runoff. Along the SFP river, NO3- is highest within the burn perimeter and decreases downstream, however this pattern is not observed in PO43- or DOC. DOC and NO3- are responsive to snowmelt but there is no apparent relationship with PO43-. Other post-fire erosion studies in the watershed (Ellett et al., 2019) show ~75% of the contribution of debris flow sediments comes from channels, with ~25% of contribution from hillslopes; this study will assess sediment yields from 2024-2025 events using a suite of remotely sensed data. Millennial-scale records from the SFP (Meyer et al., 2001) show fire-related debris flows can contribute >40,000 T/km2/yr of sediment; stratigraphic records show these events are rare, and are driven by changes in climate.  Preliminary stream chemistry results suggest that sediments sourced from hillslopes and channels are highly variable in their physical and chemical properties and responsiveness to events; future studies will link this ongoing work on water chemistry to long-term changes in sediment inputs.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7813

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