THE TUMBLING TRIUMVIRATE: QUANTIFYING FLOOD-RELATED SEDIMENT, WOOD AND CARBON (Invited Presentation)

During a four day period in September 2013, over 350 mm of precipitation fell within the North St. Vrain (NSV) basin, Front Range, Colorado resulting in a greater-than 200-year flood that produced extensive damage to downstream communities. Using field surveys, reservoir coring, and pre-and post lidar and Normalized Difference Vegetation Index (NDVI) analysis, we quantified over 300,000 m³ of sediment and wood that were delivered to a local reservoir sourced from over 30 landslides and bank erosion within a 15 km reach of the NSV. Of this total, approximately 256,000 m³ of sediment (86%) were deposited at the inlet, with 31,000 m³ of sediment (10%) deposited in the reservoir delta. Based on a reservoir core, 420 mm of sediment deposition occurred in the delta during the flood, nearly 100x background rates of 5.4 mm/yr over the 44 year life of the dam. Approximately, 4300 m³ of large wood (>1% of total flood input) formed a log jam at the inlet, the equivalent of 460 Mg of carbon (Mg C). NDVI differencing indicates a total input of carbon during the flood of 16,100 Mg C from slope failures and channel erosion. Post-flood high snowmelt discharges along North St. Vrain Creek in 2014 remobilized sediment, large wood and carbon stored in the catchment, contributing a comparable volume of sediment to the reservoir delta as was deposited during the flood, causing delta progradation of over 20 m. Additional post-flood loading of wood and carbon is in progress. Further research includes assessing continued geomorphic change within the NSV channel and tracking sediment and wood downstream to quantify ongoing loading. To our knowledge, this is the first quantification of sediment, wood, and carbon from source to sink after an extreme flood in a tectonically quiescent, semi-arid environment.