Rathburn, Sarah听1听;听Eidmann, Johanna听2
1听Department of Geosciences, Colorado State University
2听Department of Geosciences, Colorado State University
Snowmelt runoff since the 2013 Flood continues to transport sediment and wood downstream into Ralph Price Reservoir along North St. Vrain Creek, CO. Tracking the fate of sediment and wood is important for understanding channel response to floods, downstream sediment loading, and the influence of extreme events on landscapes. Over 40% of flood eroded sediment and 30% of wood remained in storage upstream from the reservoir after the flood. We seek to fill a gap in knowledge of the processes of post-flood recovery by tracking flood constituents from source to sink. We quantify the sediment and wood flux from the inlet to the reservoir using bathymetry and sediment coring, repeat topographic and wood surveys, and quadcopter-based DEM of differencing to quantify aggradation and degradation. The dominant processes of response include channel widening during snowmelt runoff as well as incision during a base level drop in the reservoir and delta aggradation. In 2014, a flood-equivalent volume of sediment was delivered to the reservoir delta. Net deposition of 67,000 m3听over 14,000 m2听(94% of the 2017 delta area) of the inlet occurred between years 1 and 3 post-flood. Over the past 4 years, the delta front prograded 180 m. We expect a decline in progradation rate as easily entrained sediment is evacuated and the channel stabilizes. Wood flux has declined by two orders of magnitude, from the 4300 m3log jam at the inlet to 500 m3听in 2014 and 20 m3听in 2017. Additional research on channel degradation through widening and minor incision upstream will be linked to delta aggradation. In addition, numerical modeling of channel change and delta evolution will be used to predict ongoing deposition and reservoir storage capacity losses under various runoff scenarios. Episodic sediment and wood transport during high discharges poses continued challenges for water management within Ralph Price Reservoir.