Geomorphic Response of Roaring River and Fall River to the September 2013 Flood

Heavy flooding associated with the September 2013 storm caused severe disturbance along many rivers draining the Colorado Front Range. In Rocky Mountain National Park, the 2013 flood destabilized steep segments of Roaring River and deposited an unusually large amount of sand- and gravel-sized sediment near the confluence with Fall River. We initiated field studies of these two river systems in May 2014 to investigate the effect of an increase in sediment supply on the geomorphology of Fall River. Measurements of water discharge and bed load were taken from May through August at three different locations to capture spatial and temporal variations in transport rates for a range of flows. Peak transport rates coincided with the peak discharge at the upstream sampling site (FR-1), but lagged behind the peak in discharge at the lower site (FR-2) by about three weeks, which is consistent with diffusive movement of sediment as observed in earlier studies. On average, 2014 transport rates were 0.022 kg/m/s and 0.035 kg/m/s at FR-1 and FR-2, respectively; these rates are 59 and 82 percent lower than rates measured previously at the same locations, but still high in comparison to other streams in the region. Erosion and deposition were also tracked by comparing temporal differences in surveyed river cross sections every ~100m along Fall River. The results of these surveys indicate 1550 m3 of erosion throughout a majority of the upstream portion of Fall River in a 50-day span. Calculations of shear stress from velocity measurements at FR1 and FR2 ranged from less than one to 1.3 times the threshold for motion, with the bed load consisting of mostly sand. The surface median grain size decreases exponentially downstream from 43 to 21mm, while remaining nearly constant for bed load from 1.3 to 0.9mm, consistent with partial transport. The rapid fining and transition to a mostly sand bed reflects the large increase in sediment supply unique to Fall River's 2014 conditions.