Differences in mean floodplain sediment turnover time associated with distinct hydroclimatic disturbance regimes in the Colorado Front Range

Abstract:

Residence time of floodplain sediment in mountainous streams influences carbon and nutrient dynamics, riparian and aquatic ecosystems, sediment yield, long-term exhumation rate, and landscape evolution. We hypothesize that differences in hydroclimatic regime on a longitudinal gradient in the Colorado Front Range result in more rapid floodplain sediment turnover rate at elevations below 2300 m. Differences in hydroclimatic regimes along the Front Range result from increased frequency and magnitude of large convective thunderstorms below 2300-m elevation. Differences in precipitation and cumulative effects of larger contributing drainage areas at lower elevations result in increased magnitude of floodplain disturbances. We use randomly selected radiocarbon ages of charcoal from the subsurface of floodplains 2 – 6 times the bankfull channel width to estimate mean floodplain sediment turnover time along three study reaches in the North Saint Vrain Creek watershed. Study reaches at 2930 m, 2360 m, and 2220 m in elevation, with drainage areas of 10.1 km², 96 km², and 200 km², have mean calibrated radiocarbon ages of 1338 ± 297, 527 ± 350, and 843 ± 589 y BP, respectively. Floodplain sediment turnover time, calculated by dividing the mass of the stored sediment by the mean radiocarbon age, result in estimates of 1, 8.1, and 7.5 g/100 y from high to low elevation sites, respectively. No observed floodplain disturbance other than overbank flow and aggradation occurred at 11 study sites above 2300 m, whereas delineation of lateral floodplain erosion from pre- and post-event aerial imagery and post-2013 flood LiDAR indicate ~15 ha of floodplain erosion below 2300 m. Preliminary results from the 10 charcoal samples suggest support for our hypothesis, but 16 – 20 additional radiocarbon ages currently being processed will reduce uncertainty associated with limited samples and confounding probability of “old” charcoal being incorporated into sites with larger drainage areas.