Abstract
High-elevation mountain streams are commonly viewed as erosive environments, but they can retain sediment along river corridors for thousands of years. In 2013, an extreme flood evacuated floodplain sediment in the Colorado Front Range, USA. We use fifty-two 14C ages collected along four streams prior to the flood to estimate mean residence time of floodplain sediment. Here we show that mountain streams above the elevation of the Pleistocene terminal moraine retain floodplain sediment for longer durations than those at lower elevation, but that wildfires may decrease floodplain sediment residence time at high elevations. Comparison of field sites and differencing of pre- and post-flood lidar show that valley confinement is a significant predictor of residence time, sediment flux, and floodplains disturbed by the 2013 flood. Elevational trends in floodplain disturbance regime also reflect differences in forest type, precipitation pattern, and wildfire regime, which are expected to shift under a changing climate.
Highlights
High-elevation mountain streams are commonly viewed as erosive environments, but they can retain sediment along river corridors for thousands of years
Erosion and sedimentation within river corridors is important for landscape evolution, natural hazards, sediment yield, the fate and transport of contaminants[1,2], organic carbon storage along mountain rivers[3,4], river ecosystems, and water quality[5]
We find that river corridors at elevations above the Pleistocene glacial terminal moraine in the North Saint Vrain Creek study basin (~ 2500 m) exhibit significantly older mean ages of floodplain sediment compared with those at lower elevations
Summary
High-elevation mountain streams are commonly viewed as erosive environments, but they can retain sediment along river corridors for thousands of years. Increased variability and uncertainty in precipitation as a result of a warming climate are projected to increase the frequency and magnitude of extreme precipitation events and potential for river flooding in the western US11,12, which could greatly influence erosion and sediment dynamics within river corridors. Recent work in the Colorado Rocky Mountains suggests that hillslope erosion during extreme events can greatly impact floodplains and may have a significant role in the exhumation rate of mountain ranges[16,17] This may be important in high-relief terrain where elevation boundaries between predominantly snowmelt and rainfall–runoff could shift[18] and glacial retreat could expose significant sediment sources to erosion by rivers[19]. Increases in rainfall and decreased glacial extent, for example, are likely to increase exposure and erosion of unconsolidated glaciated sediments in the Himalayas[20]
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