20-8 Floodplain Sediment Organic Carbon Response to River Restoration

Abstract:

Carbon sequestration by river restoration has been pitched as a potentially important natural climate solution in recent years, but we currently lack the data and knowledge to predict the response of floodplain organic carbon (OC) dynamics to river restoration. We investigated the impact of river restoration on floodplain sediment OC storage and accrual rates on Ninemile Creek (Montana, USA) which was degraded by repeated placer mining over the last century. Restoration led by Trout Unlimited has restored channel-floodplain connectivity by resetting the valley bottom along different reaches of the creek from 2014 to 2024. Using a space-for-time approach, we evaluated floodplain sediment OC accrual and changes in storage across reaches restored at different times throughout the past decade. In each reach, we delineated ecogeomorphic units (EGUs) – floodplain patches characterized by distinct geomorphology and vegetation – to account for the potential difference in local fluvial ecogeomorphic processes that influence floodplain sediment OC dynamics. We collected sediment samples from EGUs in five reaches: three restoration reaches (restored in 2014, 2018, 2022), a reach that was not restored, and a reach that was not impacted by placer mining. To determine sediment OC storage, we used an element analyzer to measure OC content (percent by mass) and, using bulk density values and alluvial thickness measurements, we calculated sediment OC stock (Mg C per ha) for each sampling location. We utilized fallout radionuclides – ⁷Be and ²¹⁰Pb – to determine deposition dates and estimate OC accrual rates of each EGU. Preliminary results show an increase in floodplain sediment OC storage with time since restoration. Total sediment OC stock in the 2014 restoration reach was greater than the unrestored reach and similar to that of the reach unaffected by placer mining. Our results provide evidence for increased floodplain OC storage as a co-benefit of river restoration. This work will inform resource management decisions regarding river restoration by providing insights into how sediment OC changes with restoration and over what timescales. 

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.