A Sediment Budget for a Sand Bed River Partitioned by Sand Fractions

• Published in: Journal of geophysical research. Earth surface 2024-07, Vol.129 (7), p.n/a
• Main Authors: Leonard, Christina M., Schmidt, John C.
• Format: Article
• Language: English
• Subjects: Accumulation; Beds (process engineering); channel change; Climate change; Disturbance; Disturbances; Downstream; Evacuation; Flood predictions; Floodplains; Global warming; Grain size; Impact prediction; Islands; Mass balance; Moisture content; River beds; River channels; Rivers; Sand; sand bed river; Sand beds; Sand transport; Sediment; sediment budget; Sediment sorting; Sediment transport; Sediments; Shaving; Water content
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2023JF007384

Sediment budgets are widely used to measure reach‐scale sediment accumulation and evacuation. Such measurements, however, cannot determine when the disturbance is major and the measured sediment mass imbalance is reflective of a river adjusting to a new equilibrium state, as opposed to situations when the disturbance is minor, and the mass imbalance is reflective of a river adjusting within its existing behavioral regime. Sediment sorting among channels and floodplains can have a large effect on how a river responds to a disturbance. Fine sediment may accumulate in the floodplains while coarser sediment erodes from the channel bed. We demonstrate that if a sediment budget does not account for the different behavior and destination of grain sizes, the budget cannot reveal important channel adjustments. In this study, we evaluated how a sand bed river responded to increases in sediment supply by partitioning a sediment budget among silt/clay and five sand fractions. On average, 12 metric tons/meter (downstream)/year of sand was evacuated from the system, but sorting caused channel margins to behave differently from vegetated islands, revealing how a river can slightly narrow while in deficit. Floodplain shaving and bed coarsening evacuated sediment while channel geometry barely changed, consistent with a river adjusting to a minor disturbance within its behavioral regime. This study is an important reminder that sediment mass imbalance does not always lead to channel change. Mechanisms such as floodplain shaving and bed textural change help rivers absorb minor disturbances and resist channel change. Plain Language Summary Rivers are experiencing increasing human interference, and global warming will significantly impact the amount of water and sediment rivers must convey, placing an even greater need to predict how rivers respond to disturbance. One important problem is predicting the susceptibility of a river to change. While measuring the amount of sediment that accumulates or evacuates from a reach provides clues to how rivers respond to disturbances, such measurements cannot determine whether reach‐average morphology, such as width, depth, slope, and planform, will significantly change or minor adjustments may occur. In this study, we evaluated how a sand bed river responded to an increase in sediment supply that occurred ∼60 years ago using continuous sediment transport measurements and aerial images to measure channel adjustment. We found that, on aver