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Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and
William J Beck1, Peter L Moore1, Keith E Schilling2
1Iowa State University, Department of Natural Resource Ecology and Management, 2310 Pammel Dr., Ames, IA 50011, USA.
The Science of the Total Environment
|February 13, 2019
Summary
Channel widening reduced floodplain storage of sediment and phosphorus. This decline in storage capacity increases watershed export of these pollutants, potentially masking conservation efforts. Channel evolution must be considered in watershed management.
Area of Science:
- Environmental Science
- Hydrology
- Geomorphology
Background:
- Floodplain storage is a major component of watershed sediment budgets.
- Channel evolution, driven by large-scale disturbances, alters channel geometry and floodplain connectivity over time.
- Changes in channel geometry can significantly impact the dynamics of sediment and nutrient fluxes to floodplains.
Purpose of the Study:
- To investigate the effects of changing channel geometry on floodplain inundation frequency.
- To quantify the impact of altered channel geometry on suspended sediment (SS) and total phosphorus (TP) flux to floodplain storage.
- To assess the implications of reduced floodplain storage on watershed export of SS and TP.
Main Methods:
- Conducted 25 in-field channel cross-section transects to measure changes in channel geometry over 16 years (1998-2014).
- Utilized hydraulic modeling (HEC-RAS) to estimate floodplain inundation frequency and volume.
- Analyzed stream gauging station data for water quality (SS, TP) and quantity to determine fluxes.
Main Results:
- Channel cross-sectional area increased by a mean of 17%, indicating a trend of degradation and widening.
- The discharge required for overbank flow increased by 15%, and floodplain inundation volume decreased by 37%.
- Annual fluxes of SS and TP to floodplain storage decreased by 61% and 62%, respectively, leading to potential increases in watershed export by 9% (SS) and 18% (TP).
Conclusions:
- Channel evolution, specifically widening and degradation, significantly reduces floodplain storage capacity for sediment and phosphorus.
- Decreased floodplain storage leads to increased watershed export of SS and TP, potentially undermining the effectiveness of edge-of-field conservation practices.
- Understanding and accounting for channel evolution stages is critical for effective watershed-scale management of sediment and phosphorus loading.
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