Changing suspended sediment in United States rivers and streams: linking sediment trends to changes in land use/cover, hydrology and climate

Abstract

Abstract. Sediment is one of the leading pollutants in rivers and streams across the United States (US) and the world. Between 1992 and 2012, concentrations of annual mean suspended sediment decreased at over half of the 137 stream sites assessed across the contiguous US. Increases occurred at less than 25 % of the sites, and the direction of change was uncertain at the remaining 25 %. Sediment trends were characterized using the Weighted Regressions on Time, Discharge, and Season (WRTDS) model, and decreases in sediment ranged from −95 % to −8.5 % of the 1992 concentration. To explore potential drivers of these changes, the sediment trends were (1) parsed into two broad contributors of change, changes in land management versus changes in the streamflow regime, and (2) grouped by land use of the watershed and correlated to concurrent changes in land use or land cover (land use/cover), hydrology and climate variables and static/long-term watershed characteristics. At 83 % of the sites, changes in land management (captured by changes in the concentration–streamflow relationship over time; C–Q relationship) contributed more to the change in the sediment trend than changes in the streamflow regime alone (i.e., any systematic change in the magnitude, frequency or timing of flows). However, at >50 % of the sites, changes in the streamflow regime contributed at least a 5 % change in sediment, and at 11 sites changes in the streamflow regime contributed over half the change in sediment, indicating that at many sites changes in streamflow were not the main driver of changes in sediment but were often an important supporting factor. Correlations between sediment trends and concurrent changes in land use/cover, hydrology and climate were often stronger at sites draining watersheds with more homogenous, human-related land uses (i.e., agricultural and urban lands) compared to mixed-use or undeveloped lands. At many sites, decreases in sediment occurred despite small-to-moderate increases in the amount of urban or agricultural land in the watershed, suggesting conservation efforts and best-management practices (BMPs) used to reduce sediment runoff to streams may be successful, up to a point, as lands are converted to urban and agricultural uses.

Incorporating Water Quality Analysis into Navigation Assessments as Demonstrated in the Mississippi River Basin

“…So, this decrease in SSC continues well beyond the timeframe expected if the decrease was exclusively related to these river management activities. That is, the decrease in SSC cannot be totally explained by river management activities, which is consistent with what was found in Allison et al (2017) and Murphy (2020). It is possible that efforts to control erosion and minimize sediment transport from erosional areas in urban and agricultural settings have been at least somewhat successful.…”

Effect of climate variability on water quality trends in New Zealand rivers

“…Trend analysis serves multiple purposes, including evaluating changes in environmental state, assessing the effectiveness of management actions and policies, evaluating relationships between environmental conditions and the factors that influence them (i.e. driver or explanatory variables) and providing early warning of environmental problems (McMellor and Underwood 2014;Tomperi et al 2016; Ministry for the Environment and Statistics New Zealand 2020; Murphy 2020).…”

The Color of Rivers

“…Trend analysis can be used to identify hotspots of change, target rivers for future experiments, and disentangle drivers of change. Limited gauge data has shown many rivers around the world have long-term trends in suspended sediment, CDOM, or chl-a concentrations due to both local and global drivers (Haaland et al, 2010;Li et al, 2020;J. C. Murphy, 2020;Roulet & Moore, 2006).…”