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.
Sediment data sources and estimated annual suspended-sediment loads of rivers and streams in Colorado