Historic land use and sedimentation in two urban reservoirs, Occoquan Reservoir and Lake Manassas, Virginia, USA.

Human population growth and subsequent land use intensification are closely linked to contemporary increases in sediment and associated contaminants fluxes to fluvial systems, lakes, reservoirs, and coastal zones worldwide. In most urban areas, reservoirs that are the main source of fresh water supply, if not effectively managed, suffer from water quality decline and loss of capacity associated with accelerated siltation. This study analyzes watershed soil losses and sediment accumulation rates in two reservoirs in the Occoquan river basin, a sub-watershed of the Chesapeake Bay in the suburbs of the greater Washington, DC area. Lake Manassas is located in the upper reaches of the basin, characterized by mixed land use and cover of mostly forest, residential areas, and agriculture, whereas Occoquan Reservoir is located in the more urbanized lower reach of the basin in the heavily populated suburban zone south of Washington, DC. Five sediment cores from each lake were used in 210Pb-based sediment accumulation rates analysis, and GIS-based Revised Soil Loss Equation (RUSLE) model and a sediment delivery ratio (SDR) were used to evaluate basin soil losses and sediment fluxes to the fluvial systems. 210Pb sediment accumulation rate estimates in Occoquan Reservoir range from 0.26 g cm-2 year-1 in the upper reaches to 0.37 g cm-2 year-1 in the lower reaches. Lake Manassas also had comparable accumulation values ranging from 0.22 to 0.40 g cm-2 year-1. RUSLE/SDR estimated watershed sediment fluxes were 0.26 Mg ha-1 year-1 (Mg-mega gram) in the upper watershed, which is significantly higher than 0.07 Mg ha-1 year-1 estimates for the lower reaches of the watershed. The variability in the reservoirs' sediment accumulation rates and basin soil losses reflects the variability of land use and cover, basin slopes, and erosion mitigation efforts within the watershed. The lower reaches, though more urbanized, have well-developed storm drain systems limiting run-off related soil losses. The well-managed riparian zones surrounding both reservoirs also limit sediment fluxes, hence the relatively low sediment accumulation rates. Although surficial sediment sources seem to be well managed, some of these efforts might be associated with the uptick in intrinsic sediment sources, leading to localized high sediment accumulation in the mouth of tributaries draining the high-intensity urban areas of the basin.

Soil erosion and sediment fluxes analysis: a watershed study of the Ni Reservoir, Spotsylvania County, VA, USA.

Anthropogenic forces that alter the physical landscape are known to cause significant soil erosion, which has negative impact on surface water bodies, such as rivers, lakes/reservoirs, and coastal zones, and thus sediment control has become one of the central aspects of catchment management planning. The revised universal soil loss equation empirical model, erosion pins, and isotopic sediment core analyses were used to evaluate watershed erosion, stream bank erosion, and reservoir sediment accumulation rates for Ni Reservoir, in central Virginia. Land-use and land cover seems to be dominant control in watershed soil erosion, with barren land and human-disturbed areas contributing the most sediment, and forest and herbaceous areas contributing the least. Results show a 7 % increase in human development from 2001 (14 %) to 2009 (21.6 %), corresponding to an increase in soil loss of 0.82 Mg ha(-1) year(-1) in the same time period. (210)Pb-based sediment accumulation rates at three locations in Ni Reservoir were 1.020, 0.364, and 0.543 g cm(-2) year(-1) respectively, indicating that sediment accumulation and distribution in the reservoir is influenced by reservoir configuration and significant contributions from bedload. All three locations indicate an increase in modern sediment accumulation rates. Erosion pin results show variability in stream bank erosion with values ranging from 4.7 to 11.3 cm year(-1). These results indicate that urban growth and the decline in vegetative cover has increased sediment fluxes from the watershed and poses a significant threat to the long-term sustainability of the Ni Reservoir as urbanization continues to increase.

Spatial analysis of soil erosion and sediment fluxes: a paired watershed study of two Rappahannock River tributaries, Stafford County, Virginia.

Soil erosion is a serious problem in areas with expanding construction, agricultural production, and improper storm water management. It is important to understand the major processes affecting sediment delivery to surficial water bodies in order to tailor effective mitigation and outreach activities. This study analyzes how naturally occurring and anthropogenic influences, such as urbanization and soil disturbance on steep slopes, are reflected in the amount of soil erosion and sediment delivery within sub-watershed-sized areas. In this study, two sub-watersheds of the Rappahannock River, Horsepen Run and Little Falls Run, were analyzed using the Revised Universal Soil Loss Equation (RUSLE) and a sediment delivery ratio (SDR) to estimate annual sediment flux rates. The RUSLE/SDR analyses for Horsepen Run and Little Falls Run predicted 298 Mg/y and 234 Mg/y, respectively, but nearly identical per-unit-area sediment flux rates of 0.15 Mg/ha/y and 0.18 Mg/ha/y. Suspended sediment sampling indicated greater amounts of sediment in Little Falls Run, which is most likely due to anthropogenic influences. Field analyses also suggest that all-terrain vehicle crossings represent the majority of sediment flux derived from forested areas of Horsepen Run. The combined RUSLE/SDR and field sampling data indicate that small-scale anthropogenic disturbances (ATV trails and construction sites) play a major role in overall sediment flux rates for both basins and that these sites must be properly accounted for when evaluating sediment flux rates at a sub-watershed scale.