What caused the Harmful Algal Blooms (HABs) during the early summer of 2023 at Smith Mountain Lake? Why did they last for so long? Are we at risk for future HABs at the lake? How can we prevent future HABs? Why didn’t we see HABs this summer? All these questions have been asked by the community, our visitors and especially the Smith Mountain Lake Association (SMLA).
While our Water Quality Monitoring Program, which has been operated by Ferrum College for 38 years, continues its important work to assess the trophic state (age) of the lake, and to evaluate in-lake water quality, we don’t have a full understanding of the dynamics of the watershed that feeds the lake. For example, we haven’t studied the conditions of the lands surrounding the lake and we don’t have a grasp yet on sources of excess nutrients and if those sources are coming from outside the lake or already are in the lake, naturally.
Watersheds are all the lands that drain to a waterbody and are organized from large to small systems. Think about the Atlantic Ocean as a macro watershed, and the Blackwater River as a sub-watershed, way down on the organizational level.
The Blackwater is fed by many small streams, and it in turn feeds the Roanoke River. The Roanoke River really begins in the Blue Ridge foothills near Christiansburg and once at the lake, starts to collect water from creeks such as Betty’s, Becky’s, Grimes, Beaverdam, and larger rivers such as the Blackwater, Gills Creek, Witcher Creek, Craddock Creek, Bull Run and many other unnamed streams. As the Roanoke continues downstream beyond Leesville Lake, it also gains water from the Staunton, Dan and Mayo Rivers, and many other feeders before it eventually reaches the Albemarle Sound at the Outer Banks of North Carolina. The Roanoke River watershed, sometimes called a basin, covers nearly 10,000 square miles.
The SMLA took a small bite out of the huge undertaking it would be to assess the conditions of the entire SML watershed. In the fall of 2023 we hired Princeton Hydro, LLC (Princeton Hydro), a professional engineering and lake management firm with vast experience in water resources engineering, lake management, floodplain and watershed modelling, dam removal and stream restoration to help us design a watershed assessment for the Blackwater arm of SML. The work was conducted throughout the spring and summer of 2024, and was completed in early September.
This mostly desk-top assessment contains several tasks, which, when compiled will provide a context and better understanding of land uses and conditions around the Blackwater, as well as provide suggestions for mitigating nutrient loads with the intent of reducing contributions to future HABs. The work will also assist the Virginia Department of Environmental Quality conduct their own watershed-based assessment of the full lake, with the collaboration of the Virginia Tech. Water Resources and Research Center, which will kick off this fall.
Princeton Hydro’s first task was to gather and analyze prior research and studies performed at the lake. Various scientific papers as well as state studies into pollutant loads were sourced. A look at the extensive database that is expanded every year by Ferrum College was also included in this task.
The overarching takeaway is that the nutrient phosphorus has been known historically to be coming from outside the lake, from non-point sources. Phosphorus is widely accepted to be the primary fuel of cyanobacteria (blue-green algae) which can overgrow to cause HABs. Sources of phosphorus in a natural system are varied but start with soils and sediments introduced through erosion and streambank destabilization.
Land use contributes large amounts of phosphorus to any system, whether it is urbanization, overuse of fertilizer, malfunctioning septic systems, or agricultural practices. Ferrum College’s phosphorus data for tributaries entering the SML system indicates that high levels of this nutrient can be found in all tributaries to Smith Mountain Lake, but within the lake, data collected in main channel locations indicate that levels of phosphorus historically have not been increasing substantially.
Princeton Hydro then compiled multiple characteristics of land use and other inputs to develop a model to estimate loading of nutrients into the Blackwater arm of the lake. The Blackwater watershed covers approximately 171,000 acres and is divided into eight sub watersheds. For each sub watershed physical characteristics were reported such as land use, soil infiltration types, slope, erosion rates, seasonal hydrology including runoff, nutrient loads, sediment and bacteria. Additionally, all houses within each sub watershed were counted, because within the Blackwater watershed, all homes are served by septic systems. More weight was given to homes within 15 meters of the lake or a stream. Animal data gathered from the USDA was entered into the model for agricultural lands. These factors, when looked at singly and together can illuminate and focus attention to primary causes of nutrient loads in the lake. Detailed charts by sub watershed provide insight into conditions around the Blackwater area.
These outputs were then used to prepare a series of specific watershed-based management techniques which when installed can reduce the nutrient loads to a lake. Detailed sheets explaining the efficacy of each technique are provided. These techniques and project types vary from individual homeowner style projects to full stream bank restoration projects and public education about the value and importance of specific management practices.
The final task Princeton Hydro completed was a sediment sampling effort to determine whether certain cyanobacteria overwinter in our sediments. Wintertime sediment samples were collected from areas in the Blackwater arm that were affected by HABs in 2023. These were incubated to encourage growth of any dormant cyanobacteria. The results of this limited sampling effort indicate that the sediment type is more critical to the overwintering abilities of cyanobacteria than any other factor.
Finally, Princeton Hydro recommended some limited next steps which could follow the watershed assessment. These include a recommendation that the internal phosphorus load of the lake be determined and compared to the external load. Additional water sampling efforts in the areas that were most affected by HABs in 2023 were suggested to understand the lake chemistry of affected coves. Added water sampling was suggested to correlate cyanobacteria cell counts to a measure of the pigment that only cyanobacteria emit could yield a useful and practical screening tool for the future. And finally, further sampling designed to align the overwintering sediment analysis with water sampling data could be valuable.
As mentioned previously, DEQ and Virginia Tech will be continuing this work for the entire lake, in a similar fashion. SMLA is participating in this effort as an engaged stakeholder and this work should begin soon. SMLA is grateful to the scientists at Princeton Hydro for their guidance and expertise while conducting this work as well as to the scientists at Ferrum College who contributed lake data to the effort.