Longitudinal variability in a small, unidirectional reservoir
Lakes and reservoirs exhibit substantial spatial variability, motivating the need to monitor water quality at multiple sites within the same waterbody to inform management. Specifically, drinking water outtakes are commonly located near reservoir dams, where water quality is highly monitored. In contrast, water quality upstream of the dam is less frequently monitored, yet could provide valuable insight into drinking water quality. For reservoirs with unidirectional flow, knowledge of upstream conditions could help identify pending downstream water quality impairment. We explored the relationship between upstream and downstream water quality in Falling Creek Reservoir (FCR), a small (0.119km2), eutrophic, primarily unidirectional reservoir in Vinton, VA. FCR is managed and operated as a drinking water supply by the Western Virginia Water Authority, with outtakes located at the dam. From 2013-2017 and in 2024, we used spatial high-frequency vertical profiling sensors (measurement every 25 centimeters) that collected conductivity, temperature, dissolved oxygen, depth, chlorophyll a, and phytoplankton fluorescence wavelengths at five sites along a linear flowpath within FCR. Starting in 2018, the dam site at FCR was outfitted with temporal high-frequency sensors (measurement every ten minutes) that collect conductivity, temperature, dissolved oxygen, and chlorophyll a every ten minutes. In 2024, we deployed a matching temporal high frequency sensor at one of the upstream sites from ~June - October. We analyzed multiple years of spatial and temporal high-frequency monitoring data at these sites in FCR. Historical data suggest that upstream sites can provide insight into downstream chlorophyll a concentrations three to four days in advance, however, data collected this past summer suggest that temperature, chlorophyll a, and dissolved oxygen are similar throughout the reservoir, which may be due to changes in water residence time, hydrology, or other factors. Regardless of the lack of spatial variability in 2024, however, upstream data could provide valuable insight into future downstream water quality conditions. Over the 2024 summer monitoring period, FCR experienced four phytoplankton blooms (chlorophyll a >50 ug/L), highlighting the need to predict water quality near the outtake. Altogether, our study is part of a long-term monitoring and water quality forecasting program that aims to advance ecosystem understanding and protect water quality in FCR.
Author Bio
Katie Hoffman is a second-year graduate student at Virginia Tech and a Virginia Lakes and Watersheds Association Scholarship recipient. She is broadly interested in freshwater ecology, with a focus on communicating science and increasing community engagement in research. Her current projects include modeling and forecasting water quality in lakes and reservoirs, co-developing forecast visualizations for managers and the public to guide decision making, and analyzing the effects of water level drawdown on phytoplankton and cyanobacteria.
