Editor’s Note: This article is authored by Eric Krueger, director of science and stewardship with the Nature Conservancy South Carolina field office. Eric and his team are working on an ecosystem flow study that will be incorporated into the ongoing Savannah River Basin Comprehensive Study (interim 2).
Managing the basin for both up and downstream needs is a daunting job. From the viewpoint of downstream fish, wildlife, forests, and other natural resources, the amount and timing of water releases from Thurmond Dam is critical for the health of the basin.
Downstream health is not always about more water; sometimes it is about less or just enough. Or, a period of flow stability may be more critical than the amount. Is it possible to meet these complex needs while also retaining values for flood control, hydropower, lake recreation and property value, water supply, the health of the harbor, and other functions?
Answering yes is more possible today than ever before. River models and analysis tools are better than ever, and a great deal of refined science has been generated to get the most out of those tools. Getting to yes requires embracing some basic principles including:
- Natural and Societal Values: River solutions must be embraced by the user community to have long-term success.
- Embracing Variability: Important river functions happen at high flows, low flows, and everything in-between. Not every function can or must happen every year.
- Recognizing Constraints: Some functions may be impossible to meet without new infrastructure, retrofits, or other unlikely near-future solutions.
- Adaptive Management: Act now based on science in hand, monitor results, and retain flexibility to make better decisions in the future.
In 2003, The Nature Conservancy and over 50 river scientists worked hand-in-hand to craft the first flow recommendations for the Savannah River downstream of Thurmond Dam. We quickly realized that we had many unknown areas about the river’s resources and identified 21 areas of needed research. In a great wave of volunteer action, many scientists returned to the river and created a substantial body of new knowledge over the last 11 years; a body that grows to this day. Much of this work was targeted at the specific effects of flow releases on river resources. The Corps of Engineers was instrumental in releasing test flows in the mid-2000’s that allowed researchers to target their work at particular flows.
The list of what was learned is long, but here are a few examples:
- In 2003, many thought the river required large floods of 50,000 cubic feet per second (cfs) or greater to maintain the cypress forests along the lower river. We now know that flows around 20,000 cfs inundate virtually the whole floodplain.
- In 2003, we knew that river fisheries were using gravel bars in the channel for spawning, but didn’t know how much water was needed for how long to make that successful. We now know that March and mid-April to mid-May are critical periods, and about7,000 cfs at the bars is needed for spawning success. Water temperature cues exactly when and how long this water is needed.
- In 2003, we knew there were over 50 old cutoff channel segments, but we didn’t know what values they may contain or how they interact with the river. Investigation has found that many of these segments support well-used local game fisheries and river access, and that access declines significantly at 4,000 cfs.
- In 2003, our best inference for flow to maintain the fresh-salt balance in the estuary was 6,000 cfs. Closer study has revealed that pulse flows of 4,500 cfs (at the estuary) timed with high tide ingress can maintain the freshwater-saltwater boundary in its historical position.
The endeavor to meet both up and downstream needs should prove much more feasible, as many of the works conducted since 2003 identify flows and timing that are much more water-efficient than the original concepts.
As part of the Savannah River Basin Comprehensive Study (Interim 2), we are incorporating all new information into a new set of recommendations that we call the Ecosystem Flow Prescription (RX). The RX will lay out the river’s needs by calendar year and system state (drought, dry, average, wet). This way, operators will know what the needs are at any given time point, and will also have a way to adjust to changing weather and water conditions.
The drought RX will be modeled as one of several flow release alternatives in the study, and parts of it may be incorporated into a master alternative that combines the best elements identified across all other alternatives.
~ Eric Krueger, Director of Science and Stewardship, The Nature Conservancy South Carolina Field Office