Environmental Flow Prescriptions

An ecologically sustainable water management program must always be built upon a foundation of knowledge about the river flows needed to sustain ecosystem health. When the water needs of a river ecosystem are clearly defined by scientists, water managers will be able to find ways of meeting human needs for water while maintaining adequate river flows for the ecosystem.

A river ecosystem’s water needs are defined in an “environmental flow prescription.” This flow prescription describes the necessary seasonal and inter-annual variation needed in low flows, high flow pulses, and floods to support native species and critically important ecological functions.

Flow Level Ecological Roles
Low (Base) Flows
•  Provide adequate habitat space for aquatic organisms
• Maintain suitable water temperatures, dissolved oxygen, and water chemistry
• Maintain water table levels in floodplain, soil moisture for plants
• Provide drinking water for terrestrial animals
• Keep fish and amphibian eggs suspended
• Enable fish to move to feeding and spawning areas
• Support hyporheic organisms (living in saturated sediments)

Extreme Low Flows
•  Enable recruitment of certain floodplain plants
• Purge invasive, introduced species from aquatic and riparian communities
• Concentrate prey into limited areas to benefit predators
High Flow Pulses
•  Shape physical character of river channel including pools, riffles
•  Determine size of stream bed substrates (sand, gravel, cobble)
•  Prevent riparian vegetation from encroaching into channel
•  Restore normal water quality conditions after prolonged low flows, flushing away waste products and pollutants
•  Aerate eggs in spawning gravels, prevents siltation
•  Maintain suitable salinity conditions in estuaries
Floods
•  Provide migration and spawning cues for fish
•  Trigger new phase in life cycle (e.g., insects)
•  Enable fish to spawn on floodplain, provide nursery area for juvenile fish
•  Provide new feeding opportunities for fish, waterfowl
•  Recharge floodplain water table
•  Maintain diversity in floodplain forest types through prolonged inundation (i.e., different plant species have different tolerances)
•  Control distribution and abundance of plants on floodplain
•  Deposit nutrients on floodplain
•  Maintain balance of species in aquatic and riparian communities
•  Create sites for recruitment of colonizing plants
•  Shape physical habitats of floodplain
•  Deposit gravel and cobbles in spawning areas
•  Flush organic materials (food) and woody debris (habitat structures) into channel
•  Purge invasive, introduced species from aquatic and riparian communities
•  Disburse seeds and fruits of riparian plants
•  Drive lateral movement of river channel, forming new habitats (secondary channels, oxbow lakes)
•  Provide plant seedlings with prolonged access to soil moisture

Scientists at The Nature Conservancy have been developing a new process for developing ecosystem flow prescriptions that emphasizes collaboration among scientists and water managers, working together to integrate human and ecosystem needs for water (see figure 1 below).

Adaptive Refinement of Flow Prescriptions


A Collaborative and Adaptive Process for Developing Environmental Flow Recommendations
This new journal paper describes our collaborative process for developing an ecosystem flow prescription "Richter, B.D., Warner, Andrew T., Meyer, Judy L., and Lutz, Kim 2004. A Collaborative and Adaptive Process for Developing Environmental Flow Recommendations. In press at River Research and Applications.  Download the paper (.pdf, 571 kb).

Examples of Environmental Flow Prescriptions
Lower Savannah River (Georgia and South Carolina) - During the past 18 months, we have been working with scientists at the University of Georgia (UGA) and 11 other agencies and organizations to develop environmental flow recommendations for the Lower Savannah River, which forms the border between Georgia and South Carolina. This project, supported by the U.S. Army Corps of Engineers and the two state governments, has engaged more than 40 scientists to date in a collaborative effort to define the flows needed to support freshwater, floodplain, and estuary ecosystems. During a workshop in early April 2003, scientists developed a flow prescription that addresses low flows, high flow pulses, and flood flows in different reaches of the river. The scientists also identified critical research and data collection that will help create a foundation for a longer term adaptive process for refining flow recommendations. Download the paper (.pdf, 500 kb).