Prescribing Flows for the Potomac River Ecosystem:

Six Recommendations

Brian Richter

The Nature Conservancy

Basic Principles

• River flows are inherently variable on daily, seasonal, and inter-annual time scales

• The life cycles and population dynamics of many aquatic, riparian, and estuarine organisms are tightly linked to natural river flow variability

Flow Regime(lows, highs, floods)

PhysicalHabitat

Water Quality

EnergySupply

Species Interactions

EcologicalIntegrity

Connectivity

Flow regime is a “master variable” in river ecosystems

Recommendation #1:Get straight about your goals

It is critically important to define the ecological goals to be supported by the flow-by

requirements -- what are you shooting for with your ecosystem flow prescription?

(Note: low flow allocation agreement calls only for “maintaining environmental conditions”)

Recommendation #2: Engage scientists from a variety of disciplines using a toolbox of

methods

Ecosystem flow prescriptions must be developed with inter-disciplinary expertise:

hydrology, water chemistry, fluvial geomorphology, fisheries biology, riparian

ecology, and other specialists

Recommendation #3: Be specific about the magnitude, duration, and timing of tolerable

extreme low flows.

Extreme low flows do not last long in natural hydrologic regimes -- they are

“events” and not a prolonged condition.

Recommendation #4: Consider more than just low flows in your prescription

Low flow events are interrupted by high flow pulses. These pulses serve many important ecological functions. An ecosystem flow prescription must consider the magnitude, frequency, and duration of necessary high

pulses.

Recommendation #5: Describe the desired inter-annual variability in low flows and high

pulses

Low flow events occur rarely under natural hydrologic conditions. By providing

“better” years following extreme low flow years, populations are able to recover.

Recommendation #6: Expect to be wrong, invest in

learning, and refine your prescription over time

Improved management of the river will only come from years of careful monitoring and adaptive refinement

of the flow prescription