Evaluating Instream Flow Needs for Potomac River, Seneca Pool to Little Falls: Initial Thoughts G. Mathias Kondolf University of California Background Points: River is basically healthy - proactive! Issue is potential effect of drought Highly irregular bedrock channel

River is basically healthy Regulation minimal (total impoundment by Jennings Randolph, Seneca, Patuxent, and Occoquan equivalent to appx 14 percent annual flow) - Water quality good - Fish populations apparently healthy

Issue is potential effects of drought - An "event" (not chronic condition) - Biological responses are key - Refugia - competition - temperature tolerance - food availability - bio-energetics - recolonization

Highly irregular bedrock channel - Contrast with alluvial rivers with regularly repeating forms - Representative sampling schemes implicitly assume repeating forms - Hydraulic models cannot be applied, empirical relations may be possible, extrapolation/interpolation risky - Role of very deep holes as refugia?

Recommended Approach: Learn from the 2002 (and 1999) drought Develop clear objectives: biological then hydrological Understand biological responses to drought Acknowledge uncertainty Consider Adaptive Management framework Think clearly/critically re modeling approaches

Learn from the 2002 (and 1999) drought - Take advantage of agency/consultant observations - Compile available information, including anecdotal - Unlike controlled flush, detective work from scraps - Any visible effect on fish populations in 2002 or 2003? - Continue/improve monitoring (start with key questions) - Determine how fish used deep pools (Mather, Am Legion) - Assess potential sources of recolonization

Develop clear objectives: biological then hydrological - Identify specific functions by fish life stage, hydro conditions to support those functions - Cannot determine flow needs for "fish habitat" - Consider factors besides flow, even if beyond your control (otherwise risk being irrelevant) e.g., nutrient loading from watershed may affect DO in deep pools

Need to understand biological responses to drought - Characterize physical conditions - Conceptual models of biological responses - Look for ways to test - PHABSIM is not likely a useful tool

Acknowledge uncertainty - Avoid trap of over-determinism -Quantify uncertainty, bound estimates Consider an Adaptive Management framework - Embrace uncertainty - Articulate conceptual models - Monitor strategically to improve understanding - Adaptive probing during next drought year (i.e., let flows drop to 300 mgd or 100 mgd)

Think clearly/critically about modeling approaches - Articulate and support biologic/hydrologic objectives -Clearly articulate conceptual models, especially flow-biological links - Clearly articulate outstanding technical questions - Monitor critical populations, habitats, relationships - PHABSIM is not likely an appropriate tool

Table ES1: Major Steps in Model Development StepNamePurpose 1.Problem IdentificationSolving the right problem 2.Define Modeling ObjectivesDefine use for model and standard of success 3.Formulation of ModelMathematical similarity to the problem system 4.Selection and Study of Numerical Solution Numerical similarity to the mathematical formulation of the problem 5.Model CalibrationSet constants to represent system behavior and characteristics 6.Model VerificationTest model based on model behavior 7.Model ValidationTest model by comparison with field data 8.Documentation of ModelMake model understandable to users 9.Update and Support of ModelMaintain and improve the model's usefulness