Report to the State Water Resources Control Board regarding flow criteria for the Delta necessary to protect public trust resources, February, 2010. Several methods for developing flow prescriptions to support desirable fish species in the Delta are compared. To be useful, flow prescriptions must respond to the changing characteristics of the Delta, including sea level rise, additional flooded islands, changes in water diversions, and new invasive species. Adaptive flow prescriptions for desirable fish species are likely to require a more causal, mechanistic, or process basis, so their effectiveness is not outstripped by underlying change and so they can be more easily modified with improvements in scientific understanding. A bottom up or process-based method for establishing Delta flow prescriptions shows promise for developing flows, where flows required for different functions are examined for independence and synergism to develop an overall flow regime. This bottom-up approach allows for the systematic organization, integration, and expansion of available scientific knowledge relating freshwater flows to native fish populations. No flow prescription approach will avoid controversy, but a bottom-up functional approach should be able to compartmentalize controversies so they are better addressed scientifically. The adoption of more causally-reasoned and quantified flow standards will require significant changes in the scientific and management institutions and infrastructure of the Delta, but ultimately should provide more effective and adaptable flow prescriptions.