The Ecological Importance of Flow Patterns to Salmon Recovery

The flow regime of rivers and streams is a major forcer of many critical processes occurring in flowing water ecosystems. Flowing water interacts with underlying geology, valley form and river geomorphology, with riparian vegetation and woody debris, and with the biological components of the ecosystem to create, maintain, and destroy habitat mosaics that influence species composition, distribution, productivity, diversity, and phenology. In this sense, flow can be termed a "master variable" and must be considered a strong determinant of the ecosystem's ecological structure. As a disturbance in the same league with fires and hurricanes, flow, in the form of floods and droughts, can reset ecosystem and population dynamics and redirect adaptive trajectories. These flow-based disturbances are an important and integral component of intact riverine--and salmonid--ecosystems. A major task for salmon recovery is to link attributes of the flow regime with direct or indirect responses by the populations. Not surprisingly, the relationships between flow and population response are exceedingly complex and have not yielded easily to quantification or precise prediction. An ecological/evolutionary perspective is necessary to integrate the effects of flow patterns and flow modifications with salmon population dynamics and recovery. Employing that perspective, we derive a set of three assumptions basic to understanding the effects of flow and flow modification on salmonid population dynamics. Next, we present a framework that links the flow regime to population and ecosystem dynamics through a set of five relatively simple propositions, or principles, that can be used to assess and evaluate the role of flow patterns in both salmon population decline and in recovery. The assumptions and propositions create a conceptual bridge between the flow regime and the ecological and evolutionary dynamics of salmon populations that is necessary for understanding the pathway of decline and for creating a successful pathway to recovery.