Wood Equilibrium: A Conceptual Framework for Assessing the Wood Regime

The wood regime describes the recruitment, transport, and retention of wood in rivers. While there are data-intensive methods for understanding these processes, such as wood budgeting and transport modeling, it remains difficult to holistically assess the wood regime, especially over large spatial and temporal scales. To address this difficulty, we present the conceptual framework of wood equilibrium. Like sediment equilibrium, wood equilibrium posits that wood load can be in equilibrium with supply, a state characterized by a consistently positive correlation between wood load and wood retention capacity (i.e., the capability of a reach to retain wood). Likewise, there also exist transport-limited and supply-limited states, in which wood retention is greater or less than capacity, respectively. We apply the wood equilibrium framework to understand the wood regime of two large, lowland rivers in Washington State, USA by comparing wood load to retention capacity and the spatial distribution of wood recruitment. We find evidence for wood equilibrium state varying in both space and time in response to changes in wood recruitment, transport, and retention capacity. We discuss how the wood equilibrium framework can be used to understand the wood regime and potential responses to wood reintroduction and river restoration.