Large wood in freshwater environments is downed, dead wood pieces in river channels, floodplains, wetlands, and lakes. Large wood was historically much more abundant in freshwaters, but decades to centuries of deforestation and direct wood removal have decreased wood loads—volumes of large wood per unit area—in freshwaters around the world. The widespread public perception that large wood is undesirable in freshwater environments contrasts with scientific understanding of the beneficial effects of large wood. Large wood tends to increase the spatial heterogeneity of hydraulics, substrate, channel planform, and the floodplain and hyporheic zone in rivers. This equates to greater habitat diversity and refugia for organisms, as well as energy dissipation and storage of materials during floods, which can increase the resilience of the river to disturbances such as wildfire, drought, and flooding. Similarly, wood in lakes increases lakeshore and lakebed heterogeneity of hydraulics, substrate, habitat, nutrient uptake, and storage of particulate organic matter and sediment. Large wood in rivers and lakes provides an array of vital ecosystem functions, and both individual species and biotic communities are adversely affected by a lack of wood in rivers and lakes that have been managed in a way that reduces wood loads. River and lake management are now more likely to include protection of existing large wood and active reintroduction of large wood, but numerous questions remain regarding appropriate targets for wood loads in different environmental settings, including potential threshold wood loads necessary to create desired effects. Large wood can also directly and indirectly enhance carbon storage in freshwater environments, but this storage remains poorly quantified.
