Dynamics of large wood in aquatic systems significantly influence physical and ecological processes in rivers. Wood mobility is notably becoming a critical issue, not only in the context of restoration, but also in terms of flooding and hazard potential. Although the number of studies focusing on instream wood has increased substantially over the last few years, physical properties of wood have rarely been measured in aquatic systems. Instead, forest industry-based standards are often used. In this study, we quantitatively assess properties of instream wood density using decayed samples from the Rhône River stored within the Génissiat Reservoir and green samples from the Ain River floodplain (France). Using in-situ and laboratory experiments, we demonstrate how wood density varies between species, how density changes with moisture sorption and decay, and how density affects buoyancy. Results illustrate that both green (e.g., 800 ± 170 kg·m− 3) and instream woods (e.g., 660 ± 200 kg·m− 3) have much greater densities than standard values used in the literature (500 kg·m− 3). Sorption processes differ in green versus instream wood; moisture desorption of green wood is faster than absorption, whereas for instream wood, absorption is faster than desorption. These findings and the related changes in density affect wood buoyancy and mobility and therefore influence wood dynamics in rivers. Finally, two case studies illustrate how more accurate density values can be used to improve wood transport modeling and wood budget estimates based on numerical simulation and ground video-imagery-based monitoring.