The power characteristics of a water wheel operating in a free-surface stream are observed using two-dimensional computational fluid dynamics simulations. The simulations are validated using an experimental model in a free-surface channel. The net power output of the wheel is decomposed in terms of the contribution of each single blade, and parametrized for comparison. This method allows the identification and quantification of phenomena such as entry splash, departure water pickup, water build-up, and interference between blades. It also helps identify and explain the effect on power output and efficiency of varying wheel depth, tip speed ratio, tip angle, or number of blades. This first analysis of free stream wheel performance using numerical tools is a first step towards optimization. (C) 2018 Elsevier Ltd. All rights reserved.