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International audienceIn this paper we study the steady uniform flows that develop when granular material is released from a hopper on top of a static pile in a channel. More specifically, we focus on the role of sidewalls by carrying out experiments in setup of different widths, from narrow channels 20 particle diameters wide to channels 600 particle diameters wide. Results show that steady flows on pile are entirely controlled by sidewall effects. A theoretical model, taking into account the wall friction and based on a simple local constitutive law recently proposed for other granular flow configurations, gives predictions in quantitative agreement with the measurements. This result gives new insights into our understanding of free-surface granular flows and strongly supports the relevance of the constitutive law proposed