In 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 set-up 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.