Our understanding of sedimentation in alluvial basins is best for very short and very long time-scales (those of bedforms to bars and basinwide deposition, respectively). Between these end members, the intermediate time-scales of stratigraphic assembly are especially hard to constrain with field data. We address these 'mesoscale' fluvial dynamics with data from an experimental alluvial system in a basin with a subsiding floor. Observations of experimental deposition over a range of time-scales illustrate two important properties of alluvial systems. First, ephemeral flows are disproportionately important in basin filling. Lack of correlation between flow occupation and sedimentation indicates that channelized flows serve mainly as conduits for sediment, while most deposition occurs via short-lived unchannelized flow events. Second, there is a characteristic time required for individual depositional events to average to basin-scale stratal patterns. This time can be scaled in terms of the time required for a single channel-depth of aggradation, and in this form is constant through a four-fold variation of experimental subsidence rate.