Side-cavities are common along natural and channelized rivers. In this paper, results of flume experiments in a rectangular side-cavity (length to width ratio L/W=2) are compared with field observations of a similar, though topographically more complex, dead zone. The main flow in the flume was kept steady, sub-critical (Fr=0.36) and turbulent (Re=3.9×104). Field measurements were performed during small floods with contrasted discharges. In the river main flow, the Froude number was about 0.5 and the Reynolds number about 2×106. In the laboratory, flow structure was investigated at three different depth levels within the sidecavity, by means of PTV (Particle Tracking Velocimetry). In situ, flow dynamics features were explored by acoustic Doppler current profiler (aDcp) and video analysis (LS-PIV). Both experimental and field cases show similar mean flow patterns. Flume experiments offer a detailed view of a two-gyre system: a primary gyre takes up three quarters of the cavity area while a secondary contra-rotating gyre occupies the upstream inner corner of the side-cavity. In the field case, the primary gyre also develops, in close connection with morphological patterns. The secondary gyre, atrophied and non-persistent, is replaced by non-eroded materials that fill in the upper corner of the cavity.