PIV measurements in side-cavity open-channel flows that simulate “Wando” in rivers, were conducted in a laboratory flume. Five types of side cavities were built by using a transparent plate at the wando flow. The pattern of the instantaneous flow fields was quite different from that of time-averaged ones. It was found that a vortex, which is located in the side cavity, is stable when the junction mouth between the side-cavity and main channel is narrow. In contrast, large-scale unstable vortices are generated semi-periodically when the junction mouth is wide. The free-surface fluctuations in and outside the wando were also measured by three sets of supersonic wave gauges. The amplitude of the watersurface fluctuations is large when the junction mouth is wide as compared with when its mouth is narrow. When the elevation of the free surface in the side-cavity is rising, its elevation in the main channel is falling, and vice versa when the junction mouth is wide.