New flooding data in a vertical rectangular channel with 5 and 10 mm gap between its main parallel plates are reported. Visual observations and fast recordings are made to determine conditions associated with the onset of flooding in the channel. For the 10 mm gap channel a combination of mechanisms, i.e,, levitation and upward transportation of laterally coherent or isolated waves, and local wave bridging, appear to be responsible for triggering flooding. The critical flooding velocity is curiously nearly independent of liquid flow rate above film Reynolds number approximately 500. In the 5 mm gap channel and for small Re-L, flooding occurs at the liquid entrance, whereas "massive" wave bridging, occurring above the middle of the test section, is the dominant flooding mechanism for relatively higher Re-L. Predictions based on small amplitude stability analysis are in good agreement with flooding data from the latter case.