The flow from a submerged bifurcated nozzle into rectangular liquid-filled cavities with width-to-thickness ratios W/T = 6.5, 11, and 18 has been studied using free surface visualization and particle tracking. When W/T = 11 and when W/T = 18, self-sustained oscillations of the submerged jets and the free surface are present. When W/T = 6.5, the self-sustained oscillations are no longer present, but oscillations with the frequency of gravity waves occur. We propose a critical value of W/T above which self-sustained jet oscillations occur, based on the spreading angle of turbulent jets. When W/T is larger than this critical value, the shear layers of the jet reach the front and back wall of the cavity before the jet can impinge the side wall, resulting in semi two-dimensional flow in the plane between the front and the back wall. Two-dimensional recirculation zones form alongside the jet leading to the jet oscillations. When W/T is smaller than this critical value, the jet can develop like a free turbulent jet up to an impingement point at the narrow side wall. When the jet impinges the side wall, flow in the directions parallel and perpendicular to the front and back walls is possible, resulting in complex three-dimensional flow patterns. The critical value for W/T, based on the known 12 deg spreading angle of turbulent jets is W/T = 10, which is in good agreement with the experimental results