If a sluice-gate is lowered just to touch the surface of a stream in an open water channel, instability may set in. A cyclic motion may ensue, in which the surface upstream, after running up the front of the gate, may descend so much that contact is lost. A wave projected forward then causes contact to be resumed and the cycle completed. The conditions necessary for this complicated sequence to occur were examined in two channels. The Froude number, calculated from the mean discharge and the gate opening, was always in the region 0.6-0.75. For a fixed gate opening the allowable ranges of discharge and outlet weir height were small. The existence downstream of a jump or cnoidal wave was essential. For all the observations the values of the dimensionless quantity (period $\times $ stream velocity/gate opening), when plotted on a base of Froude number, lay in a narrow band.