When designing and operating junctions of natural and articial streams, an important consideration is an estimation of the backwaters occuring in the upper stretch of the main channel and tributary as a result of joining open-channel flows. Laboratory investigations were carried out on a large experimental device at the Central Asian Irrigation Research Institute to determine this. The junction of two rectangular horizontal plumes was constructed on this device. The investigations showed that the joining of two open channel flows is accompanied by alternate squeezing of one flow by the other, as a result of which considerable deformation of the water surface occurs. Backwaters occur in the upper stretch of the main channel and in the tributary, and transverse slants of the water surface form at the junction below it.