Side-weirs are one of the major parts of floodwater spreading systems. Their essential role is spreading floodwater and increasing floodwater infiltration into the ground. In this paper, the hydraulic behavior of side-weir in floodwater spreading systems and the existing formulas for the prediction of side-weir discharge coefficient are discussed. In the first part of this study, the basic concepts of floodwater spreading systems are presented. Then the dynamic equation of side-weir is investigated. One of the main concerns about floodwater spreading system is that if the presented equations by the other researchers are applicable to the side-weir in floodwater spreading system or not. To answer this question a laboratory flood water spreading channel is made at the Soil Conservation and Watershed Management Research Institude and the discharge coefficients of the side-weir are measured. Afterwards, by using dimensionless analysis, the effective parameters on the discharge coefficient of the side-weir have been indicated. Finally the experimental values of discharge coefficents have been compared with other researchers’ equations. It was shown that those equations that are only a function of the upstream Froude number are impropor for use in the floodwater spreading systems. The results also indicated that side-weir discharge coefficient is influenced by four main parameters which are upstream Froude number, dimensionless weir length, the ratio of flow depth to the main channel width and the ratio of flow depth to the side-weir height.