Aquatic weeds growing in irrigation conveyance systems of the earth-lined, open-channel type retard the flow and reduce the efficiency of the systems. The aquatic weed infestations, consisting of both submerged and emergent vascular plants and algae, also reduce system capacity and may result in increased seepage and spill losses. The flow retardance effects and their impacts on the performance of the irrigation conveyance system are evaluated using a modified version of the irrigation conveyance system simulation (ICSS) model developed at the University of Calgary.Key words: aquatic plants, irrigation conveyance systems, efficiency, performance.