A washland can effectively reduce peak flow of a high flow event at a certain location. A number of washlands located in a basin influence each other's hydrological function, and hence, their hydrological role should be evaluated from a river network perspective. Here, we present an approach to determine optimal locations of constructed washlands in a basin, considering their mutual effect. The problem is formulated as a multiobjective problem with maximizing peak flow reduction effect as one objective and minimizing the total volume of washlands, which is associated with cost, as another. By simplifying modelling algorithm for hydrological routing, we show that consideration of all possible cases is feasible for the situation of a typical basin. This approach guarantees global optimum and shows a full spectrum of Pareto front, which will help decision-makers in problems of multiple objective functions. The proposed approach is demonstrated for the Anseong River basin.