The analysis of water distribution systems (WDS) is carried out under the premise that the flow that passes through the network's components remains in steady state, i.e., that its conditions are stable over time. However, flow and pressure conditions are very susceptible to changes in the hydraulic elements of the system, so that any variation that occurs in operational patterns of the network immediately modifies this initial premise. This forces us to study the flow under new conditions in which the possibility of changes in the parameters is admitted. This new state is known as transient flow. Closing regulation valves can be included in the necessary operational changes for the proper functioning of a network. Nevertheless, such activity generates pressure waves that can cause deterioration of the network. This paper attempts to find the optimal closure curve for valves in WDS minimizing both abrupt changes in pressure of system's nodes and the amount of water allowed to pass during closure. Once the results of this optimization process were obtained, the second objective of this research was to prove the existence of the marginal time in a WDS. The use of HAMMER, a computational tool to model transient flow and its effects, was required to accomplish this. Additionally, SPEA 2, a multi-objective genetic algorithm, was implemented to reach an optimal solution for the problem. The methodology was applied in a WDS of Bogota, Colombia, and the research found that the marginal time does exists in small WDS.