This study investigated the extent of urbanization that might result from the implementation of a project called Project Canal Istanbul (PCI) using the SLEUTH urban growth model and assessed its impact on the hydrological response of Ayamama watershed using the HEC-1 hydrological modelling tool. SLEUTH was run under two scenarios: Current scenario (without PCI) and PCI Scenario (with the implementation of PCI). The prediction results showed that the urban extent of Ayamama watershed would reach 44 km(2) and 63 km(2) under the current and PCI scenarios, respectively, by 2050. The HEC-1 simulation results under the indicated urban extents and various rainfall amounts showed that the urban extent of the watershed under the PCI scenario would resulted in a significant increase in the peak discharge. Such a change may increase the risk of flooding and, therefore, appropriate actions that can reduce such a risk need to be considered before the implementation of PCI.