Climate change and land use changes in urban landscapes exacerbate the runoff generation which produces economic losses and socio-environmental impacts. Urban rivers serve as blue-green infrastructure (BGI) offering ecosystem services (ESs), including runoff control and mitigation that helps in climate change adaptation, especially in arid regions where flash floods are devasting and climate models predict an increase in frequency and intensity. This study uses the InVEST urban flood risk mitigation (UFRM) model to estimate the runoff generated during precipitation events, applied to the urban section of the Chili River in Arequipa, an arid region south of Peru. The model requires information on land use/land cover, rainfall depth, and hydrological soil groups. Results from 1984 and 2022 demonstrate there is a significant reduction in runoff retention, specially in the northern section, where areas decreased their runoff retention capacity from 70-80% to 20-30%. The results highlight the river's critical role in mitigating flash flood risks in a dessert region. The methodology used can help estimate run-off mitigation provided by urban rivers as BGI and supports the conservation and restoration of urban rivers as ecological corridors, enhancing urban resilience.