Using the Storm Water Management Model (SWMM), this study evaluated the impacts of (a) 20% and 50% urbanization at the mountainous Mahabad Dam watershed in Iran, as probable future land developments, (b) the urbanization location (near the outlet, in the middle, and at the far end of the watershed), (c) climate variability (increase in evaporation and rainfall intensity), and (d) implementing vegetative swales as low impact developments (LIDs), on watershed-generated runoff and pollutant loads (total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP)). Combination of the above-mentioned factors resulted in 17 scenarios, and each scenario was run for a twelve-hour simulation in the model. The results indicated that based on land developments, areas with more dominant agricultural land generated more TN and TP, areas with more undeveloped lands generated more TSS, and more urbanized areas generated more runoff. Moreover, the 50% urbanization scenario resulted in more runoff and pollutant loads, compared with the 20% urbanization scenario. Under scenarios with climate variability, runoff and pollutant load peaks occurred earlier in time, due to the higher intensity rainfall events. Furthermore, LIDs decreased pollutant loads up to 25%, indicating their effectiveness in decreasing the impact of urbanization on receiving water bodies.