A Flash Flood is a flood that follows the causative event (excessive rain, dam or level failure…etc) within a few hours. It can be mitigated by managing and controlling the water movement by redirecting flood run-off through the use of flood walls and flood gates. The current study aims at investigating the Environmental Impact Assessment of, the flash flood and the construction of walls to reduce the movement of water and harvesting it in constructed mountainous lake. The harvested water used in cultivation Tomato and cucumber in green houses. The EIA reveal the positive impact of constructing lake and using the harvested water in cultivation on the social, economy and biodiversity items.