Hydro GIS : theory and lessons from the mega Mekong Delta -- Physical geography should be an important base for understanding the eco-system of the Mekong River water shed--geographical view of the Mekong catchment -- Hydro GIS and flood simulation -- Long history of hydraulic work development of the lowest Mekong Delta and construction policy -- Flood variation trend and environmental changes in the Mekong River Delta -- Impact evaluation and scenario driven for flood control structures -- The planning for flood mitigation and structure measures -- Conclusion and discussion.