Vietnam is located in typical monsoon climate region and therefore river are very abundant water. However, about 2/3 of water resources is originated from neighboring countries. Moreover, uneven spatial distribution and huge seasonal change are additional reasons that make Vietnam ranked low compared to other South East Asian countries in term of water resource availability (the index of water availability per capital in Vietnam is 4,170m3/s compared to 4,900 m3/s in South East Asian region). In Vietnam, there is dense network of river systems, out of which about 2,360 river have length of 10km or more with total volume of 835 billion m3. However, the flow during 6 or 7 months of dry season is counted for only 15-30% of total annual flow. As a result, every yeas, drought and water shortage have always occurred in many areas of different basins. To cope with this situation and also to meet increasing water demand, number of reservoirs has been built for water resource regulation. After many years of development, many large exploitation work systems have been constructed and operated in all the basins thought the country, such as dams, reservoirs, weirs, embankments and so on. Those systems are to supply water for all kinds of use, including: irrigation, drainage, and hydropower generation, households, industry and flood control. Those works have been played an important role in water supply for major social-economic development sectors of the country, such as irrigation, hydropower generation, domestic and industrial used.