The correct answer is - Mekong.
Mekong is the river in question that has a drainage basin twice the size of California. Mekong is the biggest river in Southeast Asia, and by far the most important one for the region. It comes down from the eastern part of the Himalayas and passes through big portion of the mainland Southeast Asia region. It is a country that serves for fishing, her floodings make the soil fertile for farming, for irrigation, travel and trade through it, and as border between few countries.