The water gets used by the local population, for drinking water, agriculture and industry. Unfortunately quite a lot less is released than arrives.
It is a bit of a different story this year in China as all the dams fill up they have no choice but to release whatever arrives. I read somewhere (CNN) that over 400 rivers are flooded through central and eastern China, with 39 of those rivers at record levels. That is a lot of water.
However, the Mekhong is a different story. A friend sent a picture from Nakhon Phanom of the Khong, shallow and crystal clear. The upstream dams have stolen not only the water but also the silt and fine nutrients that are key to the health of the river. It is increasingly a problem.