The top line change of color is where the water level is at extreme high level. A few years back the lake was at some points overflowing.
Then a bit lower you can see a second line change of color, this is where the normal level of water used to be.
Then much lower you can see how low the water is now. You can also see that nature is reclaiming the dry parts and there is something growing there.
Mind you, this water reservoir is where the water for Khon Kaen city is coming from! And this the water that a few factories nearby are depending on. And currently it about the end of what should have been a raining season.