Why do peak water levels reach the southern Mekong later than the upper reaches?

The Mekong River's flow is primarily driven by rainfall in its lower basin, which fluctuates seasonally due to monsoon winds.1

The timing of peak water levels varies by location because the river's flow increases as the rain-bearing southerly monsoon winds arrive, starting in May or June. These rains cause the water levels to rise first in the northern and eastern highlands, leading to peak levels in the upper reaches as early as August or September. As the water moves downstream and accumulates from various tributaries, the peak levels reach the southern reaches later, typically by October.1