The Mekong River’s flow comes chiefly from rainfall in its lower basin, which fluctuates seasonally with the monsoon winds. In April the flow is ordinarily at its lowest. In May or June—as the rain-bearing southerly monsoon winds arrive—the flow begins to increase, with an especially rapid increase in the eastern and northern highlands. The Mekong’s highest water levels occur as early as August or September in the upper reaches and as late as October in the southern reaches. The northeasterly monsoon wind, beginning ordinarily in November in the southern areas, brings dry weather until May. During the long dry period, rice cultivation is impossible without irrigation, and the river’s waters are vital to agricultural production.
Temperatures in the lower Mekong basin are uniformly warm throughout the year. Daily highs at Phnom Penh average 89 °F (32 °C), and lows average 74 °F (23 °C). In the upper basin, temperatures are moderated somewhat by altitude and generally are lower and exhibit more seasonal variation than those found farther south.

Mekong River

The Mekong River

The mean annual flow of the river at Krâchéh in Cambodia is about 500,000 cubic feet (14,200 cubic metres) per second, which is about twice the flow of the Columbia River in North America. The recorded minimum at Krâchéh is about one-twelfth of the mean, and the annual peak flow about four times the mean. Below Krâchéh the peak flows diminish as the water spreads out into the distributary channels and backswamps.The recorded annual sediment load is highest at Pakxé, where it amounts to some 187 million tons; it is about half that amount at the Myanmarese border and about two-thirds that at Phnom Penh.
Source: Britannica.com