From the northwest and northeast, respectively, flow the Tonle Sab and Mekong Rivers. These waters merge and split into the Basak River and the Mekong, which flow southeast to the South China Sea. The Mekong River is the 12th longest in the world, flowing 2,600 mi from western China to the Mekong Delta in southern Vietnam. Every autumn, monsoon rains are too great for the Mekong to carry, and it floods a large area of Cambodia . This area receives 152 to 203 cm of rain annually, most of which falls during the southeast monsoons from mid-May to early October.
Landsat images are effective for quantifying changes in surface water.
While the images were both acquired after the monsoon season, the 1985 image clearly shows more surface water than the 1973 image . Phnom Penh is the Mekong River's largest city. Because of the extreme instability in these decades, data on Cambodia are often fragmentary and contradictory. Many of these deaths were from hunger, disease, war, and forced work, but there were also mass executions. Canals around China's Yangtze River delta harnessed rainy-season floodwaters, carrying them out to the surrounding lowlands where in the dry season people lifted the water up into their rice fields.