the water festival celebrates an amazing natural event. normally, the tonle sap river flows from tonle sap lake into the mekong river. each summer, though, the monsoon rains come. for months, rain pours down each day. the mekong river fills with raging waters. the mekong's powerful floodwaters push the tonle sap river backward! instead of flowing into the mekong, it flows into the lake! then, this lake-the largest in southeast asia-becomes even larger. the floodwaters expand the lake to many times its normal size.