The tropical storm, ?oodwater, and the ?oodplainsediment layer of a 100-year recurrence ?ood are examined to better understand characteristics of large monsoon ?oods on medium-sized rivers in northern Thailand. Storms producing large ?oods in northern Thailand occur early or late in the summer rainy season (May–October). These storms are associated with tropical depressions evolving from typhoons in the South China Sea that travel westward across the Indochina Peninsula. In late September, 2005, the tropical depression from Typhoon Damrey swept across northern Thailand delivering 100–200mm/day at stations in mountainous areas. Peak ?ow from the 6355-km2 drainage area of the Ping River upstream of the city of Chiang Mai was 867m3s-1 (river-gage of height 4.93m) and ?ow greater than 600m3s-1 lasted for 2.5 days. Parts of the city of Chiang Mai and some parts of the ?oodplain in the intermontane Chiang Mai basin were ?ooded up to 1-km distant from the main channel. Suspended-sediment concentrations in the ?oodwater were measured and estimated to be 1000– 1300mg l-1.