Tropical mountain geomorphology and landscape evolution in north-west Thailand.

A mountainous karst landscape characterised by very steep slopes and predominantly underground drainage has evolved in the tropical mountains of northwest Thailand. The underground drainage routes havc been left hanging above the floor of the the Nam Khong valley, which has been out in noncarbonate sediments that underlie the folded limestone. Gravel deposits are widespread along the streams and relict diamictons cloak many hillsides. Deposition of these sediments has occurred since the middle Pleistoccne. Slash and burn agricultural practices have generated only fine sediment from steep hillslopes during thc late Holocene despite localised annual stripping of all the vegetation cover immediately preceeding the most intense monsoon rains. Gulleys cut into the diamictons indicate changes in the sediment load and moisture discharge ratio during the late Holocene that may be associated with increassed human population pressures in the area.