SEDIMENT ACCUMULATION IN A LARGE ROCK CUT CHANNEL: THE MEKONG RIVER IN NORTHERN LAO MOUNTAINS

Channel configuration and sediment accumulation sites in the Mekong River in the mountains of northern Lao PDR have been studied using SPOT images, the Navigation Atlas of the Mekong, and field visits. The Mekong here has a rock-cut channel with steep banks in a narrow valley. The river is nearly straight with sharp bends, indicating structural control. The shape of the channel varies among the following: (1) a trapezoidal symmetrical form, (2) a relatively narrow channel with a deep inner channel and shoulders, and (3) a wide channel with transverse rock protrusions rising up to 30 m from the channel floor.

The regional rainfall is high (2000-4000 mm), almost all of which arrives in the southwestern monsoon, a pattern which is reflected in the river discharge. Large floods that may cause 30 m change of state may occur late in the wet season. The sediment in the channel undergoes a storage and transfer process annually, a pattern heightened during high-magnitude floods.The sediment is deposited in the channel as insets against steep banks, around rock protrusions, and on rocky shoulders above the inner channel. The paper describes the seasonal pattern of sediment transfer and storage, the channel locations where such sediment accumulates, and the bar forms in the channel. It also illustrates the application of remote sensing as a substitute for field data in geomorphological investigation of large rivers in tropical mountains.