Wetlands of the Mekong River Basin, an Overview

Abstract

The Mekong River extends for 4,909 km from the Tibetan plateau, through to the Delta in Vietnam, passing through Yunnan province in China, Myanmar, Lao PDR, Thailand and Cambodia.The drainage basin of the Mekong River covers nearly 800,000 km² of which about 80% lies in the Lower Mekong Basin. Important wetland features include the Khone Falls and the braided river complex of Siphandone and Stung Treng and the Tonle Sap Great Lake. For much of its length, the Mekong flows through bedrock confined channels by or old alluvium in the river bed and banks. Meanders, oxbow lakes, cut-offs and extensive floodplains are restricted to a short stretch around Vientiane, and below Kratie, where the river develops unrestrictedalluvial channels. Throughout its length, it is characterized by about 500 deep pools which are important dry season refuges for fish.

The climate of the Lower Mekong Basin is tropical monsoonal and the Mekong flows are characterized by an annual flood pulse which drives the reverse flow that occurs in the Tonle Sap in Cambodia, so that the area of wetlands in the Great Lake increase 6-fold at the peak of the flood season. About 42% of the total land area of the LMB has been classified as wetland, of which only 56,000 km² can be classified as natural wetlands; the majority of man-made wetland areas are paddy rice fields. The natural wetlands can be divided into wet “lands”, open water, flowing water and coastal and estuarine wetlands. Most of the wetlands lie in the Central Indochina Dry Forests and Tonle Sap freshwater swamp forest Ecoregions. There are nearly 100 wetland sites of national and regional importance in the Lower Mekong Basin and of these 12 have been designated as Ramsar sites.

The biodiversity of the Mekong and its wetlands is second only to the Amazon and Congo rivers, especially its fish and mollusk diversity, with over 850 recorded fish species and over 83 species of mollusk. Key flagship species include the Giant Mekong catfish, Irrawaddy dolphin, Eastern Sarus Crane and Siamese crocodile.

Most of the 60 million people that live in the LMB live in rural areas and most live near rivers, lakes and wetlands. The collection of fish and other wetland products are very important for their livelihoods. The Mekong Basin has the largest inland fishery in the world, estimated at 2.8 Mt per year of which 1.9 Mt comes from the capture fishery, i.e., from the wetlands.

Conversion of wetlands to agriculture and loss of habitat has been the most significant threat to wetlands in the LMB with an estimated 78% of the original natural wetland area being already converted. Other threats include the construction of hydropower dams and irrigation schemes which change the hydrology of the basin, block important fish migrations and trap the sediment, leading to a significant reduction in sediment transport to the Delta. Climate change is also expected to both increase the wet season flows and reduce the dry season flows, which will have impacts upon the extent of permanent and seasonal wetlands. In the Delta, the combination of sea level rise and changing flow patterns is expected to lead to erosion of the delta and saline intrusion, including permanent inundation of some important mangrove areas