The Ayeyarwady River Basin
The Ayeyarwady River is approximately 2170 kilometres (km) long, and its catchment area of 414,000 square kilometres (km2) occupies 62% of Myanmar. Its largest tributary is the Chindwin River, representing more than a quarter of the total Ayeyarwady Basin (114,500 km2). The Ayeyarwady Basin has 13 sub-basins, encompassing all the major tributaries of the Ayeyarwady and Chindwin Rivers, each of which has distinct hydrophysical characteristics with respect to area, the main waterway, and elevation. Based on river morphology, the Ayeyarwady Basin has five distinct hydro-ecological zones (HEZs), namely the Upper Ayeyarwady (HEZ 1), Chindwin (HEZ 2), Middle Ayeyarwady (HEZ 3), Lower Ayeyarwady (HEZ 4), and Ayeyarwady Delta (HEZ 5).
Upper Ayeyarwady (HEZ 1): The Upper Ayeyarwady incorporates the upper reaches of the Ayeyarwady River and the N’Mai Hka and Mali Hka Rivers from the most northern sections of the Ayeyarwady Basin to approximately 50 km downstream of Myitkyina. It is characterized by steep rocky slopes, alpine vegetation, and semi-evergreen forest. The headwaters rise more than 1,500 m above mean sea level. Most of the drop in elevation of the Ayeyarwady River occurs within the Upper Ayeyarwady. These upper reaches of the Ayeyarwady River are very steep with a drop of approximately 3 m/km in the 450 km upstream of the N’Mai Hka/Mali Hka confluence. Downstream of the N’Mai Hka/Mali Hka confluence the Ayeyarwady River flattens out to an average slope of 0.09m/km for the remaining 1,550 km of the total river length.
Chindwin (HEZ 2): The Chindwin River is predominantly fed by easterly flowing rivers arising from the Rakhine Yoma. A number of medium-sized tributaries, including the Myittha and Uyu Rivers, form significant floodplains at their confluences with the Chindwin. The geology is mainly comprised of relatively young, thrusted and faulted sedimentary and meta-sedimentary rocks, which tend to erode quickly and produce abundant silts, muds, and sands. The Chindwin is a major contributor to the sediment load of the Ayeyarwady Basin. Although the headwaters of the Chindwin River are not as high or as steep as those of the Ayeyarwady River, the lower Chindwin River is steeper than the lower Ayeyarwady. The upper reaches of the Chindwin drop by approximately 1 m/km in its first 170 km. Thereafter, it flattens out to an average slope of 0.14 m/km for the remaining 985 km of the river’s length.
Middle Ayeyarwady (HEZ 3): The Middle Ayeyarwady is defined as the section of the Ayeyarwady Basin between the Upper Ayeyarwady (near Myitkyina) and the Ayeyarwady/Chindwin River confluence. The largest tributaries in this HEZ are the Tarpein, Shweli, Myitnge and Mu Rivers. The HEZ represents a largely flat reach, with the southern parts entering into the Central Dry Zone climatic area. The mainstem is interspersed by short reaches of confined rock-cut river channel, which partially constrains meandering and mobility. The right bank tributaries of the Middle Ayeyarwady generally comprise low-gradient, moist-broadleaf forests with large floodplains. Left bank tributaries consist of higher-elevation, moist-broad leaf forests as well as karst formations and generally smaller flood plains.
Lower Ayeyarwady (HEZ 4): The Lower Ayeyarwady extends from the Chindwin/Ayeyarwady confluence in the north to Myanaung in the south – approximately 100 km downstream of Pyay. This is the point in the Ayeyarwady River where tidal movement influences channel hydrology. Many of the tributary streams in this HEZ have a high variability index, illustrating their intermittent nature. Both the Middle Ayeyarwady (HEZ 3) and Lower Ayeyarwady (HEZ 4) are dominated by flatter, cultivated landscapes, with higher population densities and a drier climate.
Ayeyarwady Delta (HEZ 5): The southernmost HEZ is the Ayeyarwady Delta, made up by the division of the mainstem into a complex of low-gradient streams with mangroves along the southern edges. It is characterized by coastal processes, salinity gradients, high population densities and cultivated land.