Originating from the confluence of the N'mai and Mali rivers,[7] it flows from north to south before emptying through the Irrawaddy Delta in the Ayeyarwady Region into the Andaman Sea.

Its drainage basin of about 404,000 square kilometres (156,000 sq mi) covers 61% of the land area of Burma, and contains five of its largest cities.

[8] As early as the sixth century, the river was used for trade and transport, and an extensive network of irrigation canals was developed to support agriculture.

Between Myitkyina and Mandalay, the Irrawaddy flows through three well-marked defiles:[18] This sheet of lava is the Singu Plateau, a volcanic field from the Holocene.

This field consists of magma from the fissure vents and covers an area of about 62 square kilometres (24 sq mi).

[20] Leaving this plateau at Kyaukmyaung,[21] the river follows a broad, open course through the central Dry Zone[22] – the Bamar people's ancient cultural heartland[23] – where large areas consist of alluvial flats.

Below its confluence with the Chindwin, the Irrawaddy continues to meander through the petroleum producing city of Yenangyaung, below which it flows generally southward.

[32] Due to monsoonal rains, which occur between mid-May and mid-October, the volume of the Irrawaddy and its tributaries varies greatly throughout the year.

[36] Still, low water levels have caused problems for ports along the river, as in the Bamaw–Mandalay–Pyay sectors, the shallowest point is as shallow as 60 centimetres (2.0 ft).

Further, 2) There is no evidence showing that modern sediment has accumulated or is transported into the Martaban Canyon; 3) There is a mud drape/blanket wrapping around the narrow western Myanmar Shelf in the eastern Bay of Bengal.

If we assume this has mainly accumulated since the middle Holocene highstand (~6000 yr BP) like other major deltas, the historical annual mean depositional flux on the shelf would be 215 Mt/yr, which is equivalent to ~35% of the modern Ayeyarwady-Thanlwin rivers derived sediments; 5) Unlike other large river systems in Asia, such as the Yangtze and Mekong, this study indicates a bi-directional transport and depositional pattern controlled by the local currents that are influenced by tides, and seasonally varying monsoons winds and waves.

[39] No complete and precise list of all the fish in the Irrawaddy river basin currently exists, but in 1996 it was estimated that there are about 200 species.

It is found in discontinuous sub-populations near sea coasts and in estuaries and rivers in parts of the Bay of Bengal and South-East Asia.

The valleys and lower hill ranges are covered with tropical and subtropical evergreen rainforest instead of monsoon (deciduous) forest.

The predominant trees of the drier patches are the thorny Terminalia oliveri and the hardwood dahat teak (Tectona hamiltoniana) with stands of Indaing (Dipterocarpus tuberculatus) which is cut for timber.

The southern portion of the ecoregion transitions into the Burmese Coast mangroves[45] and is made up of fanlike marshes with oxbow lakes, islands, and meandering streams.

[46][47] Birds of the delta are both winter visitors and passage migrants including great cormorant (Phalacrocorax carbo), a wide variety of Anatidae, Eurasian coot (Fulica atra), about thirty species of migratory shorebirds, the whiskered tern (Chlidonias hybrida), the Caspian tern (Hydroprogne caspia), and the brown-headed gull (Larus brunnicephalus), which is very common.

One of the most numerous wintering shorebird is the lesser sand plover (Charadrius mongolus), which occurs in flocks of many thousands along the outer coast of the delta.

In the late 19th century, the spot-billed pelican (Pelecanus philippensis) nested in huge numbers in south Myanmar.

One colony on the Sittaung River plain to the east of the delta was described in November 1877 as covering 300 square kilometres (100 sq mi) and containing millions of birds.

The species was formerly abundant in coastal regions, but population numbers have decreased because of a combination of commercial skin hunting, habitat loss, drowning in fishing nets and over-collection of living animals to supply crocodile farms.

[52] Despite recent declines in the sea turtle populations, five species are known to nest in Myanmar at well known island and mainland beaches known as turtle-banks.

As they flow through the northern tip of Myanmar – the Kachin State – they cut long north–south alluvial plains and relatively narrow upland valleys between the 2,000 to 3,000 metres (7,000–10,000 ft) mountain ridges.

Despite Mandalay's position as the chief rail and highway focus in northern Myanmar, a considerable amount of passenger and goods traffic moves by river.

Other major goods that are transported from the nation's heartlands to Yangon for export are other foodstuffs, petroleum, cotton, and local commodities.

Myanmar's military junta signed an agreement with China Power Investment Corporation in May 2007 for the construction of seven hydroelectric dams along the Irrawaddy, Mali, and N'Mai River in Kachin State.

The total planned output of all seven plants will be 15,160 MWs of electricity, making it the largest hydropower project in Myanmar, surpassing the 7100 MW Tasang Dam in Shan State.

[59] ParConfluence Region Hydropower Projects, several companies have been or are currently involved in the preparation, construction and financing of the 3,600 MW Myitsone Dam.

[65] Due to its location and size, construction of the Myitsone Dam has raised significant ecological and sociological concerns.

Ecological concerns focus on the inundation of an area that is the border of the Indo-Burma and South Central China biodiversity hotspots.