Anduin.

Great River.

Langflood.

Anduin the Great River in Near Eastern Middle-Earth ( See Endor / Arda.) East of Hithaeglir and West of Mirkwood. It had its spring in the high North of Hithaeglir. Indeed its first course was called Langwell by the Éothéod. As it came down from the mountains and passed Framsburg it was joined by its first tributary the Greylin river from Ered Mithrin.
From the junction with Greylin it turned South and began its long course down in the deep and green vales of its upper and fastest course and soon the second of its tributaries; the Rhimdath from the mountains flowed into Anduin strengthening the power of the water as it passed around the Carrock before it reach the Old Ford and had run the first180 Miles. ( as a small note it is told that at the Carrock the water flowed quietly and here it was not really deep, step stones were laid across the river.)

From the Old Ford the river continued to the Gladden River where those two in contribution had made the Gladden Fields. These first 290 Miles of Anduin was called Langflood by the Éothéod.
After the Gladden Fields the river became slower in its flow, it began to turn East and West whereas it at its upper course had a straight path South allowing the water to run faster. From the Gladden the country became more flat and the narrow vales widened out a bit, the Southern eaves of Mirkwood turned a little East and then back west and the course of Anduin bent east ahead Lórien and Silverlode 170 Miles further down at the West shore.
As it reached Lórien and Silverlode it also came out of the long slopes of Southern Mirkwood and turned East towards the Brown Lands and Field of Celebrant 110 Miles away as the birds fly but 130 along the shore. When it passed the Field of Celebrant, river Limlight joined and Anduin made two great 180 degrees westward curves over the next 130 Miles; the North and South Undeeps.

After the Undeeps Anduin cut its 110 Miles long Southern way into Emyn Muil where it at great speed passed Sarn Gebir and the Argonath before it reached the lake Nen Hithoel and the Falls of Rauros.
From the falls the surrounding land was flat and wet; Nindalf was lying in the East and and from the West came Entwash flowing out through the Mouth of Entwash. When it left the falls its course turned into a South-Eastern direction and 260 Miles further down it passed the isle of Cair Andros, went South again into the narrow pass between Ephel Dúath and Ered Nimrais where the Bridge of Osgiliath 45 Miles from Cair Andros crossed the river from Minas Tirith to Ithilien. here was also the outflow of Morgulduin.

After Anduin had passed beneath Osgiliath it made a Westward curve around the heigths of Emyn Arnen in South Ithilien and then back South again where river Erui 70 Miles down joined. Now Anduin curved into a South-West direction for the 80 Miles to Pelargir where Sirith joined it. Anduin continued its course ahead the Bay of Belfalas 110 Miles away but before it reached Ethir Anduin river Poros came from the South-East and Harondor as the last of the Great Rivers tributaries.