The Mander is one of the longest and largest rivers in Westeros. The Mander and its tributaries (including the Blueburn and the Cockleswhent) extend across much of the Reach, helping make it the most fertile part of the continent. Highgarden, the seat of the ruling House Tyrell of the Reach, is located on the banks of the Mander.
The Mander’s headwaters are located south-west of King’s Landing. The river flows for several hundred miles south and west, being fed by several other rivers, until it flows into the Sunset Sea just south of the Shield Islands. Tumbleton, Bitterbridge, Long Table and Cider Hall are also located on the banks of the Mander. The Roseroad crosses the river at Bitterbridge.
In the books
In the A Song of Ice and Fire novels, the Mander is said to be one of the most impressive natural features in Westeros. However, even the Mander is utterly dwarfed by the massive Rhoyne river in western Essos. The Mander is navigable from the sea for quite some distance along its tributaries (at least as far as Bitterbridge), which speeds travel in the Reach. However, this also allowed the Reach to be raided by the ironborn until the King of the Reach fortified the Shield Islands two thousand years before the events of the series.
Some say that the Mander River is actually named for House Manderly, while others say that House Manderly took its name from the river in ancient times. House Manderly was formerly a powerful House until it disgraced itself in the eyes of the King of the Reach (perhaps not coincidentally, around the time the Rhoynar invaded Dorne to the east, a thousand years ago). The Manderlys were exiled from the Reach approximately one thousand years ago. Eventually they settled in the North and proved worthy servants of the King in the North, who made them the lords of White Harbor in return for their loyal service.
The Mander is said to be extremely wide, but also slow-moving and shallow in places, with sandbars that can ground unwary ships.