The Rhoyne is the largest and most extensive river network in the known world. It dominates the Free Cities region of the continent of Essos and serves as a trade and transport artery linking the region together. Roughly speaking, it divides the Free Cities from the Dothraki sea to the east. The Free City of Volantis sits on the mouth of the Rhoyne.
The Rhoyne and its numerous tributaries are the ancestral homeland of the Rhoynar, a powerful kingdom that was destroyed by the dragons of the Valyrian Freehold a thousand years ago. According to legend, the warrior-queen Nymeria led the remnants of her people in ten thousand ships in flight westwards, eventually landing in Dorne, in the far south of Westeros. There she made alliance with House Martell, then just one of many families feuding for control of Dorne, and helped them become the dominant family of the region.
Lady Talisa Maegyr, a native of Volantis, tells King Robb Stark about how as a child, during the hotter summers, people would swim in the Rhoyne to cool off. Her little brother nearly drowned in the river but was saved by a healer, which awoke her own interest in medicine.
In the books
The main body of the Rhoyne is formed by the confluence of the Little Rhoyne and Upper Rhoyne rivers, where they meet just east of the ruins of Ghoyan Drohe. The river then swings east and south, being met by several other tributaries before flowing into the Summer Sea through an extensive river delta. The Free City of Volantis is located on one of the mouths of the Rhoyne.
The Rhoyne tributaries are patrolled by warships from several of the Free Cities. Norvos controls the Noyne, whilst Qohor controls the Qhoyne and Darkwash. Their ships have influence as far south as Dagger Lake. Volantis controls the stretch of the river south of the Sorrows (the ruins of Chroyane). Between Dagger Lake and the Sorrows the river is a lawless no-man's land, not controlled by Volantis to the south, or Norvos and Qohor to the north. River pirates and freebooters are common along Dagger Lake. The mist-shrouded ruins of the Sorrows are basically used as a leper-colony for people infected with the dreaded disease known as greyscale (which is somewhat similar to leprosy); river traffic usually tries to move quickly through the Sorrows to avoid contact with them.
In the Rhoynar tradition, the Rhoyne is worshiped as the representative of a goddess, Mother Rhoyne, and her six major tributaries are known as her daughters.The six major tributaries of the Rhoyne are the Upper and Little Rhoynes, the Noyne, Qhoyne, Lhorulu and Selhoru. The Volaena is also a tributary of the Rhoyne, but is not counted as one of the major rivers. The Darkwash, a sub-tributary of the Qhoyne, is part of the same network but not counted as one of the major rivers either.
Rhoynish ruins dot the banks of the Rhoyne. Ghoyan Drohe, on the banks of the Little Rhoyne, was once one of the northwestern-most outposts of Rhoynish influence. Ny Sar, at the confluence of the Rhoyne and Noyne, was the capital city of the kingdom. Ar Noy, on the Qhoyne, was another major population center. Chroyane, at the confluence of the Lhorulu and the Rhoyne, was once a great city, the summer palace and festival city of the Rhoynish people, but is now a mist-shrouded ruin. The southernmost Rhoynish city was Sar Mell, not far from the mouth of the river.
The river network also sports several inhabited population centers. As well as the Free Cities of Norvos, Qohor and Volantis, the Volantene townships of Selhorys, Valysar and Volon Therys are located on the banks of the river. Selhorys, located on the eastern banks, frequently pays tribute to passing Dothraki khalasars to spare it from destruction and looting.