From A Wiki of Ice and Fire
The Riverlands is a central region of the continent of Westeros, consisting of the fertile areas between the forks of the Trident. Because of its central position in Westeros it has been a frequent battleground. The people of the Riverlands are known as "rivermen" and are a mixture of hardy warriors, peaceful fisherfolk and devout smallfolk. Bastards born in the Riverlands are given the surname Rivers.
For the last three centuries the Riverlands have been ruled by House Tully from their castle at Riverrun, until their defeat in the War of the Five Kings. Riverrun is now held by House Frey of the Twins, although Petyr Baelish has been named Lord Paramount of the Trident and the liege lord of the Riverlands with Harrenhal as his seat.
The Riverlands encompass a region of plains, forests, hills, and endless rivers. The lands extend as far north as the swamp area of the Neck, at the source of the Green Fork, east up to the kingsroad, south to the Blackwater Rush, and west to Ironman's Bay and the hills of the Westerlands. It seems that the Riverlands only has easily defensible borders in the North. Although there are mountains in the east and the west the mountains themselves seem to belong to the Vale and the Westerlands respectively. In the west the heavily fortified castle Golden Tooth guards the entrance to the Riverlands but belongs itself to the Westerlands.
The Riverlands is named for the number of rivers which are present. The Green Fork that runs south from swamps in the Neck, the Blue Fork and the Red Fork, that run from the north from the southwest, come together a short distance from Lord Harroway's Town to form the Trident. The Red Fork runs from its source in the western mountains to Riverrun where it combines with the Tumblestone and then runs to the east and combines with the Trident before pouring into the sea at Saltpans. In the south the Blackwater Rush runs through a part of the Riverlands.
The Riverlands does not contain any major cities but a number towns, such as Fairmarket, Lord Harroway's Town, Saltpans, Maidenpool and Stoney Sept. Its major castles are Riverrun, Harrenhal, Seagard, Stone Hedge, and The Twins. Additional noteworthy places in the Riverlands are the old ruins at Oldstones, where in ancient times a Riverlands dynasty used to have its castle, and the Lake God's Eye with the Isle of Faces in it.
The Riverlands are rich and fertile and populous. The numerous rivers that run across its expanse are used for trading and the transport of goods, In times of peace fisherfolk in skiffs and grain barges pole downstream and merchants on floating ships sail on the rivers. With so much trade on the rivers, villagers will haul their grain and other goods to it to see it sold and carried elsewhere by the merchants. Wine can be made from small, tart grapes grown in the Riverlands. 
During the Dawn Age the First Men settled in the Riverlands, coming into conflict with the native Children of the Forest. After centuries of fighting, a pact was signed at the Isle of Faces, the children withdrew to their forests, while the First Men raised their kingdoms in the lands ceded by the mysterious folk. The Riverlands saw the rise of the Kings of the Rivers and Hills.
During the thousand years that followed, various families ruled the Riverlands. Many dynasties claimed these lands, including the Brackens, the Blackwoods and the Mudds, who were the last of the First Men to claim the Riverlands as their own. Their time came to an end with the coming of the Andal Invasion. At this time, King Tristifer IV of House Mudd, raised his armies and met the Andal invaders. King Tristifer was said to have won 99 out of his 100 battles against the Andals; in the final battle he was killed. His son and successor, Tristifer V was not as successful as his father and the kingdom fell.
The Andals raised their own petty kingdoms and for a long time the Riverlands were the kingdom of native Andal dynasties, including the Teagues, the Fishers and the Justmans. The last native king was killed by Stormland invaders about 360 years before Aegon's Landing. The Storm Kings in turn were defeated by Iron King Harwyn Hardhand, who established his own kingdom from the Iron Islands to the Riverlands. Two generations later, Harwyn's grandson, Harren the Black, ordered the construction the immensive castle, Harrenhal, as a display of his wealth and power. Construction of Harrenhal took forty years and a huge amount of resources and money to build. Ironically, the same day the most immense castle in history was finished, Aegon I invaded Westeros with his dragons.
King Harren the Black was very unpopular within the Riverlands when Aegon invaded Westeros. Aegon approached a number of lords in the region, convincing them to rebel against Harren, including Edmyn Tully. Harren thought if he refused battle Aegon would have to lay siege to the castle. Instead, Aegon let his dragons attack. The heat produced by the dragons was so great that much of the castle burned and melted, killing Harren and his children. After the victory Aegon raised Edmyn Tully to Lord Paramount of the Riverlands, a sub-kingdom below the sovereignty of the Targaryens.
- See also: Robert's Rebellion
During Robert's Rebellion Hoster Tully, Lord Paramount of the Riverlands, took the side of the rebels; he was already connected with them through the engagement of his daughter Catelyn to Brandon Stark. After Brandon's death, Hoster had Catelyn married to Brandon's younger brother Eddard and his younger daughter Lysa married to Jon Arryn. A number of Lord Tully's vassals remained loyal to the Targaryens: Darry, Ryger, and Goodbrook. Lord Walder Frey arrived late to the Battle of the Ruby Ford in support of the rebels.
The Battle of the Trident, which ultimately determined the outcome of Robert's Rebellion, was fought in the Riverlands.
War of the Five Kings
- Main article: War of the Five Kings
The Riverlands were hit hard during the war, starting with raiders under the command of Ser Gregor Clegane. When a party commanded by Beric Dondarrion went to the Riverlands to find and kill them they were ambushed by the raiders, who resumed their killing and plundering.
A second phase set in as Jaime Lannister defeated a Riverland army under the command of Edmure Tully and besieged Riverrun. Because it was surrounded by the Tumblestone and the Red Fork, Lannister had to set up three camps to lay an effective siege. A northern army under Robb Stark moved nearby undetected and was able to defeat the different camps in turn, liberating Edmure Tully and taking Jaime Lannister prisoner.
Another Lannister army was still in the Riverlands south of the Red Fork and was able to kill and plunder at will. The Twins and Seagard were noted to be the only two Riverland settlements largely untouched by the fighting. The majority of Riverland castles were conquered (Harrenhal) and others were sacked (Darry). In revenge, the army of Robb Stark invaded the Westerlands.
After the Red Wedding the Riverland lords surrendered. Most were welcomed again into the king's peace, but the Tullys lost Riverrun to the Freys and the lordship over the Riverlands to Petyr Baelish. Even thought the war had ended, outlaw bands and broken men still roam the Riverlands, some officially fighting in the name of King Robert, but most killing at will.