From Labyrinthe Wiki
The Great Rivers of Primus are very important to the world and have some rather unexpected properties. Firstly they are conduits of supernatural power and also serve to help in washing away Entropy from the world into the ocean. They allow the faith and supernatural power that flows through the Throne of Glass to be more easily spread across the Empire and possibly Primus as a whole. They manage this to such a degree that when the Choas Wars intensified and the Magiarchs started taking ever greater and greater risks and pursing more extreme strategies to secure victory Glorianave, seated as it was in Halgar, thought to choke off the flow of power from the Throne to the rest of the continent and its enemies and thus deny the other Magiocracies the power they needed. So it was that the Magiarch Kerisdorn acted to destroy the rivers. This he achieved by defeating and binding all the River Spirits bar one, placing great seals across them so for decades there were simply no Rivers except The Alg, whose Spirit, The Great Alg, like its river, was so great it could not be bound.
It eventually fell to the Wizard Mythrandier to free the River Spirits and aided by Troll Wildething to restore the flow of the rivers and as a side effect restore the mighty Lake Halgar. For in a fashion no entirely understood the wardens of these waterways are in part the Trolls who’s bridges are said to regulate the flow of power and see that it is pure. It seems unlikely that they understand the hows and whys but act instinctively and with the traditions of their forebears.
Secondly, all the Great Rivers, to one degree or another, flow both ways, most linking Lake Halgar to the Tigrean or Xathrean, although none have this property in such abundance as The Alg which flows one direction on one bank and the other direction on the other bank.