| Category | Rivers |
| Continent | Karterus |
| Region | Maenedhel, Urizigar |
| Map | Galsour River |
Galsour is a scorching hot river fed from even deeper Underdark regions than Maenedhel and Urizigar. The origin of the river is located in Berolaz, northeast of the ancient demonic ruin Barhaz Halarax. The springs and rivulets serving as the river's source emerge from a vast cavern filled with geysers, jets and clouds of steam, as well as boiling water puddles and ponds. This abundant hot water originates from areas within Katerus's Tier 2 Underdark.
Starting from Berolaz, the river journeys southward, and on its path to the terminus at Lake Hatosvir, it traverses additional geothermal "feeder" caverns of analogous composition; high temperatures, steam, and jets of boiling water, as well as overflowing pools that form rivulets feeding into the principal waterway.
The area where the Galsour River originates is a result of water being forcibly pushed upward under immense pressure and heat from the deeper regions of Katerus's continental landmass.
Boka'tud, a member of the Marching Twenty, recounted the following from his exploration journals: "Delving Tier 2 Karterus - 'Source of the Galsour'"
Galsour's temperature cools as it nears and merges with Lake Hatosvir. Outside of dry spells, the waters of this lake are consistently overflowing into natural channels that dive downward, serving as links between Huzar-Arân and the sector below it. There are hundreds of these channels, some barely noticeable and others large enough to accommodate Underdark travelers. Galsour's overflowing waters flow down these channels, returning to the super-heated regions below where the process begins anew. The structure facilitating this water movement between a Tier 2 and Tier 1 sector of the Underdark is known as the Kossuth Aqueduct.