The toxic river is a form of drainage that glows greenish yellow. Its origins are unknown, but it presumably leaked out on Prison Island following the large explosion caused there by Dr. Eggman. The toxic river is highly poisonous, making it hazardous to any living creature.
In gameplay, the toxic river is only encountered in Prison Island where aqueducts of it make up a big part of the Stage. Touching it will cause instant damage, so the player will more often than not find themselves traversing over the toxic river with the aid of an Air Saucer. Alternatively, the player can rely on the invincibility effect granted by the Hero and Dark Shadow transformations to walk across the toxic river without constantly being damaged.