It was the product of a long-term terraforming project that was begun by Pardot Kynes, Leto's maternal great-grandfather. Kynes' vision was to transform Arrakis from a desert world into a lush paradise with open water and green forests. However, the river only became a significant natural landmark during Leto II's reign. It was named after Duncan Idaho, the loyal Atreides swordmaster.
Leto was killed when his Royal Cart plunged into the Idaho River after the bridge that crossed it was detonated by assassins.
The river ceased to exist some years later, when the sandtrout that had left Leto's worm body after his death, began the process of once again making Arrakis a desert world.