From Halopedia, the Halo wiki
The Slidr River was a murky, slow-moving river on the planet Harvest. The river began at a waterfall that originated from the Mimir River, near the Harvest Botanical Gardens. It follows the contours of the Bifrost and ultimately drains into Harvest's southern sea, Munin. It was destroyed when Harvest was glassed in the First Battle of Harvest.
The river was likely named after Slidr River from Norse mythology.