The Gilgil River drains part of the floor of the Great Rift Valley, Kenya and the plateau to the east of the valley, flowing from the north into Lake Naivasha. The river runs to the east of the town of Gilgil, which is on the height of land between the Lake Naivasha and Lake Elmenteita basins. The geologist J.W. Gregory discovered an old settlement on the Gilgil river with obsidian stone flake tools and rough pottery, predating the Iron Age. Tools included skin scrapers, borers and small knives.