The Simiyu arises on the western slopes of Mount Loolmalasin and the Ngorongoro Highlands. It flows across the Serengeti into Magu Bay of the Speke Gulf of Lake Victoria. The Simiyu has two main tributaries entering from the right, the Duma River and the Komahola River, and one from the left the Maloho River. There is considerable farming in the lower reaches of the Simiyu Basin, which causes runoff of fertilizers and pesticides. The delta of the Simiyu is a marsh that starts just downstream from the town of Magu. Upstream from Magu just below the Ndagalu Forest Reserve is the village of Samilunga.