Complex sentence: The principal tributary of the Orange is the Vaal River which flows through part of the Northern Cape from the vicinity of Warrenton, which in turn has tributaries within the province, the Harts River and the Riet River with its own major tributary, the Modder River.
Split into simpler sentences: The principal tributary of the Orange is the Vaal River, which flows through part of the Northern Cape from the vicinity of Warrenton. The Vaal, in turn, has tributaries within the province: the Harts River and the Riet River, which has its own major tributary, the Modder River.