Complex sentence: Lanzhou is situated on the upper reaches of the Yellow River where it emerges from the mountains and has been a center since early times, being at the southern end of the route leading via the Hexi Corridor across Central Asia.
Split into simpler sentences: Lanzhou is situated on the upper course of the Yellow River, where the river emerges from the mountains. It has been a center since early times, being at the southern end of the route leading via the Hexi Corridor across Central Asia.