sentences:  At the rate of a few cm per year, this river, the Middle Zambezi, started cutting back the bed of its river towards the west, aided by grabens (rift valleys) forming along its course in an east-west axis.As it did so it captured a number of south-flowing rivers such as the Luangwa and Kafue.Eventually the large lake trapped at Makgadikgadi (or a tributary of it) was captured by the Middle Zambezi cutting back towards it, and emptied eastwards.