The Vindhyas form the southern boundary of the Ganges basin, with the western part of the Ganges basin draining into the Yamuna and the eastern part directly into the Ganges itself. All the rivers, which drain into the Ganges, flow from south to north, with the Chambal, Shipra, Kali Sindh, Parbati, Kuno, Sind, Betwa, Dhasan and Ken rivers being the main tributaries of the Yamuna. Shipra River is one of the most sacred rivers of Hinduism. It is the site of the Simhastha Kumbh Mela, which is held every 12 years. The land drained by these rivers is agriculturally rich, with the natural vegetation largely consisting of grass and dry deciduous forest types, largely thorny. The eastern part of the Ganges basin consists of the Son, the Tons and the Rihand Rivers. Son, which arises in the Maikal hills around Amarkantak, is the largest tributary that goes into the Ganges on the south bank and that does not arise from the Himalayas. Son and its tributaries contribute the bulk of the monsoon flow into the Ganges, because the north bank tributaries are all snow fed. The forests in their basins are much richer than the thorn forests of the northwestern part of Madhya Pradesh.