The area along Ganga and its tributaries rivers to the south of the Ganga was known as Magadha in ancient times.

Explanation:
Magadha was an ancient Indian kingdom located in the eastern part of the Indian subcontinent, in present-day Bihar and parts of Bengal. It was one of the sixteen Mahajanapadas (great kingdoms) of ancient India. Magadha was situated in a fertile region and had access to abundant resources such as iron, timber, and minerals, which helped it to become a powerful and prosperous kingdom.

The region along Ganga and its tributaries rivers to the south of the Ganga was a part of Magadha. The kingdom's capital, Rajgir, was located in this region. Magadha was ruled by several dynasties, including the Haryanka dynasty, the Shishunaga dynasty, the Nanda dynasty, and the Maurya dynasty. The Maurya dynasty, under the leadership of Emperor Ashoka, expanded the kingdom's boundaries to include most of the Indian subcontinent.

Magadha was also an important center of learning and culture. The ancient universities of Nalanda and Vikramshila were located in this region. These universities attracted scholars and students from all over the world and were renowned for their excellence in education, philosophy, and arts.

In conclusion, the area along Ganga and its tributaries rivers to the south of the Ganga was known as Magadha in ancient times. It was a powerful and prosperous kingdom that played a significant role in the history and culture of ancient India.

Cities developed on the banks of the Ganga and its tributaries, and along the seacoasts. The area along these rivers to the south of the Ganga was known as Magadha.