The largest basin in India is the Ganga-Brahmaputra Basin, also known as the Ganges-Brahmaputra Basin. It is a vast river basin located in the northern and northeastern parts of India.

Geographical Extent:
The Ganga-Brahmaputra Basin covers an area of about 1.08 million square kilometers, extending from the Himalayas in the north to the Bay of Bengal in the south. It is one of the most fertile and densely populated regions in the country.

Formation:
The basin was formed by the Ganges and Brahmaputra rivers, which originate from the Himalayas. These rivers bring huge volumes of water and sediments from the mountains, resulting in the formation of a vast alluvial plain.

Key Features:
- Alluvial Plain: The Ganga-Brahmaputra Basin is characterized by a vast alluvial plain, which is rich in fertile soil. This makes the region highly suitable for agriculture, and it has been a major agricultural hub for centuries.
- River Systems: The basin is home to several major river systems, including the Ganga, Brahmaputra, Yamuna, and many other tributaries. These rivers not only provide water for irrigation and drinking purposes but also serve as important transportation routes.
- Biodiversity: The Ganga-Brahmaputra Basin is renowned for its rich biodiversity. The rivers and their associated wetlands support a wide variety of flora and fauna, including several endangered species like the Gangetic dolphin and the Gharial.
- Cultural Significance: The basin holds immense cultural and religious significance for the people of India. The Ganges, in particular, is considered sacred by Hindus, and millions of pilgrims visit its banks each year.

Importance:
The Ganga-Brahmaputra Basin plays a crucial role in India's economy and livelihoods of millions of people. It supports extensive agriculture, provides water for irrigation and drinking purposes, and serves as a major transportation route for trade and commerce. The basin also contributes significantly to the country's hydropower generation.

Challenges:
Despite its importance, the Ganga-Brahmaputra Basin faces several challenges. Pollution from industrial and domestic sources, deforestation, and unsustainable agricultural practices have resulted in degradation of the rivers and their ecosystems. Efforts are being made to clean and rejuvenate the rivers through various government initiatives like the Namami Gange project.

In conclusion, the Ganga-Brahmaputra Basin is the largest basin in India, covering a vast area from the Himalayas to the Bay of Bengal. It is a region of great geographical, ecological, and cultural significance, supporting agriculture, providing water resources, and serving as a transportation route. However, it also faces challenges due to pollution and degradation, requiring concerted efforts for its conservation and sustainable management.