Introduction:
The Narmada River is one of the major rivers in India, flowing through the states of Madhya Pradesh, Maharashtra, and Gujarat. It holds great significance in terms of its geological, cultural, and ecological importance. Let's analyze the given statements and determine which one is not true.

Statement 1: The Narmada River is the longest west-flowing river in India.
This statement is true. The Narmada River is indeed the longest west-flowing river in India, stretching over a length of approximately 1,312 kilometers.

Statement 2: The Narmada River flows into the Bay of Bengal.
This statement is not true. The Narmada River does not flow into the Bay of Bengal. Instead, it empties into the Arabian Sea. It forms an estuary near the Gulf of Khambhat in Gujarat.

Statement 3: The Narmada River originates from the Amarkantak Plateau.
This statement is true. The Narmada River originates from the Amarkantak Plateau in Madhya Pradesh. It is a prominent pilgrimage site and a meeting point of the Vindhya and Satpura mountain ranges.

Statement 4: The Narmada River is not a major source of irrigation in the region.
This statement is not true. The Narmada River is a significant source of irrigation in the region. The river has several dams built across its course, such as the Sardar Sarovar Dam, Bargi Dam, and Indira Sagar Dam. These dams provide water for irrigation, hydroelectric power generation, and drinking purposes.

Statement 5: The Narmada River basin is not known for its rich biodiversity.
This statement is not true. The Narmada River basin is known for its rich biodiversity. It supports diverse flora and fauna, including several endangered species. The river is home to the critically endangered gharial, mugger crocodile, and dolphins. The forests surrounding the river provide habitat for various species of plants and animals.

Conclusion:
After analyzing the given statements, it is clear that the statement "The Narmada River flows into the Bay of Bengal" is not true. The Narmada River actually empties into the Arabian Sea.