Triveni canal has been constructed on the Gandak river.

Explanation:
The Triveni canal is a significant irrigation canal located in Bihar, India. It is constructed on the Gandak river, which is a major tributary of the Ganges river. The canal plays a crucial role in providing water for irrigation to the surrounding agricultural lands.

The Gandak river, also known as the Narayani river, originates in the high Himalayas of Nepal and flows through Nepal and India. It is a transboundary river and serves as a natural boundary between Nepal and the Indian states of Bihar and Uttar Pradesh.

The construction of the Triveni canal on the Gandak river has brought numerous benefits to the region:

1. Irrigation: The primary purpose of the canal is to provide water for irrigation to the agricultural fields in Bihar. The canal facilitates the diversion of water from the Gandak river to the surrounding farmlands, ensuring a consistent supply of water for crop cultivation. This has resulted in increased agricultural productivity and improved livelihoods for the local farmers.

2. Drinking Water: Apart from irrigation, the Triveni canal also serves as a source of drinking water for the nearby towns and villages. The canal system includes distribution networks that supply clean and treated water to the local communities, ensuring access to safe drinking water.

3. Hydroelectric Power Generation: The construction of the canal has also led to the development of hydroelectric power projects. The flowing water in the canal is utilized to generate electricity, contributing to the overall energy requirements of the region.

4. Navigation and Transportation: The Triveni canal has facilitated navigation and transportation along its course. Small boats and barges can navigate through the canal, enabling the transportation of goods and commodities, thereby promoting trade and commerce in the area.

In conclusion, the Triveni canal has been constructed on the Gandak river in Bihar, India. It serves multiple purposes, including irrigation, drinking water supply, hydroelectric power generation, and transportation. The canal has significantly benefited the region by improving agricultural productivity, providing access to clean water, and supporting economic activities.