Nepal's rivers are grouped into four major systems, each named after its principal tributary as it leaves the country: the Koshi in the east, the Gandaki or Narayani in the centre, the Karnali in the west, and the Mahakali forming part of the western border with India.
These systems matter for irrigation, for hydropower potential, and as the defining feature of Nepal's north-south drainage pattern, since most major rivers rise in the Himalayas and flow south into the Ganges basin.
Questions tend to focus on river origins, on which tributaries belong to which system, and on the hydropower projects associated with each. Learning them as four groups rather than as a list of names is far more durable under exam conditions.