Define perennial and nonperennial rivers. What makes Himalayan rivers perennial?

Perennial rivers are rivers or streams that have a continuous flow of water throughout the year.

Nonperennial rivers are those rivers which do not flow throughout the year. It flows in one season. The Himalayan rivers are perennial rivers as they originate in snow-covered Himalayan mountains. During summers, the snow melts and the rivers get a continuous supply of water.