The Himalayan region abounds in glaciers. Most of the big glaciers lie in the eastern Himalayas. As the western Himalayas receive only a small amount of rainfall, barring the formation of vast snowfields, the source of some of the big rivers of Nepal are in fact glaciers. Nepal's largest glacier lies in the Mahalangur and the Kumbhakarna ranges. Khumbu is the biggest glacier and Langtang the longest. Kanchenjunga, Yalung, Nupchu and Lamtang are some other glaciers belonging to the eastern Himalayas. Tukche and Hidden Valley glacises belong to the central Himalayas but these are comparatively small. Eating into the land along their course, the glaciers have been partly responsible for changing the Himalayan landscape, and for evolving various geo-patterns ranging from big cirques and rock basins to hanging valleys and morainic ridges.

Nepal's rivers can be broadly divided into three categories in accordance with their origins. The first category comprises the three main river systems of the country – the Koshi, Gandaki and Karnali river systems, all of them originating from glaciers and show-fed lakes.

The Koshi river system consists of the Tamor, Arun, Dudhkoshi, Likhu, Tama-koshi, Sunkoshi and Indravati rivers. Of these, the Arun and Sunkoshi originate in Tibet. The confluence of these rivers is at Tribeni (near Dharan) in Sagarmatha zone. Flowing for almost 10 kilometres through a narrow gorge before entering the plains, the "Sapta Koshi" or "Koshi swollen with the waters of the seven rivers" finally merges into the Ganges.

The Gandaki river system in central Nepal consists of the Kaligandaki, Buddhigandaki, Marsyandi, Trishuli, Seti, Madi and Daraundi rivers. The Kaligandaki is the longest river amoung them and the Trishuli, the main tributary of this system. The Kaligandaki originates in Mustang and converses with the Trishuli at Deoghat in Chitwan. The river is then called the Narayani and goes on to meet the Ganges.

The Karnali river system in western Nepal consists of the Humla Karnali, Mugu Karnali, Seti and Bheri rivers and is the longest river system in the country. The Humla Karnali, which rises in Tibet, is the main tributary. After entering India, this river assumes the name Gogra.

Rivers like the Mechi, Mahakali, Bagmati, Kamala, Rapti etc. most of which have their origin in the Mahabharat range, constitute the rivers of the second category. The Bagmati, which rises at Bagdwar and drains out through the Chobhar gorge, is the principal river of the Kathmandu Valley. Streams and rivulets originating mostly from the Chure hills make up the third category. These rivers rely on monsoon rains and are otherwise dry.

There are number of lakes and tanks of glacial and tectonic origin in Nepal. Mountain lakes like Rara, Shey-Phoksundo, Phewa etc. are majestic in extent and beauty. Rara in Mugu district of western Nepal is the largest lake. Phewa at Pokhara is another equally famous lake. Shey-Phoksundo in Dolpa district is the deepest lake as well as situated at heighest altitude but is less well - known than Phewa Lake due to its difficult approach. Rupatal and Begnastal (Pokhara) Gaduwaltal (Chitwan), Satyavatital (Palpa), Khaptadtal (Doti), Barhakunetal (Dang), Surma Sarobar (Bajhang ) and Tilichotal (Manang) are some of Nepal's other well - known lakes. Gosaikund and other lakes in the Langtang Himal region are of religious significance.