the glaciers joined with the ice stream network in the Karakoram, and in the north, they joined with the former Tibetan inland ice. To the south, outflow glaciers came to an end below an elevation of . While the current valley glaciers of the Himalaya reach at most in length, several of the main valley glaciers were long during the ice age. The glacier snowline (the altitude where accumulation and ablation of a glacier are balanced) was about lower than it is today. Thus, the climate was at least colder than it is today. Hydrology Despite their scale, the Himalayas do not form a major watershed, and a number of rivers cut through the range, particularly in the eastern part of the range. As a result, the main ridge of the Himalayas is not clearly defined, and mountain passes are not as significant for traversing the range as with other mountain ranges. The rivers of the Himalayas drain into two large river systems: The western rivers combine into the Indus Basin. The Indus itself forms the northern and western boundaries of the Himalayas. It begins in Tibet at the confluence of Sengge and Gar rivers and flows north-west through India into Pakistan before turning south-west to the Arabian Sea. It is fed by several major tributaries draining the southern slopes of the Himalayas, including the Jhelum, Chenab, Ravi, Beas and Sutlej rivers, the five rivers of the Punjab. The other Himalayan rivers drain the Ganges-Brahmaputra Basin. Its main rivers are the Ganges, the Brahmaputra and the Yamuna, as well as other tributaries. The Brahmaputra originates as the Yarlung Tsangpo River in western Tibet, and flows east through Tibet and west through the plains of Assam. The Ganges and the Brahmaputra meet in Bangladesh and drain into the Bay of Bengal through the world's largest river delta, the Sunderbans. The northern slopes of Gyala Peri and the peaks beyond the Tsangpo, sometimes included in the Himalayas, drain into the Irrawaddy River, which originates in eastern Tibet and flows south through Myanmar to drain into the Andaman Sea. The Salween, Mekong, Yangtze and Yellow River all originate from parts of the Tibetan Plateau that are geologically distinct from the Himalaya mountains and are therefore not considered true Himalayan rivers. Some geologists refer to all the rivers collectively as the circum-Himalayan rivers. Glaciers The great ranges of central Asia, including the Himalayas, contain