One is a shining blue river. The other is dirty green. Both of them originate in the high Himalayas. While the Indus originates in the Changthang Cold Desert, the Zanskar river originates in the Zanskar ranges. Both these Himalayan rivers join at a scenic setting near the town of Nimmu in Ladakh.
If you visit this place during the summer months, you will see a fast turgid river in Zanskar and a relatively calmer river in Indus. But, if you see it in the winter, it is just the opposite.
The Zanskar, instead of being the fast mountain river is the one that slows down and starts to freeze, thus making the flow very less, while the Indus moves relatively fast and with a lot of ice floating on it. While the Zanskar freezes completely during some part of the winter to enable the Chadar trek, the Indus keeps flowing though it too would have started to freeze in patches. Summer or winter, this confluence makes for a great sight.