This is easy to find, as the water of Zanskar is brown, whilst that of the Indus is a little pink.
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4,846 m
12.5 km (one-way), 1,200 m altitude gain—it's the pass that leads to Markha Valley. You might be able to do it in one day from Stok if you are fit, otherwise you might have to camp along the way. Note, people say that this is proper mountaineering and you will need some experience.
Gyamche La Pass, 4,810 m
If you take an early morning bus (8AM?) to Phyang village (3,600 m) in the parallel valley west of Leh, this trail leads you by the Gyamche La Pass and back into Leh from the north. This is a proper full day hike of 20 km, and considering its 1,200 m altitude gain, it will require some time. On the way, you can also visit Phyang Monastery.
4,133 m
A 2-hr hike (3 km) with a great view of Leh and the valley. The trail starts in the very north of Leh.
This is an easy full day hike of 12 km (loop) going from 3,500–3,950 m just west of Stok.