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Can someone direct me to some good information on where this might have happened? And when?

The monsoons were over, I believe and they don't usually reach that region anyway.

I do know of the "Western Disturbance", a weather pattern that originates much further west, from the Med, even the Atlantic, which happens in December and January and brings moisture (rain/snowstorms) to the NW subcontinent.

Anyway, any suggestions would be appreciated.

If we go with Aurel Stein's route and check out Strabo 15.2.6, then the flash flood most probably occurred in the general vicinity of the Kolwa depression and the Kech River valley, since it preceded Alexander's striking out for the coast. Arrian, Anabasis 6.24-25 places it broadly during the sixty day march between Ora and Pura, but also implicitly favours the early stages of the route by embroiling the beasts of burden, which had been slaughtered and eaten before the final stages of the march. The monsoon had not ended - it was perhaps early September 325BC.

Best wishes,

Andrew