> How do you divert a river and put it back on demand, with not even medieval technology?
Now I am wondering how Kurus handled this situation after he took Babylon.
Now I am wondering how Kurus handled this situation after he took Babylon.
On a small scale you can improve a stream by forcing the water to turn, using stones or posts driven into the water. If you have the ability to build a bridge, you have some ability to change the direction of the water.
In a floodplain or alluvial fan, all of the land is sediment from the river, which means that at flood stage the river has been everywhere and during a flood, or over geologic time periods, the banks of the river can shift. Oxbow lakes are old bends in the river that straightened themselves out.
In theory, if you had a city that was built beside the new banks of a river, you could go upstream and rechannel the river to an old course, either to expand the city or for nefarious purposes.
Also Babylon is, I think, a weird case anyway. If you look at a map of the Fertile Crescent, you see that the Euphrates forks, and it forks a LOT. And Babylon is located between two of the outer forks. Convincing a river that forks to chose another fork might just require some megalithic technology - throw some big rocks in until the flow rate slows rather than stops, and evaporation takes care of some of the rest. Or a phalanx of pilons all driven into the shadow of the next one up river (think geese in flight), which is how you divert a stream.
Even if the river doesn't stop, it might not have enough clear water in it to sustain a city under siege. Especially if you can get them to surrender before the next rainy season washes away all your hydraulic engineering.
The priests of Marduk in Babylon hated Nabonidis due to his abandonment of Marduk worship and making another city his headquarters.
It may have been more acceptable to the population to tell them that the Persians diverted a river at night and entered rather than saying the gates were opened by insiders.