So is this a big giant fail? I see no lush vegetation or produce fields.
Did they shoot it flat with some alien prehistoric laser and then terre firma tilt and make the water run backwards?
Realistically (and I’ll eyeball it overall later), the canals, if such, were probably put in and utilized 1 or 2 at a time, moving in their parallel direction, and abandoned as the soil suffered saline toxicity from the evaporation rate inherent in the area.
They were probably building one or two while utilizing one or two and I’d bet the salinity issue kept the system just barely keeping up with itself.
In that situation, it probably only fed on the high side maybe 5-10 million people. Once it hit either too far a distance to get the available flow there (evaporation and infiltration), if the original starting point hadn’t been relieved of it’s salt concentrations by rains/flooding, the society responsible for it would have collapsed.