Bronze age alluvial civilization in Amu Darya and Syr Darya

What would have happened if either or both the Amu Darya and Syr Darya in Central Asia had supported an early Bronze age type (eg 3rd and 2nd millennium BC) civilization, similar to the ones that grew up around the Nile, Tigris/ Euphrates, Indus, and Huange He?

The "Bronze Age" refers to the period and style, so they don't actually have to use Bronze.

Why did this not happen?
 
There was a civilization around this time in the 2000s BCE on the Oxus river, which is essentially Central Asia. We however, have little knowledge (that I'm aware of) as to its fate. It certainly would not have survived past the Harappan to its near south in the Indus River valley.

More information is sure to be gained though, if we ever find a way to decipher Harappan writings or discover more writing (easier to read) from the Harappan.
 
As John explained, this is OTL. The Bactria-Marginia Archeological Complex fits your bill. Did it perish? Was it simply transformed? Or did its bearers go a-conquering somewhere else, appearing in history as one of the many groups whose origins are obscure to us?
I don't know.
 
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