In particular, one that at least in part defines “greatest” in terms of how much water is moving through them (flow rate), as opposed to longest or most relevant to human history etc.
It’s a gift-request from my Dad, who seems to delight in asking for highly specific things that may or may not exist. This time around I’m sure plenty of them do, but it’s less than compellingly obvious how I’d search for them.
The key words you would want are “volume” and/or “flow rate” combined with “river” and some superlative to get the record-holders.
Clearly the Amazon is the year-round record-holder among all rivers, but it appears that, with the possible exception of an Encyclopedia Brittanica article from which one only gets a teaser without a subscription, a listing of what rivers are its nearest rivals is not available in one place online, at least to my moderate Googling skills.
You’re looking for a gift-type book on something like “Great Rivers of the World”?
Putting “Great Rivers of the World” into amazon.com turns up a variety of selections with that title. Your public library probably has some of them on the shelves, you could go check 'em out and see if they were worth buying.
BTW, some years ago, when South America and Africa were one landmass, the prehistoric Congo River flowed into the prehistoric Amazon, which emptied out somewhere on the coast of maybe modern day Peru? Anyway, that river system had the combined drainage of the Amazon (greatest discharge today) and the Congo (second greatest). I think it would have been rather impressive.
I would think the formula for volume(space)would work,height x depth x width x speed, or something like that,the actual formula is somewhere in my old math book.You could roughly guestimate with the info thats out there on any river. Good luck.