During wet season its widest point can reach 30 miles. This means that if you're on a boat in the middle of the river, you can't see nothing but water in any direction. To cross it on foot, over some imaginary bridge, would take you over 8 hours.

I wonder what the breakdown in sources for the disparity is. I can think of three potential differences, that are all likely contributors:

- Rainfall levels in the drainage area

- Evaporation over the course of the river

- Diversion for human use

I have to think the third is a substantial factor with the Nile vs the Amazon, considering that the Nile is essentially the only water source for two high-population countries (agriculture in Brazil is ostensibly supported by rainfall as well as by the river).