Because of its high flow rate, why isn't the Amazon River one of the top rivers with the highest potential for hydro electric power potential in the world?
It has exceptionally little elevation change. 2000 miles (3218 km) from the mouth to the Peru border there is only 300 feet (91 m) of elevation change; an average grade of 0.15 inch per mile (1 in 35200). Apparently that is why so much area if flooded in Brazil during the wet season , there is very little elevation to push the water down river. So although the volume of the Amazon is huge there is no elevation to develop pressure to turn a turbine in a dam.
Typically, a dam creates a stretched basin, not only wide, or / and deep, but long. Between the point water enters this newly created container, and the exit gates of the dam, water will slow down, and any thing which was floating earlier has plenty time to settle. While this sedimentation is wanted e.g., in waste water facilities past the biological treatment to remove the activated sludge, it is not good for dams because over time, this build-up of deposit reduces the water (intake) capacity of the dam. And apparently (reference), the Amazon river carries a lot of mud.
In addition, the Amazon is an important route of transportation for goods, population as well as animals. Thus building such road blocks equally demands the construction (and maintenance) of locks for ships, and fish ladders to compensate at least a little for the impact of a dam.
According to Worldatlas.com, the Amazon doesn't even make 4th onto the list of the deepest rivers in the world, it's not the longest, although closely trailing the Nile.
@Buttonwoods Answer also provides good reasons for why the amazon is not the place for dams,
"In addition, the Amazon is an important route of transportation for goods, population as well as animals. Thus building such road blocks equally demands the construction (and maintenance) of locks for ships, and fish ladders to compensate at least a little for the impact of a dam."
Flow turbines - also called power buoys, would be the most suitable form of hydro power for the Amazon and its many tributaries. With a maximum of 70 kW per buoy at the moment, the Amazon needs a lot of these power plants to become world champion in hydro power.