I assume a planet like earth. Obviously the Amazon river is fed into the Andes mountains as well as other rivers that give water to the system. Could the same amount of water come from a specific climate setting? Would a lighter terrain slope be enough to distribute that water and create such an environment? Would ocean currents change the climate making it less homogeneous?
The Andes Mountains are the dominant topographic feature in South America, extending over 7,000 km from *7N to 45S, with Andean plateau elevations as high as *4,000 m over a large portion of the central Andes. Despite its enormity, previous studies have suggested the Andes Mountains have only a minor influence on large-scale atmospheric patterns over South America. The major stationary features, including the Bolivian High and the Nordeste Low, the low-level northerly flow over northern and central South America, and the large-scale precipitation are mainly products of diabatic heating over the Amazon Basin and are only marginally affected by the Andes.
The Amazon rainforest is the result of the moisture-laden equitorial region. If you look at a Koppen-Geiger climate map, you'll note the Amazon, African, Southeast Asian and Oceanian tropical forests all occupy the same latitudes. A mountain range like the Andes simply isn't necessary factor.