In which part of a river is a waterfall usually found ?I remember it was something like near a tributary or something....
Waterfalls can happen in any part of a river's system where massive erosion or an abrupt steepening of the river's channel exists.
Waterfalls are very erosive. The power with which the water pours over a waterfall continually erodes away at both the material the water pours past and the material the water lands upon.
"Blockage of a preexisting channel" can cause waterfalls to form. Debris from land erosion or the hardness or softness of the underlying rock can cause diversion of water flow and produce a waterfall.
Essentially, waterfalls are the result of massive erosion. Waterfalls, then, can exist in any part of a river where the erosion has been massive enough to allow the water of the river to fall.