What part of rivers are waterfalls usually found?

The answer is: waterfalls can usually be found wherever there is lots of erosion or where there is steepness in the river's channel. 

The us is first  the  u.s. economy  remains the  largest in the world  in terms of nominal  gdp. however, the  u.s. economy  loses its spot as the number one  economy  to china when  measured  in terms of  gdp  based on ppp. in these terms, china's  gdp  is $23.19 trillion exceeds the  u.s. gdp  of $19.42 trillion