Check out this site www.ojohaven.com and click on crossword solver. I use it loads. It comes up with: farce/forge/force/forte and furze.............I would say it will be either force or forge. Good luck.
Because 'force' - from the old Norse word 'fors' - simply does mean a 'waterfall'...ie a cascade of water. In addition, of course, 'force' may also be 'energy'...in this case from the Latin 'fortia' meaning 'strength'.