One picture behind this word is water coming down all at once: a whole river going over an edge, too much of it, too fast, and loud enough to hear before you see it. That is the picture the waterfall and the flood senses share. The eye sense stands somewhere else, and Wiktionary files it under a separate origin — there the picture is a heavy gate lowered across a gateway, shutting the way.
From the word’s history
Wiktionary traces the waterfall noun to Ancient Greek katar(rh)aktes, rushing downwards, from a verb meaning to pour down or rush downwards. The falling water is the oldest layer of the word.
Wiktionary 把「瀑布」這個名詞追到古希臘文 καταρράκτης，意思是「向下衝的」，來自一個表示「往下傾瀉」的動詞。也就是說，這個字最早記錄的是一個動作，不是一個地點。先有「往下衝」，才有「大瀑布」這個東西。
Wiktionary entry 'cataract' etymology 1 (kaikki:3f6f69a190036ae4:503017532): 'from Latin cataracta (floodgate; waterfall), from Ancient Greek καταρ(ρ)άκτης (rushing downwards).'
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