What is a plash? These days, it’s a rather precious splash: a pleasant plop, a pretty slap into the water; a word made for prose and poetry that is perused in plush places. I do not think an author could use it without seeming self-conscious. What, simply strip that sloppy starting /s/ from splash to make it a bit less conventionalized – or a bit more archaic-seeming? It may not please as planned.
But plash was not formed by taking the s off splash. No, in fact, quite the opposite: splash was formed in the late 1600s by adding the s to plash, which had already been around at least since the early 1500s. And somehow the sloppier, wetter spl version has prevailed, to make the set with splat, splatter, splodge, splotch, splutter, and the similarly sloppy splay and splurge. It is true that when you slap the surface of a pool of water, or drop a single thing into it, what you hear may be more like “plash” or “plook,” but we seem now to prefer our wetness less tidy and contemplative and more slap-dash. Or at least more conforming to other wet words.
There is actually an even older word plash, a noun meaning (according to Oxford) ‘area of shallow standing water’ or ‘marshy pool’. It’s been in the English language since before there was an English language for it to be in. It’s only used in certain regions of England now (Yorkshire, for instance). It has cognates in other Germanic languages; it may have an onomatopoeic origin – hardly surprising if it does. It may or may not have been the source of plash meaning ‘splash (but not so messily)’, or they may or may not have come from the same root. But really, when we have all these crashing, dashing, smashing -ash words and all these plopping or plucking pl- words, it really is an inevitable formation, isn’t it?
The more interesting thing, indeed, is just how splash has taken over by force of analogy, and plash has acquired a bit of a precious air in consequence. The sound symbolism may be the initial splash into the plash, but the splattered spots of mud and marsh will sometimes drain or dry in unpredictable ways and become more a part of the paint than mere fluid dynamics.