I'm just gonna come right out and say it: you have to be a certain kind of reader to 'get' Undercurrent. If you approach this book the same way you approach any other young adult book, looking for the same linear storytelling and plot itself being the point of the book, you're going to be sorely disappointed. Me? I'm an avid of fan of classic sci-fi shows like The Twilight Zone and The Outer Limits, so for me at least it was fairly easy to guess what Paul Blackwell was aiming for with this one. Knowing what I do, I must say I quite enjoyed the old school sci-fi feel that he's captured precisely because of how differently this is written from your run of the mill young adult book.
For those who don't know how the sci-fi anthology series I mentioned work, each hour long episode tells a different story - but rather than being the kind of story with a beginning, a middle, and an end, where some sort of plot happens or the protagonist has to do something, they're stories that explore abstract ideas. Sure, they still have characters, setting, plot, but those considerations are secondary and only explored if they help advance the idea, otherwise the episodes are more like vignettes that make you think, where the ending makes you go hmmm on some sort of lesson, rather than delivering a plot where things happen and you're entertained and the end. One of my favorites, in fact, involve a reptilian species that has gifted Earth with a transporter system with an occasional flaw that sometimes it'll make a duplicate of the person using it on the sending side, and the entire episode revolves around the transporter operator being pressured by the aliens to kill the duplicate. You can look at that story as either one of a man who just spent sixty minutes waffling over whether to shove a lady out of an airlock - or, you can look at it as a rumination on the value of human life, or even whether a copy is every bit as 'alive' as the original. I took the philosophical road, which is why I 'got' Undercurrent.
Now, I'm not saying Undercurrent has anywhere near the existential quality of a Twilight Zone or an Outer Limits episode (I have a feeling such a book would be extremely poorly received by the young adult community at large), but rather that it gives off the vibes and has the feel of such an episode. What Paul Blackwell really does well is use all the familiar sci-fi conventions to great effect, starting with how we're thrust right into the middle of the story with Cal waking up in a hospital from a coma having fallen off a waterfall and immediately recognizing that something is off. What's really great is how Blackwell's writing really helps to set off Cal's post-waterfall reality from the pre-waterfall reality of his memories - now Cal himself is pretty dense as to what happens, whereas it's incredibly obvious to me what's going on, but the flip side of that is that he's a lot more casual about describing things that are different between the two realities, and that style really helps to keep the tone detached and mystery interesting in my mind, allowing me to catalog the differences and figure things out on my own, whereas a more genre savvy protagonist would probably have obsessively poured over every last detail, looking for the latest in hard science to explain his predicament, and completely ruined the whole thing in my eyes because this isn't that kind of story (although a closer balance between the two extremes would've made for an even better book).
No, what this is is a story that leaves almost everything hanging in the air, which is one of the best things about hard sci-fi. Because you're not supposed to get what happened, the point of the book is supposed to leave you with a feeling of 'what the hell happened', and then from what the book tells you you're supposed to go back and fill in the blanks. That, I think, is the strongest part of the book, although if it were up to me I'd have left out the last scene, because I actually feel it was tacked on to give readers who might not be familiar with science fiction conventions a 'firmer' ending. The abstract idea in this case is how the few little things, include one seemingly small decision Cal made regarding his brother Cole, that happened differently between these two realities ripples out to have these huge effects, not only Cal's character, but every little thing about the town, and as long as you can see it on a metaphysical level after the last page goes by, it's actually a brilliant form of writing. The takeaways could've been stronger and more developed, but as with most serious sci-fi you just have to be able to fill in a few blanks to see how Blackwell's trying to say how one or two simple decisions can create this drastically different character and outcome, connect all the dots along the way, and the satisfaction comes from realizing the point here is not why or how things happen, those aren't important and just kills any sense of a greater mystery, but why this story.
The real problem, I think, is that an Outer Limits or Twilight Zone episode doesn't make for a very good book - there simply isn't enough material. As a result, Undercurrent feels a bit on the thin side with not much of a plot. Still, it works as a piece of speculative fiction, and I'm rating it as such, just don't expect your average young adult experience and read this with an open mind.
EDIT - Also, I don't have a shelf called covers to brown paper bag, but this would be a prime candidate for such a shelf. Just sayin'