I know lots of folks have taken their tiny ones to the river starting from an early age with no problems...
That being said, I'd ask my pediatrician first. Babies that young are still developing their body temperature regulation mechanisms. Also, note that Pedialyte says right on the bottle, "Use under the care of a doctor" so I'd also ask my pediatrician before using Pedialyte (too many electrolytes is bad just like depleted electrolytes). By asking your pedi first, you're also covering your ass -- if you don't ask and God forbid something happens, your pedi can't say, "What the F were you thinking?" On the other hand, your pedi might say it's fine and give you some good tips and advice for the trip!
Also, remember that you can't use sunscreen on a baby less than six months, so that makes the shade all that more important (and don't forget about reflected UV).
Either way, have fun and good luck.