Yes, it's a rock. Now tell us what makes this rock more special than the billions other rocks :-)
First of all there's no billions of rocks there. Just one.
Secondly, this in itself is strange being a river bank and all. The rock obviously came off the top of the bank (it's quite large and angular, so no distant transport, rolling, etc), there's none other the same size around, and it appears to have been there long enough that trees grew behind it, perhaps hiding the path it went down on, so a one-off event. The thick vegetation suggests that is not a very active erosional spot, so I would ask myself, Why did this rock come down alone (and not accompanied by many others everywhere else as in a normal erosional setting which is a river bank)? What mechanism triggered it coming down? I would be looking for a discontinuity in the ground below the rock's place of origin (say a fault) in response to the first question and I would be thinking something along the lines of a small earthquake for the second.
I would also look at what the rock actually is compared to its host at origin. If it's very different, you might have stumbled on an olistolith (there's a PhD dissertation right there), if not, why did it break in one big piece like that rather than a bunch of small ones?
How am I doing?