Firete and Yishim contemplate tactics on entering the waterfall. Not that I'm a tactical genius, but our only chance for a surprise round is if the kobolds on the other side aren't expecting us. If they are, I'd guess almost nothing will help once we poke our heads through.
If they aren't however, we could try a few things. The spell I used to freeze the ground will likely not be powerful enough to stop a 20' wide waterfall, or even create a wider barrier for us to slip through.
My soundblast of thunder might be useful, either to push a volume of water into the caverns beyond, or once we're just inside to push creatures around. That, of course, puts me in the thick of action and hopes I can get a spell off quickly - and accurately. After my last miserable performance, I wouldn't bet on either...
Maybe we just head in? Yishim dons his pack again and looks to the more experienced members of the party for guidance.