All the little questions

[S'mon]

Because the players don't really have any significant knowledge of them? If the players weren't cottoning on to the idea of a flash flood being really dangerous here, I'd probably bring it up to any character with ranks in Know (Nature), Know (Geography), or Survival.

Yeah, for these Cimmerian Barbarian PCs, I'd be giving them a bunch of checks or just flat-out telling them. If they've grown up in the local area they should almost certainly be aware of such dangers.

 

[Dannyalcatraz]

Anthro Lycanthropic Lungfish!!

Hmmm...when the moon is full, they have to find mud and dig themselves a hole?

 

[JRRNeiklot]

If the dry creek is filling with water, the water has to come from somewhere. Every small stream, creek, or branch in the radius of the storm is also flooding and overflowing. Presumably, some of that water ends up in the (formerly) dry creek. So, yeah, there could be fish, but they won't be concerned with food, so catching any wouldn't be likely.
They should be more concerned with getting the hell out of there. I once went on about a 2 hour hike. Crossed a creek that was little more than ankle deep. It rained. 2 hours later it was a rushing river at least 6 feet deep. My 2 hour hike became an 8 hour odyssey, as the nearest place to cross was many miles away. The water level in those canyons can rise 40 feet in just a few hours. There's even a sign in the parking lot at one trail head warning against parking there in inclement weather. You can return to find your car either gone or under twenty feet of water. Also, the drier the climate, the quicker it floods.

 

[Ed_Laprade]

If you want to give them food, give them some dead animals that have drowned and been washed downstream. Maybe that will give them a hint not to hang around, too.