• I've been up and down both rivers and fished every inch of them, as far as I can tell, and still haven't found that last fish, that that guidebook says "prefers calm river waters". Anybody have a hint?
• There is a little boarded-up thing halfway up the tall waterfall river, like the barriers I smashed with the pick to open up tunnels, but I can't smash this one. Is there something else to do there?
• Up in the icy area, there is a tunnel in a hot spring, but when I try to go through it, I am told the passage is blocked. Did I miss the other end of the tunnel (maybe it's the one mentioned above), or is this just a dead-end in the game?
• Does anybody know if something happens when you get all the bouncy flowers to bloom? I'm sure I haven't managed to do that yet.
• For completeness's sake, I'll mention that I haven't managed the parkour race to the top yet; I keep running into interactable things just as I need to jump and getting stuck! It's rather annoying. :-P

Hi! I do have some of the answers you seek! :) 

I had a really hard time finding that exact fish myself! I eventually had to look it up. As it turns out, the river in the forest (the river in front of the lighthouse, which ends in a waterfall near Aunt May's house) is the one where you can find this fish, as it is apparently slower than the rest of the rivers on the island. 

I don't think there is a passageway in the river area you mentioned. I was also confused by all the boards around there, but I don't think they have a function. I tried to enter through the top of the river, but it is not a passageway.

As for the icy passage, yes, there is another end of the tunnel! =) If you go to Aunt May's house, then north through the water (just staying near the edge of the ledges will do), it won't take many seconds until you reach the other passage! Hope you find it :D 

And I've also been wondering what happens if you get all the bouncy flowers to bloom! I've been trying to find them all myself, and I'm not sure if I have found them all. I don't know if anything happens if you do get them all. I did a google search and there doesn't seem to be any guides for this yet, unfortunately.

I've seen other people having problems with accidentally interacting with things during the parkour races, and unfortunately I can't help with that as I never really had this problem. The technique I tend to use is to fly as high as I can from the get-go and swoop to build speed while I float over all the obstacles. Maybe that would help! (Tip: get the silver feather(s) first for better height and climbing speed to maximize this advantage!)

I hope all of this helps! :D Good luck! 

Wow thank you so much! I had fished in the spot you mentioned, but, between the waterfall and the bridge or on the other side of the big pillar. I found that if I went to where I couldn't see Claire, I got the fish! Then I tried fishing off of the bridge itself, so that the bobber was behind the pillar, and I eventually got the fish there too. Now I have the Golden Fishing Rod! (Where did you look it up? I didn't think a game this small would have a walkthrough or reference. :-)

It looks like the board up the waterfall is a section of one of the broken bridges from up higher and not a barrier to a tunnel.

I did find the other end of the icy passage too. Thanks for that! I might have found it eventually but it's easy to miss.

I have 20 golden feathers and 2 silver feathers. I found that having the bouncy flowers opened helps with the parkour race so I will keep at it. I think that and the flowers are all I need to finish in this game.

By the way did you find the little island with the girl selling sun hats? I had stumbled upon it early in the game but couldn't find it again for the longest time. It's offshore northeast from the boat.

Thanks again!