Method 1 - Cucco Edit
discovered by YautjaElder
Grab the cucco at the beginning of the river and bring it all the way up to the waterfall. Go to the very left of the closest ledge to the waterfall. Now face the wall sligtly left of the water fall and jump at it. Hug the wall and go a little bit right until you get in the tunnel.
Method 2 - Megaflip Edit
discovered by ZeldaXMaster
You can do it with a megaflip without the need for the cucco. Its much faster but also harder.
Method 3 - Hover Boots Edit
discovered by ???
As adult simply using the hover boots like the cucco method will do.