Is this the GameCube or Wii Version?
GameCube world map:
http://i247.photobucket.com/albums/g...twilightGC.png
Wii world map:
http://i247.photobucket.com/albums/g...wilightWii.png
As you can see, there are multiple rivers heading south from Zora's Domain and the fishing hole. The one furthest from Zora's Domain is not enterable but the other two are: one will lead you to northern Hyrule Field in the Lanayru Province, from which you can see Hyrule Castle to the south, and the other longer one will take you to Lake Hylia. But have you had the cutscene on the bridge over Lake Hylia yet? If not I recommend going to the bridge. There's also another shortcut to Lake Hylia, which is falling into the deep pits just south of the castle town in the grassy field.