After a friendly battle with Professor Sycamore in Couriway Town, you continue south to Grande Vallée Way.
Route Information
In the flowers and grass of this route, you might find Drapion, Weepinbell, Sliggoo, Karrablast, Shelmet, Quagsire, or more rarely, Haunter or Carnivine. You may also encounter hordes of Arbok, Weepinbell, or Gligar. When walking on puddles, you might encounter Haunter, Quagsire, Carnivine, Karrablast, Shelmet, Stunfisk, or Sliggoo. If you Surf in the water here, you might encounter Quagsire, Stunfisk, or Sligoo. If you use the Old Rod, you'll find Poliwag. If you use the Good Rod, you might find Poliwhirl or Barboach. If you use the Super Rod, you might find Poliwhirl, Politoed, or Whiscash.
To find everything in this route, bring a pokémon that knows Surf and a pokémon that knows Strength.
Battle and Catch
Go south from the gatehouse and you'll see Hex Maniac Josette. She sends out a level 50 Pumpkaboo, followed by another level 50 Pumpkaboo.
Use the Dowsing Machine to find a hidden Net Ball on the rock to the right of the Hex Maniac.
Use the stairs west of the Hex Maniac and continue down the next set of stairs.
Use Surf in the dark green water. Go directly south and you'll reach some shallow water. Use the Dowsing Machine here to find a hidden Damp Rock on the leaves.
West of there, you can have a double battle with Pokémon Rangers Ivy and Orrick. They have a level 53 Tauros and a level 53 Miltank.
Go west and south. Swimmer Coral is standing in the water. She has a level 52 Stunfisk.
Go south from the swimmer in the narrow strip of water to find a Rare Bone. This item has no use, but you might be able to sell it to someone for a high price.
West and north of the Pokémon Rangers, there is a hidden Escape Rope on a rock in some deep water.
Near the hidden Escape Rope, you find Pokémon Ranger Clementine. She has a level 54 Alomomola.
Go north from there and Surf northwest. Land on the shallow water to the north. You can see Pokémon Ranger Ambre running back and forth there. She has a level 51 Emolga and a level 51 Grumpig.
Go north from her to find a rock that you can push with Strength. First push the boulder south, then push it east, then north twice, then east until it falls into the hole. Go east from there to find a Toxic Plate.
Go south, back to where the Pokémon Ranger is running around. Go west from her and go up the stairs. Go south through the flowers to find a PP Up.
Go north and then east across the bridge. You'll find Pokémon Ranger Shinobu, who has a level 51 Zangoose and a level 51 Beartic.
East of Pokémon Ranger Shinobu, there is a Yache Berry tree.
Now go south and up the stairs. To the east, you find TM36 Sludge Bomb.
Go east over the ledge. You'll be back near the gatehouse to Couriway Town. Go east from here and go up the stairs. Go east through the flowers. On a ledge to the east, you find Sky Trainer Sera, who has a level 50 Noctowl and a level 52 Aerodactyl.
In the flowers to the east, turn on the Dowsing Machine to find a hidden Antidote.
Go south, then west down the stairs. You can hop on the rocks to the north to reach the Max Revive in the center.
South of those rocks, you find Fairy Girl Lovelyn, who has a level 49 Mr. Mime, a level 49 Azumarill, and a level 49 Aromatisse.
There are some purple flowers to the south. Go into the southwest corner of those flowers and then go south, then east, then north to find an HP Up.
Battle your Friends
From here, go west onto the bridge. Shauna will appear and she will challenge you to a battle. She has a level 51 starter that is weak to your starter, a level 49 Delcatty (weak to Fighting), and a level 49 Goodra (weak to Ice, Dragon, and Fairy).
As soon as the battle is over, Tierno challenges you to a battle. He has a level 48 Talonflame (very weak to Rock, weak to Water and Electric), a level 52 Crawdaunt (weak to Grass, Electric, Fighting, Bug, and Fairy), and a level 49 Roserade (weak to Fire, Ice, Flying, and Psychic).
After that, Trevor heals up your pokémon, then challenges you to a battle. He has a level 49 Raichu (weak to Ground), a level 49 Aerodactyl (weak to Water, Electric, Ice, Rock, and Steel), and a level 51 Florges (weak to Poison and Steel).
After all of that, Shauna gives you HM05 Waterfall. You can't use it outside of battle until you get the next gym badge. Here is a list of places that you have already been to where you will be able to get new items using Waterfall: Couriway Town, Frost Cavern (outside), Route 15, and Route 16.
Explore More
After your friends depart, go west on the bridge, then turn on the Dowsing Machine to find a hidden Timer Ball in the northeast corner of the purple flowers.
Now go southwest to reach Snowbelle City.