Here is the full list of all 57 Riverbond achievements.
Find the secret beach
Defeat Great Cartos
Speak with Brother Honey
Kill 80 Slimes
Die by Traps 20 Times
Begin the Ninth Adventure
Defeat All Bosses
Destroy The Knight
Speak with Eldra Highest of High
Speak with Eldra Rex
Collect 35 Stars
Speak with Eldra the Spirit
Begin the eighth adventure
Speak with Spirit-Running-Softly
Begin the seventh adventure
Exterminate Column XVII
Collect 25 Stars
Find the secret room in the museum
Begin the sixth adventure
Speak with Hamdalf
Defeat Puffy the Lurker
Speak with Hamdini
Find the secret cave in the desert
Speak with Ham Solo
Unlock 35 Skins
Defeat 20 enemies with any Slap Weapon
Collect 15 Stars
Begin the fifth adventure
Slay Pumpiron Pengo
Speak with Fur
Speak with Hoota tha Skoota
Begin the fourth adventure
Shatter Romeo Ur-Ribitt
Speak with with Mr. Oopay
Unlock 25 Skins
Speak with BOMB GIRL in the Swamp
Find the secret storage area in the swamp
Begin the third adventure
Wipe Out Rabbitus Maximus
Speak with Larry the Lost
Defeat 20 enemies with any Dual Weapon
Defeat 20 enemies with any Club
Defeat 20 enemies with any Spear
Speak with Peaceful Pig
Begin the second adventure
Unlock 15 Skins
Wreck Captain Barbacus
Defeat 20 enemies with any Gun
Speak with Big Bob
Speak with Waterkeeper Wyn
Collect 5 Stars
Begin the first adventure
Unlock 5 Skins
Complete the tutorial
Defeat 20 enemies with any Sword
Waste Koke the Crab
Speak with Sunlight-Rising-Boldly