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