Mouth of the River
by Imagine Dragons
is most likely in the Key of G#/Ab Major
Notes in G#/Ab Major:
A#/Bb, C, C#/Db, D#/Eb, F, G, G#/Ab
Chords in G#/Ab Major:
Ab, Bbm, Cm, Db, Eb, Fm, Gdim
This key can also be known as F minor.
When it's used as a minor key, the minor key note is stressed more than the major key note. The same notes and same chords still apply.