Snake Rivers
The adverb is mainly. It modifies the verb "drain" to say that the rivers do not entirely drain into the valley.
Gulf of Mexico
Columbia and the Snake Rivers
All rivers drain into oceans.Although, due to humans drawing huge amounts of water for our cities, some rivers dry up before they reach the see nowadays.
nile river
Lena River
The Colorado River.
drought
the Magdalena and Cauca
The Rio Grande and other rivers dry up before reaching the sea because humans take so much water from them. Some rivers may also dry up naturally, or drain into a salt lake with no ocean connection.
your mothers river, and your mothers fathers river