Sometimes a river

Does anyone know what the phrase "...but sometimes a river" means? Like why would someone conclude a sentence with that? It's more or less driving me crazy trying to get to the bottom of that.

"Usually I swim in a lake, but sometimes a river." Is that the sort of sentence you mean or is there more to it?

I'm confused, too. What's the complete sentence, and what is it from?
SS

Maybe it's "Sometime ... or never." Someone could be declining to do a task that was asked of him: "Sure, I'll get around to that ... sometime .... [then under the breath:] or never."

Sometimes A River The String Cheese Incident lyrics
Artist: The String Cheese Incident
Album: One Step Closer
Year: 2005

I was cleaning the ash from the fire
and putting the party away
helping out all of my lost friends
to see the light of day

I was feeling the wind through the windows
Sometimes it crashed through the wall
seems like it comes out of nowhere
theres no warning at all

chorus
sometimes a river
sometimes a river
runs dry
sometimes a river
sometimes a river
runs high
..........
There's lots more. See, inter alia,

[Dead link removed - ed]: SS

Sometimes I live in the country -
Sometimes I live in town -
Sometimes I get a great notion
To jump in the river and drown.

Irene, good night, Irene, good night -
Good night, Irene, good night, Irene -
I'll see you in my dreams.

(Words and music by Leadbelly (Huddie Ledbetter))

If the river was whiskey and I was a duck,
I'd dive to the bottom and I'd never come up -

Oh tell me how long do I have to wait,
Can I get you now, or must I hesitate?

(Traditional song)