The line that connects them both

The two of them stood at the river's edge
The current slipping away, quick and cold.
Casting towards shadows, where the sunlight fails,
And fish shelter in the undergrowth.
And when the unseen strikes, how all else pales
Beside the bright-dark struggle, the rainbow wroth,
Life and death weighed in the shining scales,
The invisible line pulled taut that links them both.