The difference between the present reservoir, with its silent sterile shores and debris-choked side canyons, and the original Glen Canyon, is the difference between death and life. Glen Canyon was alive. Lake Powell is a graveyard. Which phrase best contributes to the author's sad tone?

"debris-choked".

Explanation:

In the given passage, the speaker/ author presents the difference between the Glen Canyon and Lake Powell. This contrasting image/ description is made in such a way that the difference is felt and projects a drastic image.

The comparison of these reservoirs is so drastic that the author uses life and death to compare it. While "Glen Canyon was alive. Lake Powell is a graveyard." And the one phrase that best contributes to the author's sad tone is "debris-choked", which presents an image of a place that is dying, or on the verge of death.