GR translation: the dark increase of the river

[Mike Jing]

V444.18-30, P451.28-40   In a dream from this time, his father has
come to find him. Slothrop has been wandering at sundown by the
Mungahannock, near a rotting old paper mill, abandoned back in the
nineties. A heron rises in silhouette against luminous and dying
orange. “Son,” a falling tower of words tumbling over and over
themselves, “the president died three months ago.” Slothrop stands and
curses him. “Why didn’t you tell me? Pop, I loved him. You only wanted
to sell me to the IG. You sold me out.” The old man’s eyes fill with
tears. “Oh son . . .” trying to take his hand. But the sky is dark,
the heron gone, the empty skeleton of the mill and the dark increase
of the river saying it is time to go . . . then his father is gone
too, no time to say good-by, though his face stays, Broderick who sold
him out, long after waking, and the sadness Slothrop brought into it,
fool loudmouth kid.

What does "increase" mean here?
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