By the touch of that poison the huge trees on the banks were withered and being touched by the waters raised by the wind the birds were scorched. Beholding that dreadful serpent, like the second mouth of death, the illustrious Slayer of Madhu thought—"Forsooth, the wicked and poisonous serpent Kāliya lives here, who, being defeated by me, was constrained to leave the ocean whose waters were defiled.

In plain words: The poison had withered the big trees on the shore and burned birds touched by the spray. Seeing the serpent, as frightening as death itself, Krishna thought: this is the wicked snake Kaliya, whom I defeated and forced to leave the ocean.