I too many and many a time crossed the river, the sun half an hour high, I watched the Twelfth Month sea-gulls—I saw them high in the air, floating with motionless wings, oscillating their bodies, I saw how the glistening yellow lit up parts of their bodies, and left the rest in strong shadow, I saw the slow-wheeling circles, and the gradual edging toward the south.
Now Whitman begins using one of his favorite and most famous phrases: "I too."
He's being very sneaky and trying to draw us into his experience…by pretending that it was really our experience. He's like, "Oh, so you've crossed a river many times a half hour before sunset. How funny – me too!" And some of us might be thinking, "Well, actually no, I use the Brooklyn Brid-," but we don't complete our thought, because that would make Whitman sad. Instead you've got to play along.
So, just like us, Whitman has crossed the river and watched the seagulls performing aerial acrobatics.
If you're a fan of gorgeous descriptions of nature, you might want to linger on these lines.