A big up-bound steamboat had run down and sunk a smaller boat of some kind.

That much he could see, and he was filled with horror at the nearness and magnitude of the disaster.
He had heard agonized screams, and knew that lives had been sacrificed.
One shrill cry that came to his ears with startling distinctness sounded as though uttered by a woman or a girl, and Winn shuddered at the thought of her fate.
The raft was drifting rapidly away from the scene of the catastrophe, and the dimly discerned steamboat was just disappearing from his view, when the boy thought he heard a gurgling cry from the water.

Could some bold swimmer have escaped?
He bent his head to the water's edge and listened.

Again he heard the cry.

And this time it seemed nearer.
Some human being was struggling in the river.