Push and pole as he would, he seemed to have no control whatever over the boat.

He had had no idea that the current would be so strong.
On he went, right down toward the bridge, and as the boat swept over it, one end struck an upright beam that projected above the water, and the clumsy craft was jerked around with such violence that Harry nearly tumbled into the creek.
He heard Lewston and Harvey shouting to him, but he paid no attention to them.

He was working with all his strength to get the boat out of the current and into shallower water.

But as he found that he was not able to do that, he made desperate efforts to stop the boat by thrusting his pole into the bottom.

It was not easy to get the pole into the mud, the current was so strong; but he succeeded at last, by pushing it out in front of him, in forcing it into the bottom; and then, in a moment, it was jerked out of his hand, as the boat swept on, and, a second time, he came near tumbling overboard.
Now he was helpless.