Unclear question. But I inferred you are referring to the story "INTO THE RAPIDS" by Bradford H. Robie.
Answer:
There's wasn't any support for him to grab hold of.
Explanation:
Remember, we are told the force of the water popped the boat back up and Wyatt was thrown backward into the water. To make a good description of his difficulties the narrator uses the sight of a type of building that is very tall called- Skyscraper.
In the view of Gregor it was difficult to grab the walls of the canyon just as it is literally impossible to be on the ground and still tourch the walls of a skyscraper.
