From the Niagara Falls Review:
The Niagara Parks Commission will begin its annual rock scaling along the Great Gorge wall on the Canadian side of the Horseshoe Falls from April 21-25. The rock scaling will continue through the week, weather permitting, or until completion.
Niagara Parks staff scale rocks that have become loose during the winter to protect visitors from rocks that might fall on the observation deck at the Journey Behind the Falls attraction.