Construction on Reservoir 5, 1910.
City of Portland (OR) Archives, Reservoir 5 construction, A2008-009, 1910.
Reservoir 5, 1910
Vintage Portland
Construction on Reservoir 5, 1910.
City of Portland (OR) Archives, Reservoir 5 construction, A2008-009, 1910.
Vintage Portland 
I note the triangle cuts in the toe of the slope, I imagine to lessen slippages and slides.
Looks like a nice mild day in mid-October if the markings on the bottom corner are indeed what I think they might be: 17-October-1910. The angle of the shadows seems to support a lower position of the sun in the sky, consistent with fall.
The men pictured look like a supervisor and 3 laborers. It looks like they’ve positioned shoots under the rock pile so the men can use their shovels to slide the rocks down the shoot into the wheelbarrows at the bottom. Damn hard work loading and transporting these loads to where they need to go.
Looking at the related photos above the comments one photo shows what looks like 17-b in the lower right corner.