Spring Runoff in August?

There is plenty of public discussion concerning global climate change, and yet some historical data does not seem readily accessible. Recently rereading the classic primary text, Adventures of the First Settlers on the Oregon or Columbia River, 1810-1813 by Alexander Ross, I was caught by surprise by the date of a seemingly ordinary observation. While traveling upstream in 1811, Ross missed the great Columbia Falls (Celilo Falls) due to high water.

Having ascended about seven miles, we arrived at the falls--the great Columbia Falls, as they are generally called; but, from the high floods this year, they were scarcely perceptible, and we passed them without ever getting out of our canoes. In seasons of low water, however, the break or fall is about twenty feet high and runs across the whole breadth of the river, in an oblique direction. (132)*

This passage seems a perfectly ordinary description of spring floods in June or even early July, but Ross was traveling upriver 5 August! Had the flood levels dropped and yet remained high enough to obscure the falls? Was spring runoff in 1811 more of a mid-summer runoff than we experience today?

According to the Bonneville Power Administration and other government agencies, winter snows melt and flow downriver in the months May through July.

The moisture that is stored during the winter in the snowpack is released in the spring and early summer, and about 60 percent of the natural runoff in the basin occurs during May, June, and July. "The Columbia River System Inside Story," 2nd edition (2001)*

Has the flood schedule changed since the nineteenth century? Dams can delay high flows, but they do not make the snow melt faster. Did snows melt slower in 1811 than they do today? Are these questions for historians, or only for climatologists?


*I am using the Northwest Reprints edition: Alexander Ross, Adventures of the First Settlers on the Oregon or Columbia River, 1810-1813 (Corvallis: Oregon State University Press, 2000 [1904]).

Bonneville Power Administration, U.S. Bureau of Reclamation, U.S. Army Corps of Engineers, "The Columbia River System Inside Story," 2nd edition (April 2001), http://www.bpa.gov/power/pg/columbia_river_inside_story.pdf.