Sections of the fill in Columbia canyon near Grand Coulee Dam, together with the stratigraphy and morphology of related features downstream, record the expansion and later shrinkage of the Okanogan glacier lobe during the last glaciation. Three stratigraphic zones are recognized: (1) A Basal Sequence of lacustrine fines inter-bedded with fluvial sediments, recording oscillatory ponding of the Columbia River as the glacier dammed it downstream. At first the lake was shallow and drained west around the ice margin. Later it was more effectively dammed, deepened, and forced to discharge south through Grand Coulee. (2) A Till Zone, recording the arrival of ice, with fluctuations, at Grand Coulee Dam during the glacial climax. (3) Later deposits stratigraphically (but not everywhere vertically) overlying the Till Zone, recording oscillatory deglaciation accompanied by transition from lacustrine to fluvial conditions.
The San Poil glacier lobe entered the lake-filled Columbia canyon at Keller Ferry, 15 miles upstream from Grand Coulee Dam, and contributed sediment to the lake. The maximum of this lobe appears to have been approximately contemporaneous with the maximum of the neighboring Okanogan lobe.
While residual ice was still present locally in Columbia canyon downstream, drain-age abandoned Grand Coulee and resumed its former (and present) course. Thereafter, grading of the long profile of Columbia River proceeded both by erosion of the lake fill and by deposition in depressions partly of ice-block origin and was marked by at least one delay recorded by a particularly conspicuous stream terrace.