LAST GREAT ICE-AGE FLOOD DOWN GRAND COULEE

The last Ice Age flood down Grand Coulee resulted from the sudden lowering of glacial Lake Columbia that breached a flood-debris dam located midway along the coulee. The presence of hundreds of lacustrine varves has long been recognized as evidence for a quiescent Lake Columbia residing in the upper coulee for up to several centuries after the last Lake Missoula flood. During this time glacial ice of the Okanogan Lobe blocked the present course of the Columbia River - diverting it down Grand Coulee. Glacio-lacustrine deposits and deltas in the vicinity of the upper Grand Coulee rise up to 60 m above a bedrock-controlled outlet (460 m elev) at Coulee City. This discrepancy in elevation has been attributed to isostatic rebound after retreat of the Okanogan Lobe. Under this scenario the Columbia River drained out of Lake Columbia over the bedrock spillway. However, the lack of evidence for a Columbia River channel anywhere downstream, is inconsistent with centuries of erosion from a powerful, meltwater-gorged river.

Previously unrecognized is a 100-m tall flood-expansion bar that accumulated in the Hartline Basin at the mouth of upper Grand Coulee during earlier Missoula floods, similar to Ephrata Fan at the coulee terminus. While the central portion of the expansion bar is missing, remnants exist near Coulee City to an elevation of 550 m. Maximum elevation of glacio-lacustrine deposits in the upper coulee approaches the top of the Coulee City expansion bar. Thus, while some isostatic rebound might have occurred during deglaciation it is not required to reconcile the height of these deposits. We surmise the central portion of the bar was breached and removed during a Lake Columbia flood, lowering the lake level by 100 m before encountering resistant bedrock below. Furthermore, extensive scabland-type erosion by a Lake Columbia flood downstream of the debris dam would explain the curious absence for an ancient Columbia River channel. The mechanism(s) for dam failure could have been: 1) overtopping of the dam by late-glacial rapidly rising lake levels, 2) hydraulic piping, or 3) seismicity leading to liquefaction or perhaps a seiche wave impacting the debris dam. Soon after the Lake Columbia flood down Grand Coulee the very last Lake Columbia flood escaped down the deeper Columbia Valley upon failure of the wasting Okanogan Lobe ice dam.