LAST GREAT ICE-AGE FLOOD DOWN GRAND COULEE
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.