 The Wallula Gap was widened about the 14th millennium BC, by the historic flow of the Salmon, Snake, and Columbia rivers combined with the glacial waters that poured across the Channeled Scablands during the Missoula floods.The Wallula Gap constrained the flow such that less than 1/5 of the 800 km³ (192 mi³) of water per day entering could be discharged as it passed into Lake Condon downstream.As a result, the floods filled the Pasco Basin and formed, for a short period, Lake Lewis.