Complex sentence: Since Lake Columbia was impounded behind the Okanogan lobe, which rose to, this lobe effectively blocked the normal course of the Columbia River, blocking the Missoula Floods and diverting water to flow across much of eastern Washington state.
Split into simpler sentences: Since Lake Columbia was impounded behind the Okanogan lobe, which rose to. This lobe effectively blocked the normal course of the Columbia River, blocking the Missoula Floods and diverting water to flow across much of eastern Washington state.