North of Diamond Craters, a late Pleistocene basaltic lava flow, called the Voltage Flow, forms a high-standing subdued ridge that marks the eastern boundary of the pluvial lake basin that is now occupied by Malheur, Mud, and Harney lakes. The Voltage Flow was erupted about 1.9 million years ago from several volcanic vents located north of Diamond Craters. At that time, the Voltage Flow created the pluvial lake basin that the Malheur National Wildlife Refuge currently occupies by filling the paleovalley of and damming the prehistoric course of Malheur River at that time.