The Horse Heaven Hills, by Richland Washington, are a remarkable stop in the Lake Missoula story, showing the vast differences between the landscapes that felt the wrath of the floods and those that did not. As they churned their way to the Wallula Gap, where their might and power are on full display, vast amounts of muddy debris plugged the course of the Yakima River, and left the graveyard of its previous valley in their wake.