Marmes Rockshelter, near the confluence of the Snake and Palouse rivers in southeastern Washington, became one of North America's most important prehistoric archaeological sites because of its deep, well-preserved deposits and extremely ancient human remains. Excavated chiefly during the 1960s after its discovery drew scholarly attention, it revealed evidence of human use stretching back more than 11,000 years and a long record of river life, burials, tools, trade, and environmental change. Its remains were once considered the oldest then known in North America and remain the oldest found in Washington. Though designated a National Historic Landmark, it was inundated in 1969 by the reservoir behind Lower Monumental Dam.