| Field Trip Description: This six-hour hike follows a 13-kilometer-round-trip route to an extraordinary geologic location called “Breached Dam Overlook” just seven kilometers north of the crater of Mount St. Helens. The trail goes from the Johnston Ridge Observatory onto the largest landslide deposit to have accumulated during human history. This debris avalanche deposit of May 18, 1980, forms one of the earth’s newest landscapes of 45 square kilometers area within the headwaters of the North Fork of the Toutle River. The objectives of the trip are (1) to identify, classify and name individual landforms within the upper North Fork Toutle River landscape, (2) to relate the landforms to the sequence of events and processes that have occurred next to the volcano, and (3) to ponder questions about how the landscape at a volcano changes through time. Landforms on the debris avalanche landscape are relicts that have been impacted significantly by geomorphic processes that exceed a certain minimum energy threshold. Following the debris avalanche of May 18, 1980, the most significant event was the mudflow of March 19, 1982. That mudflow event breached the natural debris dam, caused adjustment within the drainage basin, and derived the present landscape. Now that the power of geomorphic processes has diminished, finer sediment is what is being moved. Channels are incised and armored with coarser clasts, and valleys are now plugging with sediment. Johnston Ridge Observatory on the west side of Mount St. Helens Volcano National Monument is the staging area this roundtrip hike. |