Sentinel Gap is where the Columbia River punches through the Saddle Mountains in a dramatic water gap that the Missoula Floods likely widened from a narrow notch into the broad passage visible today. The gap's basalt walls rise steeply on...
Sentinel Gap is where the Columbia River punches through the Saddle Mountains in a dramatic water gap that the Missoula Floods likely widened from a narrow notch into the broad passage visible today. The gap's basalt walls rise steeply on both sides, and scour marks at elevations far above the modern river testify to the extraordinary depth of floodwaters that once surged through this constriction. During the floods, water was concentrated as it passed through the gap, accelerating to enormous velocities and applying tremendous erosive force to the canyon walls. The Saddle Mountains themselves are an anticlinal ridge -- a fold in the basalt created by tectonic compression -- and Sentinel Gap is where the Columbia River won its battle against the rising rock over millions of years, a battle that the floods finished in spectacular fashion. The name 'Sentinel' is apt: the gap stands guard over the transition between the Quincy Basin to the north and the wider Columbia Plain to the south. Drive through Sentinel Gap and you pass through a geological gateway carved by the patient river and blown wide open by the most powerful floods in Earth's recorded geological history.
Sentinel Gap is the water gap where the Columbia River cuts through the Saddle Mountains anticline, located between Wanapum Dam (north) and Priest Rapids Dam (south) near Mattawa, WA. SR-243 runs along the east side of the river through the gap with multiple pullouts. The Wanapum Dam Overlook (Grant PUD) sits just north of the gap, with ADA viewing platform, interpretive signage that includes Ice Age Floods content, vault toilet, and shelter; free and open daily. Hiking access onto the cliffs themselves is limited, Sentinel Mountain trails (7-mile out-and-back and 10.6-mile loop) are challenging and exposed.
Sentinel Gap is the southern Columbia River chokepoint between the Saddle Mountains' eastern and western noses. Missoula floodwaters routed from the Quincy Basin and Crab Creek system funneled through this narrow opening on their way to the Pasco Basin and Wallula Gap, accelerating dramatically because of the constriction. Strandlines (horizontal high-water terraces) on the basalt walls record successive flood levels; large boulder fields below the gap reflect plucking and deposition on the immediate downstream side. Like Wallula Gap, this was a pre-existing structural feature, a water gap carved by the Columbia River across an actively growing anticline, that the floods reamed out and steepened with each pulse. The cliffs on the north side rise nearly 2,000 feet above the river.
The 2020 USGS/Waitt review covers Sentinel Gap as part of the Mid-Columbia routing system. No new site-specific peer-reviewed work since.
Covered by the IAFI Lake Lewis Chapter and Ellensburg Chapter; included in regional field-trip itineraries. The Wanapum Dam Overlook carries Ice Age Floods interpretive panels.
Best in spring and fall; summer temperatures regularly exceed 100°F with no shade. The Wanapum Dam Overlook is the recommended quick stop; drive SR-243 south through the gap for the through-canyon perspective.
Flood-extent overlay shows the maximum reach of the Missoula Floods.
The site in its place along the flood path, with the maximum flood extent drawn over the modern map.
View on the interactive map Searchable sites · 360° trails · flood reconstruction