The Columbia River drifts lazily through the gap in the Saddle Mountains and curves to form Wahluke Slope's southern and western boundaries.  It is the only appellation in the Pacific Northwest that is a single geological landform.  The region is a giant alluvial fan created by repetitive flooding events.

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        Of the four newest Washington appellations within the Columbia Valley AVA, the Wahluke Slop may hold the most untapped potential.  An 81,000-acre triangular slice of desolate scrubland, it is nestled above the Columbia River in south-central Washington and wins high marks for the way its concentrated heat ripens Merlot and Cabernet Franc.

         Much of the appellation was created by The Great Missoula Floods that exploded through ice dams thousands of years ago, ripping land and rocks from their paths during the final stages of the last ice age.  Unimaginable volumes of floodwaters propelled icebergs and boulders across the Columbia Basin.  Locally, floods burst through Sentinel Gap near Mattawa on the Wahluke Slope, grinding basaltic lava flows into tiny particles and dumping boulders, some the size of houses, across the area.  Layers of silt and wind-blown sand form the surface through which grapevines send their roots.  Gravelly, rocky soil, combined with a dry, warm climate, make Wahluke Slope one of the best places in Washington to grow wine grapes.