South Prairie Lake

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South Prairie is a seasonal lake in Gifford Pinchot National Forest, Washington. Most of the year it’s a dry meadow, but every spring it fills to about 85 acres. The exact cause of the flooding isn’t fully understood, but one idea is that ice in the nearby Big Lava Bed blocks drainage until it melts. A small stand of quaking aspen grows at the edge of the flooded area, and large black cottonwoods stand in the meadow itself when it’s flooded. The site hosts the largest known population of pale blue-eyed grass (Sisyrinchium sarmentosum), a rare blue-eyed iris first found here in 1893. This is where the species was first described, and it has the most genetically diverse population of this plant.