Helen H. Richardson, The Denver Post
Elizabeth Fowler, 16, from Texas, tries to balance herself in waves created by surge flow in Medano Creek at the Great Sand Dunes in Mosca, Colorado, on June 19, 2019. 
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Each spring Medano Creek surges, creating a beach at the base of the dunes in Colorado’s Great Sand Dunes National Park and Preserve. The flows begin on the bottom of the creek when the current pushes sand grains into small underwater ridges called “anti-dunes.” Like dams, anti-dunes trap water until they finally break, sending waves of water downstream to create a beach playground for visitors.
The park comprise 30 square miles of dunes and include 750-foot-elevation-gains. Also in the area is the backcountry Medano Pass Primitive Road winds through a canyon toward the Sangre de Cristo Mountains.
Helen H. Richardson, The Denver Post
The Great Sand Dunes are lit by the setting sun near Mosca, Colorado, on June 19, 2019. (Helen H. Richardson/The Denver Post)
Helen H. Richardson, The Denver Post
Dozens of park visitors hike around the base of the sand dunes at Great Sand Dunes National Park in Mosca, Colorado, on June 20, 2019.
Helen H. Richardson, The Denver Post
Footprints lead towards the top of the dunes at the Great Sand Dunes in Mosca, Colorado, on June 20, 2019.
Helen H. Richardson, The Denver Post
Issac Nelson, 16, gets a push on his inner tube from his mother Elizabeth Nelson Hulse as they swim in the surge flows in Medano Creek at the Great Sand Dunes National Park and Preserve on June 20, 2019.
Helen H. Richardson, The Denver Post
From left to right Emily, Bianca, and Sammi, who each declined to give their last name, wade through the waves of the surge flows in Medano Creek on June 20, 2019.
Helen H. Richardson, The Denver Post
Isaac Nelson catches waves in the surge flows in Medano Creek on June 20, 2019.
Helen H. Richardson, The Denver Post
Jeff Fruin and his daughter Calla, 2, right, check out the sand beach in Medano Creek as the sun sets at the Great Sand Dunes in Mosca, Colorado, on June 19, 2019 in Mosca.
Helen H. Richardson, The Denver Post
A park visitor hikes one of the many dunes early in the morning at Great Sand Dunes National Park on June 20, 2019.