There are only a few places in the United States where creeks have beach-like waves due to a phenomenon known as “surge flow.”
What are surge flows?
Surge flows are a phenomenon characterized by rhythmic, wave-like flows that surge forward, similar to mini tidal waves. Also known as pulsating periodic bores, it’s the right mix of sand or sediment, relatively shallow water, and steep topography that creates conditions ideal for surge flows to form.
Where do surge flows form?
Not many rivers or creeks in the world experienced surge flows. There are two locations in the U.S. National Parks System where surge flows are known to form.
Medano Creek in Colorado
The most well-known place where surge flows form is Medano Creek, a seasonal stream that flows at the base of the Great Sand Dunes in Colorado. Set against the stunning backdrop of the tallest sand dunes in North America, Medano Creek starts flowing each year once the snow fields in the Sangre de Cristo Mountains, the southernmost subrange of the Rocky Mountains, starts to melt. The water collects in Medano Lake before eventually flowing down toward the base of the dunes as Medano Creek.
The flowing water creates sand dams or antidunes. These small mounds create a dam-like barrier to water flow. After enough pressure from the flowing water, the dams break, sending the water crashing over the mounds. This rhythmic cycle happens every 20 seconds or so, mimicking small ocean waves crashing into a beach.
Late spring and early summer are the optimal times to experience surge flows at Madano Creek. The National Park Service has a short video explaining surge flows at Medano Creek:
Niobrara River in Nebraska
The Niobrara is a Great Plains river located in north-central Nebraska. In 1991, 76 miles (120 kilometers) of the Niobrara was designated a National Scenic River. Each winter, elevated water flows and sediment loads create conditions where surge flows form on the Niobrara.
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Fonte : National Geographic
