The Creek That Runs Across the Great Sand Dunes

Spring comes to the San Luis Valley as wind before it comes as warmth. The sagebrush hills stay brown, the high peaks keep their snow, and the valley floor rattles with gusts that come off the plain and run into the mountains. The wind is the reason the Great Sand Dunes exist. It gathers sand from the dry valley floor and carries it eastward, until the Sangre de Cristo rises abruptly out of the valley. The range is a wall. The wind cannot climb it all at once, and it drops its load.

That load has been piling up for a long time. The result is a dunefield not at the center of a desert but pressed hard against the base of a mountain range, a wedge of gold between the valley grass and the dark talus slopes above. The steep wall behind the dunes makes the sand look smaller than it is; a human figure standing below the high ridge of the nearest dune is a speck. These are the tallest dunes in North America.

They are not static. The wind keeps them in motion, building them, combing their surfaces, moving grains from the flanks to the crests and over the tops. The dunes are always being rebuilt. But wind alone did not make this particular place, and wind alone would not keep it. The dunes require water, too.

The Creek That Shares the Name

Medano Creek comes down from the mountains and meets the dunes at their base. It is not a large creek. In most photographs it looks like a wide, shallow stream in a sand wash, and in much of the year that is exactly what it is. But in spring, when the snow begins to genuinely melt in the high basins behind the Sangre de Cristo, the creek rises and spreads. It crosses the sand.

The creek's name remembers the landform it runs against. Medano is the Spanish word for a dune. The creek and the dunes belong together, and not only by name. In spring, the water arrives with enough force to shift the sand itself. It does not carve a channel in the ordinary sense, because a channel in sand is only a suggestion. One day the stream runs along the north side of a small hill of sand; the next day it has undercut that hill and found a route through it.

The water spreads into broad shallow sheets. It looks less like a creek than a flooding of the flat ground along the dune margin. The water is clear and very cold, carrying a fine clouded load of mineral and sand. Where it slows, it leaves a skim of silt on the surface; where it moves, it exposes dark grains of heavier material that glitter in the sun.

At the edge of the dune field, the creek does its most dramatic work. It has, over time, removed sand from the base of the dunes, built a broad wash, and kept the dunefield from advancing too far into the valley. The same water that looks so temporary has a permanent job.

The Surge

The strangest behavior of Medano Creek happens in the afternoons of its high-water weeks. The water level does not stay level. It rises, falls, and rises again in a rhythm that has nothing to do with rain or the moon. The creek pulses.

The pulse is the result of the creek working against itself. As the water runs through a shallow, sandy reach, it erodes the bank. A section of sand slides into the stream, and the water, unable to push through all at once, begins to build against the loose material. The sand behaves like a low dam. More water backs up behind it. Then an extra wedge, perhaps just a bit steeper than the rest, pushes through the dam, and the whole accumulation gives way in a sudden release of water and sediment.

That release travels downstream as a low rolling wave. It is not an ocean wave; it does not break. It moves over the flat sand as a coherent front, a step of water that can be a few inches high and several yards long. The front pushes up the beach, covers sand that had been dry, then passes. A second surge follows a few minutes later, and then another, sometimes in a steady series that keeps the whole strand line moving.

Standing in Medano Creek during a surge is a strange experience. The water goes from a gentle ankle-bath to a pushing, shin-deep current, then back. The bottom shifts as the water passes. Sand that was solid underfoot is washed away; a sandbar that was behind is suddenly in front. No one can stand in the same place without noticing that the creek is alive.

The surges are not a separate spectacle. They are the creek moving sand. Each pulse carries sediment a little farther down the wash. In that way, the creek is doing what the wind does, only in the opposite direction. The wind piles sand up against the mountains; the creek pulls it back. The dunes stand in the middle of this tug of war.

A Spring of Cold Water on Hot Sand

For the few weeks of high water, the edge of the dune field inside Great Sand Dunes National Park and Preserve becomes a shore. People come to it the way they would go to a lake, carrying chairs, towels, umbrellas and floats. There is no salt in the water, and there is a mountain of sand instead of open sea, but the scene has the temperature and light of a beach. The water is snowmelt, and it is cold. The air at this elevation is thin, and the sun is strong; a day at Medano Creek can include both sunburn and cold feet.

The beach is not a strip but a length of dune front. It stretches for a good distance, and the water is shallow over so much sand that small children can wade far out. The Sangre de Cristo rise so close on the east that the skyline is mostly rock and snow. The wind still blows, but the water keeps the sand at the shoreline wet, so the dust stays down.

By the end of spring, the high water leaves. The snow in the mountains shrinks to the deep shaded gullies. The creek loses its source, and the broad sheet of water narrows, slows, and finally disappears into the sand or blows away in the dry air. The beach goes with it.

What remains is the dune field itself, dry and immense, with the mountains behind it and the wind doing its slow arithmetic across the slope. The dry bed of Medano Creek is not much to look at in late summer: a wide, flat, sandy wash that might not seem like water at all. But it is the same sand that the wind has been stacking for a long time. The water will come back, spread again, and start the work of taking some of it down. The Great Sand Dunes are not a fixed thing. They are a balance, and in spring the balance is visible in moving water.

Great Sand Dunes National Park and Preserve

Photos: mark byzewski, James St. John (CC BY 2.0)