At Great Sand Dunes Medano
Creek flows around the east and south sides of the dune mass. A unique
phenomena occurs along this sandy reach of the creek during periods
of high water. At regular intervals, roughly every 15 seconds, a bore
(a pulse of water) passes downstream.
In this vertically exaggerated diagram, low amplitude underwater
dunes called antidunes form as water carries sand grains from trough to
crest in the same way that wind action creates sand ripples on dry land.
As the height of the antidune increases, the volume of water dammed behind
the antidune crest eventually becomes heavy enough to break the antidune
and spill over into the next trough downstream. A chain reaction of antidune
breaks occurs downstream, creating one bore of water. Subsequent bores
occur as antidunes rebuild in the streambed and break once again.
You may observe pulsating flow on a few other streams on
Earth, but Medano and Sand Creeks are the best examples in North America.
