Things like ripples in sand are a result of this dynamic relationship. I've got some more detailed posts on different kinds of bedforms, but today I want to share a video of one particularly intriguing type: antidunes.
Any bedform is going to be a byproduct of the depth and velocity of the moving fluid plus the type of material it is flowing over. For most sand, slowly flowing water will produce small ripples - you have probably seen these in the sand along the shore of a lake or river.
However, once you start trying to move fluids faster and faster over this surface, the fluid exhibits some interesting behaviors. The ratio between the velocity of the fluid to gravity and the flow's depth is referred to as the "Froude" number Fr. I've heard it pronounced "FROOD" and "FROWD." I like the sound of the former myself.
Fr = (mean velocity) / (g * depth)1/2
For low velocities, or deep streams, this number is small (typically less than one). But, when a shallow stream is flowing very fast, it can be higher than one. When Fr < 1, it is referred to as "subcritical flow." When it is equal to 1, it is called "critical flow," and if Fr > 1, it is called "supercritical flow." Most ripples and dunes form as a result of subcritical flow. With supercritical flow, however, we start to get other bedforms, including antidunes.
Antidunes are relatively easy to create in an artificial flume. They are more difficult to see in nature:
The above video was taken by a friend of mine while in Hawaii this winter. Water was draining down the beach towards the ocean. The shallow stream started picking up speed and began to form antidunes - which appear in the flowing water as rapids that migrate upstream. Recognizing these processes in the sedimentary record requires some understanding of how they work. I'll have to spend some more time talking about sedimentary structures.
I want to thank Kelly McCullough for capturing the above video - my camera had started to act a little funny and I didn't get many good pictures. In this case, video was the right tool for the job. Kelly's a sci-fi author and all-around neat guy. You should check it out sometime.