Hydraulic criteria for sand‐waves

Sand‐waves on rivers are rhythmic successions of waves which occur at flood‐stages of streams heavily loaded with sediments. They take their name from the fact that sand and associated silts and gravels form a large part of the load transported by a river at such times. They seem to be peculiar to the Southwest and many vivid descriptions of them can be found in the literature of that region. R. C. PIERCE [see 1 of “References” at end of paper[, who observed many sand‐waves on the San Juan River in Utah, has described them as resembling in appearance “the waves thrown up by a stern‐wheel river steamboat.” He further describes their appearances as follows: “The sand‐waves are not continuous, but follow a rhythmic movement. At one moment the stream is running smoothly for a distance of perhaps several hundred yards. Then suddenly a number of waves, usually from six to ten, appear. They reach their full size in a few seconds, flow for perhaps two or three minutes, then suddenly disappear. Often, for perhaps half a minute before disappearing, the crests of the waves go through a combing movement, accompanied by a roaring sound. On first appearance it seems that the wave‐forms occupy fixed positions, but by watching them closely it is seen that they move slowly upstream. In the narrow parts of the stream the waves may reach nearly the width of the river, but in the wider parts they occupy smaller proportional widths. Usually they are at right‐angles to the axis of the stream, but at some places, particularly in the wider parts of the river, they may suddenly assume a diagonal position, moving rather rapidly across the stream in the direction toward which the upstream side of the wave has turned.” Many such descriptions may be found which in the main bear out PIERCE'S account, varying, however, as to size of wave, rate, and sometimes as to direction of movement.