What is a Non-alluvial River?
With glassy purpose,
Water shoots over smooth rock;
Then rests in shadows.
-Paul Bakke, River haiku No. 976
When I was a boy, my parents loved to take us on picnics in the Cascade Mountains. And almost always, our destination would be a pretty stream. My mother loved these places because they reminded her of her childhood in Oregon. My father loved them because they were so remarkably different from his childhood on the dry prairies of North Dakota. These mountain streams were full of rocks of all sizes, and water moving around and over these rocks, noisily, in every form imaginable. The water was always clear and cool, and in those days, we used to drink it straight out of the creek. When I had had my fill of this refreshing libation, I would walk around in the creek with my sandals on, picking up rocks and looking at the interesting bugs living on them, marveling at the way the water plunged over the larger rocks, swirling and making curtains of bubbles. I wish I could say that this was when I decided to become a river scientist, but really, I had no idea then that my life would follow this course. That was all to come much later.
These mountain streams were all ”non-alluvial.” This means that the rocks making up the framework of their bed and banks were not carried there by moving water. These rocks had fallen off the local hillside individually or been carried there in landslides. Material that originates locally on the hill slope is called colluvium, and so these non-alluvial streams are often called colluvial streams. Areas with boulders may alternate with areas flowing over bedrock. Trees that fall into the channel often end up spanning the channel, not touching the water most of the year. Those trees that do end up in the water can become steps that the water plunges over, immobile for many years. And because these logs are generally quite long relative to the width of the channel, the water usually can’t move them very much. The channels are usually closely confined by the valley walls, since these channels formed by water eroding downward into the landscape over centuries and millennia. The channel inherits its structure from what was uncovered as it eroded downward, and what fell into it afterwards.
The smallest, and steepest of these colluvial streams don’t seem to have a much organization at all to their structure. Sometimes called boulder cascades, they may seem to consist of continuous turbulent white water. But as these streams grow in size, further down the hill slope, they often develop a regular sequence of steps and pools, with the pools spaced 1 – 3 channel widths apart. Evidently, in these step pool streams, the boulders do move, at least once in a great while, during the largest floods, and in doing so they organize themselves into clusters spaced apart in this manner. These boulders probably move only once in 10 years or more.
Moving further down the valley, the stream begins its transition from colluvial to alluvial character. Alluvial means carried by, and deposited by, moving water. Small pockets of alluvial (sand and gravel) sediment appear along the edges or in the protected areas behind boulders or logs. The channel may no longer have the appearance of discrete steps and pools, but rather be like a continuous, dispersed boulder field with pools scattered here and there, none of them large enough to span the whole channel. There may even be places where there is a flat area of gravelly or sandy soil, a rudimentary floodplain, forming along the channel edges, although the valley sides closely confine the stream, not giving it room to meander back and forth. Sometimes scientists call these plane-bed streams. In any case, almost all of the sediment that moves in from upstream, is simply carried on through without accumulating. For this reason, scientists also refer to these streams as “transport reaches.”
Logs that fall into these plane-bed streams are often moved by floods and can end up clustered together into logjams. Sometimes it is the logs in and on the streambed that provide most of the framework for holding the bed together. If these logs disappear, as they often have when people remove logs from streams or remove the trees that would naturally fall in, the channel can gradually lose its streambed sediment and become a bedrock channel.
Because these streams are confined by their valley walls, and have abundant large material creating friction with the moving water, they are generally not as responsive to changes in the sediment load or water discharge as are alluvial streams. They can, and do, incise or down cut, as when logs are removed from them. But they usually don’t incise very far, because bedrock is often not very deeply covered beneath the streambed. And since they don’t have a floodplain, they don’t tend to erode their banks and widen as much as an alluvial stream would under similar conditions of down cutting. Colluvial streams tend to be resilient, that is, resistant to changing their form when outside forces disturb them.