streams are dynamic systems and constantly change their channel patterns and the amount of water (discharge) and sediment being transported in the system. streams may transport orders of magnitude more water and sediment in times of spring floods, as compared with low-flow times of winter or drought. since streams are dynamic systems, as the amount of water flowing through the channel changes, the channel responds by changing its size and shape to accommodate the extra flow. For instance, in a gradually changing climate scenario, the discharge and load of a river may gradually change, and the river may be able to make small changes accordingly to account for these variables. At some point, however, the balance of controlling forces in the river may exceed a critical threshold value, and the channel may suddenly make a dramatic change into a completely different configuration. In another scenario a river may gradually downcut its gradient through a mountain range, starting as a juvenile high-gradient stream, and over the course of many years gradually decrease its gradient (slope) as the bed is eroded. At different stages in this evolution, the stream may make transitions, perhaps rapidly, through different channel types and flow regimes. The following five factors control how a stream behaves:
• width and depth of channel, measured in feet (meters)
• gradient, measured as change in elevation in feet per mile (m/km)
• average velocity, measured in feet per second (m/sec)
• discharge, measured in cubic feet per second (m3/s)
• load, measured as tons per cubic yard (metric tons/m3)
All of these factors are continually interplaying to determine how a stream system behaves. As one factor, such as discharge changes, so do the others, expressed as
Q = w x d x v where Q represents discharge, w represents width, d represents depth, and v represents velocity. Other factors may also play a role, though are less important. These include the mean annual flood, meander wavelength, width-depth ratio, and sinuosity. These secondary variables are not totally independent; for instance, sinuosity and gradient are related, the mean annual flood and discharge are related, and so on. The main point is that the variables are all interrelated, and changing one can lead to changes in the others.
All factors vary across the stream, so they are expressed as averages. If one term changes, then all or one of the others must change too. For example, with increased discharge, the stream erodes, widens, and deepens its channel. With increased discharge, the stream may also respond by increasing its sinuosity through the development of meanders, effectively creating more space for the water to flow in and occupy by adding length to the stream. The meanders may develop quickly during floods because the increased stream velocity adds more energy to the stream system, and this can rapidly erode the cut banks, enhancing the meanders.
The amount of sediment load available to the stream is also independent of the stream's discharge, so different types of stream channels develop in response to different amounts of sediment load availability. If the sediment load is low, streams tend to have simple channels, whereas braided stream channels develop where the sediment load is greater. If a large amount of sediment is dumped into a stream, the stream will respond by straightening, thus increasing the gradient and stream velocity, and increasing the stream's ability to remove the added sediment.
When streams enter lakes or reservoirs along their path to the sea, the velocity of the stream suddenly decreases. This causes the sediment load of the stream or river to be dropped as a delta on the lake bottom, and the stream attempts in this way to fill the entire lake with sediment. The stream is effectively attempting to regain its gradient by filling the lake, then eroding the dam or ridge that created the lake in the first place. When the water of the stream flows over the dam, it does so without its sediment load and therefore has greater erosive power and can erode the dam more effectively.
The concept of a graded stream is widely used by geomorphologists to describe how a river may adjust its environment to transport its sedimentary load with the least energy required. In this concept the stream gradually (over many years) erodes its bed to attain an equilibrium gradient just right for transporting the sedimentary load when balanced with the types of channels characteristics and velocity available in the area. This graded profile is typically concave up, steeper in the headwaters, and with a low slope near the mouth of the river. Graded streams are thought to be in a state of relative equilibrium; changes in one variable will be accommodated by changes in the other to keep the balance of forces.
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