Through time, a stream alters the landscape of the basin through which it flows. Gradient changes explain the pattern and complex of landforms that develop. The model of these changes is called the LIFE CYCLE of the stream.
We have already discussed how water landing on the surface first cuts little rills and that these rills merge together. The merges increase the amount of water flowing through the rills, so they can cut deeper, and that these rills merge into gullies, and these gullies eventually grow large enough to reach a supply of ground water and become a stream. At first these streams may be intermittent, only contain water during the wet season; but as the processes continue, they may eventually downcut sufficiently or have enough tributaries to become a permanent or perennial stream. Such streams are characterized by a steep V-shaped cross section (profile). They have a V-SHAPED VALLEY. These are the characteristics of YOUTH or a YOUTHFUL STREAM. The gradient will be steep, and the number of tributaries will be limited. The stream erodes downward faster than water flowing over the banks into the stream can erode the sides of the valley. However, as the stream cuts downward, it is lowering the RISE in the RISE OVER RUN equation. This means that the gradient is decreasing. Decreasing the gradient reduces the velocity and, thus, reduces the power of the stream to erode. With time, the erosion of the sides of the valley, the lateral erosion, becomes more dominant than the downcutting. The valley, then, grows wider. Youthful streams have irregularities like water falls and rapids that have not yet had time to be worn away. The time spent in youth is less than one tenth the life of a stream.
By the time the irregularities are worn away, the lateral erosion has become dominant over the downcutting. At this point, the stream is considered MATURE. The stream valley now has a U-shaped cross section or profile. This U-SHAPED VALLEY is not particularly broad, but as the erosion progresses, it grows wider. Tributaries have become integrated and established within the watershed so that water is effectively drained through the system. The gradient continues to decrease until the stream has a graded bed or is just steep enough to transport its load.
In LATE MATURITY, the stream begins to form MEANDERS. One impact of meandering is the widening of the stream valley to a width that is in equilibrium with the size of the stream. As time progresses, these meanders become more complex. The impact of meandering is to lengthen the distance the stream is flowing as it drops its elevation, thereby, lowering the gradient. With meandering, the stream cannot carry as large of a load, so deposition becomes more pronounced.
After reaching the maximum valley width, the stream has reached OLD AGE. In old age, the stream has a very low gradient, and a very slow flow. The mouth or lowest part of the stream is practically at base level, the lowest elevation to which the stream can erode. Old age is noted for its wide, alluvial covered valley floor with many complex meanders. This wide, alluvial covered valley floor is the FLOODPLAIN. The valley cross-section is a very broad deep U-shaped valley, and downcutting and lateral erosion have practically stropped. This means that stream is no longer flowing on the bedrock surface beneath that valley, but rather, it is moving back and forth across alluvial fill that it has been deposited on top of that stream-eroded surface.
The channel changes positions as it develops and transforms its great meander loops. As the bending begins, the flow of the water is forced towards the outside of the curve, eroding the bank and forming an UNDERCUT SLOPE. On the inside of the bend, deposition in the gentler, slower water is depositing materials building a SLIPOFF SLOPE. With time, this forces the bend to move outward and develop a loop shape. The inside of the well-developed loop has a narrow strip of land, the MEANDER NECK, that separates the upstream end of the loop from the downstream end of the loop. As erosion continues on the outside of the bends, this meander neck may become narrowed and then truncated or breached. When the stream cuts through the meander neck, the main force of the water will bypass the loop, because, this is the shortest distance or steepest gradient. Some water will continue to go around the loop, but this will be moving slower and less capable of carrying the stream load. This feature, a CUTOFF, is slowly filled in by dropped alluvium. Since the greatest velocity changes within the channel are at the opening and outlet of the loop, these areas fill in fastest and close off the loop. When the loop becomes isolated, water will be retained in the body of the loop creating a lake called an OXBOW LAKE because of the crescent shape. As runoff flows into this lake and as flooding inundates the floodplain, alluvial fill collects in this low area. Eventually, the lake changes to a shallow, swampy feature called an OXBOW SWAMP. The infilling process continues through time so that eventually no water may be retained. Differences in alluvial compositions and surface variations may still distinguish this feature on the surrounding floodplain, so it remains identifiable as a MEANDER SCAR.
The mature floodplain of an old age stream has other distinctive landform features besides the complex meanders. When the stream floods, water overflows the channel inundating the floodplain. The fastest flow of water is within the channel. As water escapes from the channel, the velocity drops as does the load carrying capacity. This deposit of sediment along side the channel beneath the flood acts to raise the level of the banks of the channel. This ridge of alluvium along side the channel is called a NATURAL LEVEE. The next flood may have to reach a higher level before breaching this deposit and inundating the floodplain. Some natural levees have been built quite high and when the water exceeds this level, catastrophic flooding can occur. The floodplain may also contain YAZOO STREAMS. Consider a tributary flowing into the main stream. During a flood, the tributary no longer will join the stream in the channel, but rather at the side of the flood. Meanwhile, back at the original mouth, the main stream is depositing the alluvium that builds the natural levee. When the flood subsides, the tributary valley may have become blocked. The water of the tributary will be forced along the floodplain until it can find a low or weak place in the natural levee through which it can join the main stream. Flood after flood, this process may force the yazoo stream to flow further along the floodplain before joining the main stream. Another floodplain assocociated landform is a BACKSWAMP, a poorly drained area on the floodplain that retains water.
A stream may progress from youth, to maturity, to old age as it erodes its basin and changes its gradient. However, sometimes natural processes external to the stream system invoke changes to this development. Envision lowering the base level (a drop in sea level or faulting may produce this effect). This change will cause an increase in gradient. Also, envision faulting raising the level of the head of the stream, the highest part of the stream. Again, this will cause an increase in the gradient. If the change is sufficiently significant, the stream will be "returned" to a previous stage in its life cycle. The stream will begin to downcut. TWO landforms indicate that rejuvenation has occurred. ENTRENCHED MEANDERS develop when a meandering stream begins to downcut. The water is flowing along a meandering route when rejuvenation forces the stream to begin eroding downward. This results in the looping channel cutting a V-shaped cross section. TERRACES are remnant floodplains. If a stream has developed a floodplain and then is forced to downcut, it may not erode the entire width of the old valley floor. This flattened, alluvial surface along the stream elevated above the current channel may be found on one or both sides of the stream. A stream may exhibit characteristics of youth in its head or near its source, maturity further downstream, and old age near its mouth. We will discuss issues related to this in the next lesson.
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