2 - Fluvial Erosion
The Yadkin Valley near North Wilkesboro, Wilkes County
Introduction
|
River action - derived from the abundant rainfall which runs off over the
surface to the oceans - is the main process creating the landforms of North
Carolina. The focus here is on the processes by which this running water
creates the landforms. The companion section, River Landscapes,
considers the results. The present section is in 3 parts:
2 - Fluvial Erosion |
The Yadkin Valley near North Wilkesboro, Wilkes County |
Soil particles and broken down rock fragments (see the section "Weathering and mass wasting") fall, roll or are carried by rain-water run-off downslope into a stream. The slower downward percolation of sub-surface water dissolves portions of the rock through which is passes and thus also brings material into the stream. The running water in this stream transports the stream load seaward. The load itself is of three types:
Types of load:
1 - dissolved - this load consists of the chemical elements dissolved mainly by subsurface flow. It has little impact on landforms, so is not considered here. It is, of course, very important in many other contexts. IN many places human activity has introduced many kinds of contaminants. These need removal, often in municipal or industrial water treatment plants, before stream water can be used by humans. 2 - suspended - this load comprises small particles (sediment) suspended in the river current. These particles are picked up from and deposited back to the stream bed as stream velocity and turbulence changes. 3 - bed - the bed load consists of larger particles, up to boulder size, which are either rolled along the bed (traction) or moved using a kind of hopping motion (saltation). These are not necessarily continuous movements, and again depend on stream velocity and turbulence. |
The Tar River at Louisburg, Franklin County, carrying a large suspended load. |
Variability of load amount
The amount of load transported by a given stream at a given time depends on the 'force' of the river - the flow volume, turbulence, and speed. The amount is influenced by:
Stream gradient (the effect of gravity) - A steep slope induces more turbulence and more transport. Similarly, for a given size of stream, a steep slope gives faster flow and thus a more rapid movement of load. However, size of stream is important.
Stream size - for a given speed of flow, a bigger stream is obviously capable of transporting more material, but speed depends not only on the gradient, but also on the effect of the stream cross-section.
Stream cross-section (the effect of friction) - A large stream has a smaller proportion of its volume slowed by the frictional drag of the stream bed and walls. The center of a stream is always flowing faster than the water near the banks (still pools rarely form in the middle of a river!), so that the bigger that "center" area, the faster the overall average speed of the stream will be.
Combining these three effects, the overall result is that the load varies along the river course (steepness & size change downstream) at any given time, and the load varies with time (flood and non-flood conditions) at any given spot.
Rapids on the Cullasaja River between Franklin and Highlands, Macon County. |
As the load carrying ability increases (e.g. along a steep reach) degradation - the wearing away of the river channel - occurs.
Alternately, as the load carrying ability decreases (e.g. along a more gentle downstream reach) aggradation - the deposition of load into the river channel - occurs. |
Calm reach of the Cullasaja River between Franklin and Highlands, Macon County.
|
 |
The load carrying ability changes with time. During
the rapid, turbulent flow of a flood much more material, including large
boulders, can be carried downstream. As the flood recedes, part of
the suspended load is deposited on the bed, while most of the bed load,
especially the largest objects, stop moving.
This river is the Eno in Durham County |
| The load also depends on the nature of the source
material. Some rocks weather easily and produce a great amount of
material. Streams flowing over another rock type, however, may be
carrying a much smaller load because the rock does not weather as easily.
To the right is shown a portion of the Sandhills region of the state (this being a stream in Weymouth Woods State Park, near Southern Pines, Moore County. The sandy soil yields rather little sediment in this area. |
|
2 - Fluvial Erosion
A stream not only transports load (as described above) as it moves, but it also erodes -wears away - its channel. Streams erode as the result of:
air/water pressure - any air bubbles carried into cracks, even minute ones, on the stream bed may become trapped there. Then, when turbulence squashes the bubble and pressure in the crack increases, the crack may be forced open slightly. Repeat this millions of times over thousands of years, and the result is that the rock bed gets fragmented, creating new material to be carried away as part of the bed load;
interaction between load and bed - the traction and saltation action bumps the bed load against the stream bed. The load may act as a chisel and chip away part of the bed. Alternately, the bed may cause a fragment to be chipped from the boulder or stone being moved along.
interaction between load particles - the load particles, particularly those of the suspended load will grate together, thus slowly reducing the particle size.
(Note that although the term 'erosion' implies simply a wearing away or down, as far as running water is concerned, it is impossible to separate the erosive and the transportation activities, so that the one term 'erosion' is usually used to cover both aspects)
The result of these activities:
(1) The particle size distribution decreases downstream, accounting for the observed fact that there are few big boulders occurring naturally in Coastal Plain rivers.
The partly canalized Oconaluftee River in Cherokee, Swain County |
(2) The ratio of bed load to suspended load decreases downstream. Thus most mountain streams are rather clear, while the Coastal Plain ones are full of suspended load, and appear brown. |
 |
[N.B. it appears that the transport of topsoil into the Piedmont and Coastal Plain rivers from nearby agricultural land may be vital in creating much of the suspended load, and thus the brown rivers. There is some evidence that, before extensive agricultural activity, the Coastal Plain rivers were rather clear, although some suspended load must have been present.]
Some coastal streams have organic matter as load, which tends to be rather dark in color. These are the 'black water' rivers.
By way of contrast, streams in areas where upstream glaciers, rather than rivers, provide the suspended load, often have a white color derived from the ground-up rock with little soil or organic matter intermixed.
In our area, however, the result is that streams erode their beds and banks, which leads to the formation of valleys.
Stream erosion leads first to 'down cutting' which creates valleys, and for 'lateral cutting', which influences the shape and size of these valleys.
Down cutting:
| Any minor irregularity on a sloping surface is likely to
cause the overland flow of water to concentrate at particular locations,
and flow as a stream rather than as 'sheet flow'. As soon as this
happen, the fluvial erosion begins. On the smallest scale - the
headwaters of streams or small tributaries - rills and gullies are formed.
(The gully on the right is on the bank of the Tar River in Louisburg, Franklin County) |
|
 |
Downward erosion continues and the little gully gets deeper and deeper and soon there is something we could regard as a true (if very small) valley. Over geological time this continues to get deeper. In most cases the river carving the valley will have reaches where degradation is dominant and places were aggradation dominates. The resultant erosion and deposition leads to the slow development of a fairly smooth long profile - the graph of the altitude of the stream above sea level plotted against the distance downstream. Indeed, if we consider the dynamics of water flow and material movement, we can say that the river is working towards a graded longitudinal profile, which is the smooth profile which creates just the right velocity/size conditions to transport the load (the result of weathering and mass wasting) downstream. A true permanent graded profile can never be achieved because environmental conditions - notable floods and droughts - are themselves constantly changing. The most that can be achieved is a dynamic equilibrium, where constant adjustments maintain a channel which, for most of the time, appears to change very little. |
There is a limit to how much downcutting can take place. This is ultimately controlled by the nature of the forces at work dictating the energy (especially the speed, volume and turbulence) of water and material motions downstream. In practical - and perhaps rather obvious - terms, however, it means that the stream cannot cut down below sea level. Sea level thus becomes the ultimate base level for erosion. There may be other local base levels - where a stream flows into a lake (whether a natural one or an artificial reservoir), where a band of resistant rock prevents downcutting, so that the graded profile starts to develop backwards from that point
Lateral cutting:
|
A meander on a small creek north of Winston-Salem, Forsyth County. The undercut bank (where the erosion is taking place) is on the left, with the slip-off slope (and deposition) on the right |
Lateral - sideways - cutting starts as soon as there is a valley in which erosion can take place. However, it becomes especially important once a river begins to approach grade. Any irregularity in the channel (a big boulder in the way may be all that is needed) forces the water to bump into a bank, and erosion starts. The water is 'reflected' off the bank after the collision and soon collides with the opposite bank. The result is the development of a sinuous course with a series of meanders. |
A bend - meander - on the downstream portion of a large river. The Neuse River from Cliffs of the Neuse State Park, Wayne County |
|
The flat valley floor and distant valley wall of a mountain stream. The is the Valley River in Cherokee County. Note that the floor provides good agricultural land - but it is also the flood plain, and so liable to flood from time to time. |
The erosive action on the undercut bank of a meander is usually directed slightly downstream, meaning that the meanders themselves migrate downstream. Eventually they have migrated enough to produce a flat valley floor which is called the flood plain. |
This is also a valley floor and valley wall, but on the Coastal Plain, the Neuse valley just south of Goldsboro, Wayne County. The area as far as the line of trees to the left of the road was covered with water during the 1999 floods associated with Hurricanes Dennis and Floyd. |
