16.1 Drainage basin
systems
1. drainage basin
A drainage basin is the spatial geomorphic unit occupied by a river system. A drainage basin is defined by ridges that form drainage divides; so drainage divides are the boundary of a drainage basin. the catchment area of the drainage basin is called as a watershed. (Figure 1).
The
A major drainage basin system , such as the one created by the Mississippi-Missouri-Ohio river system, is made up of many smaller drainage basins, which in turn comprise even smaller basins.
2. Stream Orders
to quantitatively describe the hierarchies of the stream size and relationship in a given drainage basin, we use stream orders, now let’s talk about stream orders.
In 1945, the hydrologist Robert Horton introduced this quantitative methodology, which was modified by the geomorphologist Arthur Strahler in the 1950s, and subsequently by others.
Think of a nesting of streams, from smallest to largest, as they join to form a main stream. we can divide them into different orders. A first-order stream has no tributaries and therefore is without smaller branches; it is the smallest stream in the system and has the smallest drainage basin. A second-order stream receives at least two first-order tributaries. A third-order stream is formed by the joining of two second-order stream; a fouth-order stream is formed by the joining of two third-order streams; and so forth. However, note that the addition of a first-order stream to a second-order stream does not result in a third -order stream. Now, can you tell me what order this stream is ? and how about this ?
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3.Drainage Density
To measure the overall efficiency of a drainage basin, we use drainage density
Drainage density = the length of all the stream channels/ the area of their drainage basin. The unit of drainage density is km per square km or mile per square mile.
Now, let’s compare diagram (a) with (b), can you tell me which one has a greater drainage density. Yes, diagram (a), Why, area shown in diagram (a) has clayey rock, so surface water is difficult to infiltrate to the underground, therefore, more water is running on the surface and more rivers; while the area shown in diagram (b) has bedrock type of limestone, as you may know, in limestone area, there are a number of sinkholes and caves, they drain a lot of surface water to underground. as a result, less surface water running on the surface and has fewer rivers.
In addition, the factors affecting drainage densities include relief, vegetation and climate.
4.Drainage patterns.
Not only the drainage density may be varied from region to region but also the overall arrangement patterns of those streams may be varied from region to region due to the difference in climate, rock type, structural controls. we call it drainage patterns. Now let’s talk about drainage patterns, There are seven most common drainage patterns in the natural world.
(a) Dendritc: The most familiar pattern is dendritic. Is is often associated with the area of uniformly diping strata. This pattern looks like a tree. Energy expended by this drainage system is efficient because the overall length of the branches is minimized.
(b)Parallel The second pattern is Parallel. The streams more or less parallel with each other. Parallel drainage is associated with steep slopes and some relief.
©Trellis The third one is trellis drainage pattern. The trellis drainage pattern is characteristic of dipping or folded topography. it can be developed in the region with parallel mountains. and valleys. The principle streams are directed by the parallel folded structures, whereas smaller streams are at work on nearby slopes, joining the main stream at right angles.
(d)Rectangular The fourth one is called rectangular pattern, it is developed by a faulted and jointed landscape, directing stream courses in patterns of right-angle turns.
(e)Radial The fifth is called radial drainage pattern. It results from streams flowing off a central peak or dome, such as occurs on a volcanic mountain.
(f)Annular The
sixth pattern is annular pattern, it is produced by structural domes,
with concentric patterns of rock strata guiding stream courses.
(g)Deranged The
seventh, the last pattern is deranged pattern, with no clear geometry in the
drainage and no true stream valley pattern, This pattern can be seen in the
glaciated shield regions of
16.2 Streamflow
characteristics
1. Flow velocity Look at a stream in cross-section, if we measured streamflow velocity at different depth across the channel, can you tell me where the greatest flow velocity might be ? Is it near the bottom of channel or close to the river bank or near the surface at the center ? The greatest velocities in a stream are near the surface at the center, corresponding with the deepest part of the stream channel. a similar pattern would occur laterally. Velocities decrease closer to the sides and bottom of the channel. (fig.xx).
The reason for this is that the material in the channel and banks, together with the sediment being dragged or washed along the channel, exerts a frictional drag on the water which reduces its velocity.
2. Flow discharge The volume of flow in a river passing a given cross section of a river in a given unit of time is called flow discharge.
Discharge is calculated by multiplying the velocity of stream by its width and depth for a specific cross section of the channel:
Q = wdv
where Q = discharge; w = channel width; d = channel depth; and v = stream velocity. Discharge is expresses either in cubic meters per second (m^3/s) or cubic feet per second (cfs). From this equation, we can see that as Q increases, some combination of channel width, depth, and stream velocity increases.
The discharge of the stream varies over both time and space. Generally it increases downstream as more water enters the channel from tributaries.
It is commonly believed that stream velocity declines downstream, the water becoming slower and more sluggish., but this is rarely true. It is a misconception that arises because the streams tend to be less turbulent and more muddy in their lower reaches. In most cases, velocity increases slightly downstream; depth and width increase more markedly (fig.xx).
16.3 Stream channel
processes
Stream channel processes may include stream erosion, stream
transport and stream deposition.
1. Stream erosion
The erosional work of a stream carves and shapes the landscapes through which it flows
Streamflow dissolves, dislodge, and move materials from the streambed and stream banks.
(1)Hydrolic action hydraulic action is the work of turbulence in the water- the eddies of motion.
(2)Abrasion: A hydraulic squeeze-and-release action works to loosen and lift rocks. As this debris moves along, it mechanically erodes the streambed further, through a process of abrasion, with rock particles grinding and carving the streambed.
2.Stream transport
You may have watched a river or creek after a heavy rainfall, the water colored brown by the heavy sediment load being transported. The maximum possible load that a stream can transport is its capacity. The materials transported by a stream can be divided into dissolved load, suspended load and bed load.
(1)dissolved load consists of
those materials derived from minerals such as limestone through chemical
weathering.
(2)The suspended load consists of fined-grained, clastic particles physically aloft in the stream. Turbulence in the water, with random upward motions, is an important mechanical factor in holding a load of sediment in suspension.
(3)The bed load refers to those coarser
materials that are dragged along the bed of the stream by traction or are
rolled and bounced along by saltation.
The maximum possible load riverflow can transport is called streamflow
transport capacity.
When the flow velocities decreases, the streamflows, therefore, do not have enough energy to carry all the transported sediments. As a result, part of the carried sediments in the stream began to deposit in the river bed, this is called river or stream deposition.
16.4 STREAM
LANDSCAPES
1. Nickpoint In the river channel, when the stream gradient
experiences an abrupt change over very short distance. the point of
interruption is called Nickpoint. the
typiccal example for nickpoint development is waterfall. like Niagara Fall. The
hard, resistant rock strata underlain by less resistent rocks. As the less resistant material continues to
wheather away, the overlying rock strata collapse, allowing the falls to erode
farther upstream. Like
2.Meandering channel
and Braided channel
looked at from the air or on a map, meandering stream channels can be seen to be highly sinuous, like a snake
many channels are not only sinuous but also braided. Braiding refers to the separation of the main channel into a number of smaller, interlocking channels.
3.Floodplains
The low-lying area near a stream channel that is subjected to recurrent flooding is a floodplain. It is formed when the river leaves its channel during times of high flow. Thus, when the river channel changes course or when floods occurs, the floodplain is inundated.. When the water recedes, alluvial deposits generally mask the underlying rock.
4.Alluvial Terraces When stream cut downward with increased erosion, the floodplain itself becomes the topographic steps above the river, this is called alluvial terraces, new floodplain may develop at the low-lying area near the river channel.
5. River deltas. At the mouth of a river where river join the ocean, because streamflow meets a large body of standing water, the flow velocities suddenly slow down. the reduced velocity causing its transported load to be in excess of its capacity. Coarse sediments drop out first, with finer clays carried to the extreme end of the deposit. This depositional plain formed at the mouth of a river is called a delta, named after the Greek letter delta: D because of the triangular shape.
Each flood stage deposits a new layer of alluvium over the
surface of the delta so that it grows outward.
There are several types of delta.
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