GRADATION, FLUVIAL PROCESSES AND
soon as tectonic forces (mountain building) occurs,
gradational processes start to work at leveling.
rarely waits for gradation to finish.
goal of gradation is to achieve base level, a surface so flat that erosional forces can no longer act on it ultimate base
level is sea level.
includes the picking up and removal of loose materials, transport to another
location, and deposition.
Erosion is the wearing away of the land and the transport of
Deposition is the filling in of depressions and the
raising up the land surface.
water exerts a greater influence in shaping the landforms of our planet than
any other gradational process.
both erosion and deposition, running water modifies existing landforms as it
flows over land surfaces or in confined channels.
Streams are bodies of water that flow in well-defined channels.
process associated with the work of streams are known
as fluvial processes.
flow of water on the land surface is called run-off. Run-off is affected by:
short distances, run-off may occur in sheets called sheetwash.
major stream and its tributaries (the smaller streams that feed it) make
up a stream system.
land surface drained by a stream system is called a drainage basin, watershed,
or catchment basement.
river systems consist of smaller drainage basins.
higher land between two tributary valleys of a drainage basin is referred to as
an interfluve that separates two drainage
basins, there is an imaginary line called a drainage divide.
WATER FLOW IN
Perennial streams flow all year.
Intermittent streams only flow during the rain storms or during
streams have a base level below which they cannot erode. The level of a lake or sea level.
slope of a stream bed is called its gradient. In general, steeper gradients
produce greater velocity.
gradients are usually steepest at the headwaters and diminish downstream.
Headward erosion occurs at the
headwaters of a stream as gullies are developed.
erosion continues, an idealized gradient is reached and the stream is described
as a graded stream.
greater the stream flow, the greater the amount of energy available to shape
ability of a stream to pick up and carry materials is largely determined by the
velocity and the degree of stream turbulence.
rough channel bottom increases the intensity of turbulent flow and a small
increase in velocity can result in a significant increase in turbulence.
particles transported by a stream are gradually reduced in size and their shape
changes from angular to rounded. This is called attrition.
materials transported by fluvial processes are called stream load. There
are several ways that streams transport material.
dissolved in the water are carried by solution.
finest solid particles are carried in suspension.
Sediment particles, too large to be carried in
suspension, slide and roll along the bottom by traction.
Saltation is when particles hop
and bounce along the channel bottom.
load of a stream is measured by the weight of the material it is transporting.
Suspended Load carried in suspension
Bed load the particles that roll (traction) and saltation
Dissolved load portion held in solution
relative proportion of each type of load varies with flow rates and the nature
of the drainage basin.
Most work done during flood stages.
Alluvium is the general name
given to fluvial deposits regardless of the type or size of the material.
deposition occurs in locations where velocity is slowed
bends of meanders
flattens (leaving mountain areas)
changes cause a stream to sort particles by size, transporting the sizes it can
and depositing the sizes it cant.
a stream is trying to move a heavy bed load (gravels and sand), it deposits
its load in strands of gravel and sand that interweave, separate and rejoin
which gives the stream a braided look braided stream.
looking at a typical river course from its headwaters to its mouth, you see
changes that occur.
general, erosion tends to be more significant in the upper course and
deposition is more important in the lower course.
gradient is steepest in the upper course and more vertical erosion or
down-cutting is done resulting in steep sided valleys and no flood plain
gives you a V-shaped valley; the more resistant the rock the steeper the
valley walls are.
gradient is reduced, and the stream is approaching its base level vertical
erosion becomes less significant and lateral erosion of the channel sides is
more important and a narrow flood plain develops.
development of the floodplain is from the stream meandering back and forth
across the floodplain. On the outside of the meander loop, the stream erodes
forming a cutbank. On the inside of the
meanders, alluvium deposits forming point bars.
becomes more significant in the lower course of a river. The river is traveling
over an alluvial plain that was deposited by the river.
flat area along a river is called the floodplain.
depositional features include:
in the base level of a stream can occur as a result of either tectonic
processes or climatic changes. This causes streams to start down-cutting.
the uplift occurs gradually after the formation of the meanders, the meanders
uplift occurs in stages, stream terraces can form.
primary factors that influence stream pattern are geologic structure and the
nature of the land surface.
dendritic stream pattern is an irregular
branching pattern with tributaries joining large streams at acute angles (less
than 90°), most common
Dendritic patterns form where the underlying
geologic structure does not strongly control the position of the stream
trellis pattern forms long parallel streams, linked by short
right-angled tributaries. Found in areas where the rocks have been folded.
radial pattern develops where streams flow away from a common high point
cone (volcano) or dome-shaped geologic structure.
centripetal pattern is where streams converge on a central area, in a
basin interior drainage.
Rectangular patterns occur where streams follow sets of
fractures to produce a blocky network of straight channels with right angels.
stream patterns take tens of thousands of years to become well established. In
areas recently glaciated (10,000 years ago) streams flow on low gradients
connecting marshes and lakes in a chaotic pattern called deranged drainage.
streams avoid geologic structures in their way but in some cases streams have
apparently cut straight through mountains. These are called transverse
Antecedent streams are ones that existed before the
formation of the mountains. They flow through/over cutting down as the uplift
River - Royal Gorge CO
Superimposed streams are ones that originated on buried
structure. With erosion, these streams have cut through the buried structure,
thus creating water gaps.