CHAPTER 17

GRADATION, FLUVIAL PROCESSES AND LANDFORMS

 

INTRODUCTION

As soon as tectonic forces (mountain building) occurs, gradational processes start to work at leveling.

Tectonics rarely waits for gradation to finish.

The 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.

 

THE GRADATIONAL PROCESSES

Gradation 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 material.

 

Deposition is the filling in of depressions and the raising up the land surface.

 

INTRODUCTION

Running water exerts a greater influence in shaping the landforms of our planet than any other gradational process.

Through both erosion and deposition, running water modifies existing landforms as it flows over land surfaces or in confined channels.

•      Erosion – Grand Canyon

•      Deposition – Mississippi River

 

THE STREAM SYSTEM

Streams are bodies of water that flow in well-defined channels.

•      Different terms

 

The process associated with the work of streams are known as fluvial processes.

 

RUN-OFF

The flow of water on the land surface is called run-off.  Run-off is affected by:

•      Vegetation (agriculture)

•      Slope                                                           

•      Deforestation

•      Urbanization                                    

 

For short distances, run-off may occur in sheets called sheetwash.

 

STREAM SYSTEM

Each major stream and its tributaries (the smaller streams that feed it) make up a stream system.

 

The land surface drained by a stream system is called a drainage basin, watershed, or catchment basement.

 

Large river systems consist of smaller drainage basins.

 

STREAM SYSTEM

The higher land between two tributary valleys of a drainage basin is referred to as an interfluve.

 

On an interfluve that separates two drainage basins, there is an imaginary line called a drainage divide.

•      Continental Divide

 

WATER FLOW IN STREAMS

Perennial streams flow all year.

 

Intermittent streams only flow during the rain storms or during certain seasons.

 

BASE LEVEL

All streams have a base level below which they cannot erode. The level of a lake or sea level.

 

The slope of a stream bed is called its gradient. In general, steeper gradients produce greater velocity.

 

Stream gradients are usually steepest at the headwaters and diminish downstream.

 

GRADED STREAM

Headward erosion occurs at the headwaters of a stream as gullies are developed.

 

As erosion continues, an idealized gradient is reached and the stream is described as a graded stream.

 

EROSION BY STREAMS

The greater the stream flow, the greater the amount of energy available to shape the land.

 The ability of a stream to pick up and carry materials is largely determined by the velocity and the degree of stream turbulence.

A rough channel bottom increases the intensity of turbulent flow and a small increase in velocity can result in a significant increase in turbulence.

The particles transported by a stream are gradually reduced in size and their shape changes from angular to rounded. This is called attrition.

 

STREAM LOAD

The materials transported by fluvial processes are called stream load. There are several ways that streams transport material.

•      Minerals dissolved in the water are carried by solution.

•      The 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.

 

 

 

 

 

 

STREAM LOAD

The load of a stream is measured by the weight of the material it is transporting.

This includes:

•       Suspended Load – carried in suspension

•      Bed load – the particles that roll (traction) and saltation

•      Dissolved load – portion held in solution 

The relative proportion of each type of load varies with flow rates and the nature of the drainage basin.

Most work done during flood stages.

 

STREAM DEPOSITION

Alluvium is the general name given to fluvial deposits regardless of the type or size of the material.

Stream deposition occurs in locations where velocity is slowed

•      Inside bends of meanders

•      Flood plains

•      River mouths – deltas

•      Gradient flattens (leaving mountain areas)

 

BRAIDED STREAM

Velocity changes cause a stream to sort particles by size, transporting the sizes it can and depositing the sizes it can’t.

 

When a stream is trying to move a heavy bed load (gravels and sand), it deposits it’s load in strands of gravel and sand that interweave, separate and rejoin which gives the stream a braided look – braided stream.

 

LAND SCULPTURE BY STREAMS

When looking at a typical river course from its headwaters to its mouth, you see changes that occur.

 

In general, erosion tends to be more significant in the upper course and deposition is more important in the lower course.

 

UPPER COURSE

The 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

 

This gives you a V-shaped valley; the more resistant the rock the steeper the valley walls are.

 

 

MIDDLE COURSE

As 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.

The 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.

 

 

LOWER COURSE

Deposition becomes more significant in the lower course of a river. The river is traveling over an alluvial plain that was deposited by the river.

 

The flat area along a river is called the floodplain.

 

The depositional features include:

•      Oxbow lakes

•      Yazoo streams

•      Natural levees

•      Meander scars

•      Deltas

 

REJUVINATION

Changes 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.

 

If the uplift occurs gradually after the formation of the meanders, the meanders become entrenched.

 

When uplift occurs in stages, stream terraces can form.

 

STREAM PATTERNS

The primary factors that influence stream pattern are geologic structure and the nature of the land surface.

 A dendritic stream pattern is an irregular branching pattern with tributaries joining large streams at acute angles (less than 90°), most common – tree-like.

Dendritic patterns form where the underlying geologic structure does not strongly control the position of the stream channel.

 

STREAM PATTERNS

A trellis pattern forms long parallel streams, linked by short right-angled tributaries. Found in areas where the rocks have been folded.

 

A radial pattern develops where streams flow away from a common high point – cone (volcano) or dome-shaped geologic structure.

 

STREAM PATTERNS

A 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

Most 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.

 

 

TRANSVERSE STREAMS

Most streams avoid geologic structures in their way but in some cases streams have apparently cut straight through mountains. These are called transverse streams.

 

Antecedent streams are ones that existed before the formation of the mountains. They flow through/over cutting down as the uplift occurs.

•       Arkansas River - Royal Gorge – CO

 

SUPERIMPOSED STREAMS

Superimposed streams are ones that originated on buried structure. With erosion, these streams have cut through the buried structure, thus creating water gaps.

 

•      Susquehanna River