I. INTRODUCTION

Stream is a term commonly used for any flowing body of water and rivers are the major channels in the system.

Streams are important for transportation of rocks and minerals material- both as solid particles and in dissolved form.

Streams are major geomorphic forces- most of the landscape is caused by surface runoff- (in the case of Florida, it also has to do with groundwater flow.

Can erode high areas and deposit sediment in low areas.

Streams cone in many forms and shapes, generally controlled by the physics of flow through the channel

II. STREAM FLOW AND SEDIMENT MOVEMENT

A. Types of flow:

There are two types of flow in a stream channel (or any type of channel or pipe)- laminar and turbulent:

***** Look at Fig. 13.1 for diagram of laminar versus turbulent flow

Laminar flow- stream lines are parallel to each other

Turbulent flow- stream lines cross each other

Controls on these types of flow include: (1) velocity of flow (2) depth of flow, and (3) fluid viscosity (its resistance to flow, just like magmas).

Turbulent flow occurs with rapid flow, deep flow, or flow of low viscosity fluid.

Most water at surface temperatures has low viscosity so most flow is turbulent.

B. Sediment movement

(1) Some terms:

Two ways that rivers may carry solid material- suspended load and bed load

Suspended load- the material that is permenantly carried in suspension- a physical mixture of particles and the water in the stream.

Bed load- the material that is carried along the bottom of the stream by sliding and rolling.

***** Look at Fig. 13.2 for diagram of suspended and bed load

Two ways to characterize the sediment load- competence and capacity

Competence- The maximum size of particle (is it a boulder or clay size particle??) that a stream can carry

Capacity- The total load of all sediment

This is a very important difference- "thought" examples: a small mountain stream may have a large competence, but only a small capacity, while a large river (e.g. Mississippi) may have huge capacity, but only small competence

(2) Controls on sediment movement:

Different grains settle at different rates- a characteristic called settling velocity

The major control of settling velocity is the size of the particle.

Other factors that control settling velocity are (a) the density of the particle and (b) viscosity of fluid. Because (a) and (b) are more or less the same in streams, grain size is the most important factor for settling in streams.

What this means is that small particles settle much more slowly than large particles

Another important factor for how much sediment a stream carries is the velocity of the water, because it adds bouyant forces to the particles.

Some large particles will be in suspension intermittendly- that is they will bounce off the bottom of the bed and then slowly settle out of the stream. A process called saltation

**** Fig. 13.3 shows a diagram of saltating particles.

Summary- Possible to make a figure of how changing flow velocity and particle size will be important for transportation of particles in a stream.

***** Hjulstrom's diagram- Fig. 13.4- very important description of exactly what current speed and grain size will result in particles being eroded from the stream bed and what particles will settle out and be deposited.

C. Bedforms: Ripples and Dunes

The bottom of streams often have small "hills" or mounds that result from the motion of grains of sand downstream- These mounds are called ripples or dunes, depending on their size.

Ripples- small low narrow ridges, separated by wide troughs, may be only a few cm tall. Very common features in sand dunes. They are asymmetric

Dunes- Larger features, they typically have small ripples climbing up their backs, which is how much of the sediment moves.

Ripples and dunes form by having the grains pushed up the less steep side of the dune by the current. When the grain reaches the top it falls back to the bottom by gravity.

This pattern of forming dunes and ripples is ultimately what causes the sedimentary structures we have already talked about- cross bedding.

*** See fig. 13.5 for a diagram of dune formation.

III. THE RIVER AS A SYSTEM- VALLEYS, CAHNNELS, AND FLOODPLAINS

While channels erode the land surface, they form valleys. The valleys contain channels, the location where the water normally flows, and floodplains- flat areas that are about the elevation as the top of the channel where the river floods when the river spills out of its channel.

A. Channel Patterns- Map (or "birds-eye") view

(1) Meanders- These are more or less regular bends in streams.

They are fundamental aspect of streams. and are characteristic of all channel patterns, including the flows like the jet stream and gulf stream.

The channel itself contains meanders: the line of maximum velolcity (also the maximum depth of the channel) meanders from either side of the channel. This maximum velocity is called the Thalweg.

Deposition on the shallow side forms a feature called a Point bar- a curved sandbar that is on the inside bend of the meander.

Meanders continue to grow as deposition and erosion continue. Eventually, the meander loops become large enough that they are unstable. They will be cut off, usually during a flood, and create an Oxbow lake.

A special type of meander is one that has cut down into bedrock so that it becomes incised into the bedrock. Oddly enough, they are called "Incised Meanders"

**** Look at Fig. 13.11 for diagram of origin and evolution of meanders, culminating with the formation of an oxbow lake.

(2) Braided streams-

These are steams that have many channels instead of only one channel.

Occur where the sediment load exceeds the capacity of the stream. Generally in places with large amount of sediment and rapidly changing stream flows.

B. The Floodplain

As rivers meander across valley floors they leave behind deposits that are called the flood plain- essentially, the land area that is inundated during floods.

The sediments deposited include remnants of point bar material (coarse grained), oxbow lake sediments (fine grained), and flood deposits (fine grained).

However, when rivers flood, they tend to drop most of the sediment at the edge of the channel. The deposition tends to create small hills along the channels. These small hills are called levees.

*** Look at Fig. 13.13 for diagram of how levees form

IV. CHANGES IN STREAMS WITH TIME AND SPACE

A. Discharge

As you all know from the lab exercise, discharge is the amount of water flowing through a stream channel at any given time.

It is determined by multiplying the channel width, depth of the water and the velocity of the water. Obiously the dimensions have to be volume per time (e.g. cubic feet or cubic meters per second).

In most streams, discharge decreases downstream. This is not always the case. Where there are influent streams, discharge decreases because it flows back into the ground water.

An example of such a stream is the Suwannee river.

B. Floods

An extreme and short term increase in discharge of a river is a flood. With high discharge, the rivers spill over their banks.

There can be a range of size of floods and a range of the frequency which they occur for any particular river.

Geologists use historical and geological information to determine the number of times that a stream has flooded in the past. This provides a probability that a flood will occur within any particular year.

example- If a river has flooded 5 times in a 100 year history, then there is a 5 out of 100, or 5%, chance that it will flood in a given year.

This probability is called a recurrence interval.

This type of probability can be broken down further into probabilities for floods of a given size.

Note- these recurrence intervals are only probabilities, they do not predicit that a flood will occur within that interval. In fact, if a flood occurs in any one year, the probability that the same flood will occur the following year is exactly the same as the preceding year.

C. Profile:

The slope of streams change systematically from the headwaters to their mouths. In all cases there is a Longitudinal profile that is concave upward. The streams are steep near their headwaters, and a more shallow near their mouths.

***** Look at Fig. 13.16 to see a diagram of a concave upward longitudinal profile

The reason for this profile is that water always runs downhill, which means that the headwaters are always at a higher elevation than the mouth.

The higher the elevation, there greater the amount of erosion than at the downstream sections. The sediment that is eroded at the upper reaches is deposited at the lower reaches, thereby contributing to the flat portion of the profile.

The lower portion of the stream is controlled by the base level.

Base level is the lowest part of the stream- where it enters a standing body of water (either the ocean or lake).

Base level can be changed artificially- that is by damming a stream. This has important effects on the longitudinal profile of the stream- It divides the stream into two sections, the upper one is "too steep" and the lower one is "too flat". The result is that the stream deposits sediment in the upper part and erodes sediment in the lower part.

***** Look at Fig. 13.18 to see how building a dam changes the profile of a stream by altering base level.

Base level can also be changed by natural processes, especially by changing sealevel by glaciation and tectonics. A good example of the effects of changing sea level is the formation of the Chesapeake Bay in Virginia, which is the old river channel of the Susquehanna River.

When a stream profile is in equilibrium- that is neither eroding or depositing sediment- it is called a graded stream

V DRAINAGE NETWORKS

A. Drainage Divides and Basins

An imaginary line that separates the direction that streams flow is called a divide- the physiographic feature that they follow are called ridges.

The biggest example in North America is called the continental divide the line that separates water flowing to the Pacific ocean and the Atlantic ocean.

This is not the only divide, every stream can be subdivided into a drainage basin that is surrounded by divides

Drainage basin- The system of rivers that are surrounded by a continuous divide. these may be any size, and there are smaller drainage basins that are totally within larger basins.

The divides are not permanent features. They are continually being eroded headward, and will eventually breach. This process is called stream capture, or stream piracy

B. Drainage patterns-

The angle that tributaries run into streams can divide stream basins into various types of drainage networks

The shape of the drainage networks is usually controlled by the surface topography or fracture systems in the rocks.

**** Look at Fig. 13.23 for example of the various types of drainage systems

Note that these drainage patterns are very big features- it would be dificult to see them from the ground. May be possible to see them from several miles elevation.

(1) Dendritic drainage- look like tree branches "dendron" and occurs when the underlying rock is homogeneous.

(2) Rectangular pattern- occurs when there is an underlying pattern of fractures in the rock.

(3) Trellis pattern- Occurs when there are long ridges (such as Appalachian mountains) that form from erosion.

(4) Radial drainage- forms when several streams drain from a central high point such as a volcano.

***** Look at Fig. 13.23 to see the differences in the different types of drainage patters, and particularly what controls the patterns.

There are two special types of streams that reflect geologic phenomenon:

(1) Antecedent streams- These are streams that continue to erode through mountains as they are being uplifted. In this case, there is no relationship between the underlying geology and the stream pattern.

(2) Superposed stream- these streams develop their drainage network in one layer of strata, and then erode down into another layer, but retain the pattern of the previous layer.

******* Look at Fig. 13.24 and 13.25 to see the differences between antecedent and superposed streams and what controls them.

VI. DELTAS

Rivers always reach a local base level, such as a lake, or the ultimate base level, which is sealevel.

When this happens, their velocity goes to zero, so the competence also goes to zero- if the water doesn't move, then it doesn't carry sediment, and it is deposited.

Special sedimentary deposits are formed at the local base level- these deposits are called Deltas. Because rivers carry such a large amount of material, deltas are major sedimentary deposits.

Note that the work delta comes from the greek description of the shape of the Nile River delta, which looked like the greek letter delta.

The shape of a delta is controlled by branches in the stream channel similar to its tributaries, but now the branches flow away from the main channel. These branches are called distributaries- they essential distribute water (and sediment) away from the main channel.

Not all deltas have this shape however, the shape of a delta is controled by three main physical factors. And the shape of deltas is how they are classificed.

The three important factors are:

(1) how big is the stream,

(2) how large is the tidal range where the delta forms, and

(3) how big are the waves where the delta forms?

Based on these factors, deltas can be classified based on which factor is most important:

(1) River dominated (e.g. Mississippi) form "bird's foot deltas"- they have many distributary channels, they extend far out into the body of water.

(2) Wave dominated- The coast is straighter because longshore drift carries the sediment parrallel to the coast, ie redistributes the sediment.

(3) Tide dominated- The body of water can cause the river to reverse its flow, the sediment will be trapped in tidal flats all along the river channel. This also produces a relatively straight margin.