## Streams and sediment transportation

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Including...

Introduction

Streams and energy

Floods and floodplains

The 3 stages of stream development

### Introduction

REVIEW: Hydro cycle

Weathering

Erosion, and now...

Transportation

What is a river? A stream? A creek?

Is a brook bigger or smaller than a rill?

And if so, by how much?

Are there exact values for each, and if so, what are they based on?

Discharge? DEFINE

Channel size? DEFINE

Drainage basin size DEFINE

How about daily and seasonal variations?

All in all, a seemingly nebulous classification scheme!

We'll use the term "stream" for all bodies of moving water which are confined in a definable channel

Which way does a stream flow? (downhill)

How far does a stream flow? (until it reaches the bottom)

DEFINE: Base level: Temporary vs. Permanent

### Streams and energy

Need to get the mechanical & chemical sediments to the ocean

What is the transportation system?

In most cases moving water

The purpose of a stream is to transport the weathered and eroded materials to the beach

Energy is the determining factor

Need to get the sediments moving

There are 3 interconnected formulas dealing with how this is accomplished

Formula #1: Need to impart an "acceleration" to the sediment

Newton defined this in the 1600's in his 2nd Law of Motion

a = F / m (acceleration = force / mass of the sediment)

DIGRESS: Fractions (numerator vs. denominator)

In this case, mass of the sediment is critical

Greater mass requires greater force

Where do we get the force?

Several sources

Water, wind, gravity, geologists

Formula #2: Moving water has energy

Kinetic energy: Ek = 1/2 mv2

Where "m" is the mass of the water, and "v" is its velocity

DIGRESS: velocity is a vector

But since water usually moves in the same direction we can concentrate on the speed

What happens if we double the mass (discharge) of the stream?

What happens if we double the velocity of the stream?

Velocity is the clearly most important variable

So if we are trying to maximize sediment transport, it is important to get the water moving as fast as possible

Formula #3: What affects the velocity of a stream?

Gradient: how steep the stream bed is

Discharge: how much water is in the stream

Flood stage - lots of water, usually moving fast

Channel configuration: wide vs. narrow; shallow vs. deep

V = Q / A (velocity = discharge / cross-sectional area)

If we assume that discharge in a constant locally

Then any change in channel morphology has to affect the stream's velocity

Lots of examples of this

Riffles

River delta

Alluvial fan

Meander vs. point bar

Several types of load are possible

Material moving downstream while dissolved in the water

Generally produced by chemical weathering processes

Cannot be seen, and will not settle out of the water

Can usually be separated from the water by evaporation of the water

Resulting in the precipitation of the dissolved material

These are actual clasts of weathered rock

Generally produced by mechanical weathering processes

But chemically weathered clay also falls into this category

Can be seen, and will settle out of the water

There are 2 varieties

Generally based on the size of the clast relative to the velocity of the stream

"Larger" material moving downstream without losing contact with the river bed

"Smaller" material moving downstream while suspended in the water

### Sediment load: an evolving balance

There are dynamic terms

Change as the velocity changes

Many level to this

Local variations in channel shape and size

Riffles, meanders, holes, etc.

Normal seasonal variations in discharge

Wet season vs. dry season

Not always winter vs. summer!

Longer term variations in discharge

Bed load can become suspended in times of flood

### Floods and floodplains

Floods occur when the amount of discharge exceeds the size of the "normal" channel

Given names based on frequency

50 year flood, 100 year flood, 500 year flood, and so on

Floodplains: Sediment-filled areas along the borders of most streams

Basically serve two (2) purposed:

Store excess water in times of high discharge

Store excess sediment in times of low discharge

Unfortunately, they are also convenient areas for settlement and agriculture

Humans have historically built on floodplains

And regretted it when the water rises

Only to rebuild as soon as the floodwaters recede!

Remember: the floodplain is part of the stream

### The 3 stages of stream development

Youthful : generally in the mountains

V-shaped valleys with steep canyon walls

Higher velocity

Relatively lower discharge

Small to non-existent floodplains

Mature

Rounded hills and valleys

Moderate velocity

Moderate discharge

Small to locally well developed floodplains

Old age: generally found near the mouths of river systems

Very low relief topography (essentially flat)

Very slow velocity

Increased discharge

Generally extensive floodplains

Meanders and point bars common

Oxbow lakes

Groundwater and surface water are part of the same system

Lots of factors can force the water to leak onto the surface

Climates where there is too much precipitation for the ground to hold

Tropics vs. arid lands

Climates where the weathering and/or erosion processes are incomplete

Lack of fractures and/or soil

Places where there is some sort of blockage which forces the water out

Springs - places where water flows or seeps onto the surface

Occur where the water table intersects the surface

Can be caused by many different sub-surface conditions

Effluent stream - Gets its water from the water table

Common to temperate climates

Actually, effluent streams are just springs with a lot of water!

Associated with relatively stable water tables (in a natural setting)

Directly reflects the water table

Deer Creek bridge example

Influent stream - Adds water to the groundwater supply

Common in arid regions

The water is usually from more humid areas upstream which are destined to flow down into a desert