Revised 8 / 06 (Monroe 6th ed.)

Streams and Stream Dynamics (Ch. 15)




The Hydrologic Cycle

Streams and Energy

Velocity, Channel Morphology, and Discharge

Transportation of Sediments

Deposition of Sediments

Graded Streams

Evolution of Drainage Systems

Linkage to Groundwater

Humans and the Rational Use of Streams


Click here for a list of vocabulary associated with this topic



Rivers and water are pretty important

"Givers of life"

Nile, Tigris & Euphrates, Congo

Some of the earliest human construction projects were associated with the transportation and storage of water

Dams and canals

Also some of the earliest legal disputes

Code of Hammurabi - 1700 B.C.

Natural boundaries throughout history

Communication routes

Routes into the interiors of the continents

Most of the world's leading cities are either on rivers or at their mouths

Act as the transportation systems to get sediments to the ocean

Clears the land of weathered and eroded materials

Rivers and river systems are intensely studied

In an effort to "understand and control" the movement of water

"Understand?" Maybe. "Control?" I doubt it!

Refer to Strickler's 4th Law of GeoFantasy

Actually, the concept of controlling ANY earth process gives me trouble


The Hydrologic Cycle

The overall volume of water on earth, while clearly not static in the long term, can be considered constant at human time scales

However, the spatial distribution of water can and does change

Constantly moving between numerous "temporary storage units"

Both "long" and "short" term fluctuations

Ice ages vs. Pepsi cans

This movement is called the Hydrologic Cycle (Monroe: Fig. 15-3, pg. 460)

The cycle has 4 main parts

Each involves a change of state or the interaction of water and gravity

Evaporation: liquid to vapor

Condensation: vapor to liquid

Precipitation: liquid and gravity

Collection: liquid and gravity

The water is usually purified as it changes state

This is good, since we tend to mess it up, no matter what state it's in

Click here for more information on the hydrologic cycle


Streams and Energy

Need to get weathered sediments to the ocean (Strickler's 3rd Law of GeoFantasy)

What is the transportation system?

In most cases moving water

Energy is the determining factor

Need to get the sediments moving, but inertia tries to keep them stationary

Acceleration = force / mass

In this case, mass of the sediment is critical

Greater mass requires greater force to achieve an acceleration

Click here for more on Newton and the Laws of Motion

Where do we get the force?

Several sources: Water, wind, gravity, geologists and other bozos

Moving water has kinetic energy (the energy of motion)

Kinetic energy: Ek = 1/2 mv2

Velocity is the most important variable (EXPLAIN)

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


Velocity, Channel Morphology, and Discharge

Start with the basic equation of flow

Discharge = Area of channel X Average velocity of flow (Q=AV)

We'll find that all 3 variables are almost impossible to define accurately

And if any one variable is suspect, real values for all 3 are in question

Streams are active systems, and don't stop long enough to be measured

And it's even worse than that:

When they're doing the most work it can be downright unsafe to be anywhere near them!

Stream flow is mainly turbulent (Monroe: Fig. 15-4, pg. 461)

As opposed to laminar (like glaciers)

Water molecules go every which way

Up and down

Side to side

Even back upstream

This makes any attempt to determine average velocity difficult

Stream velocity... a definition

"The direction and magnitude of a portion of the stream per unit of time"

Translation - how fast and in what direction

Quite a range of values

Up to 20 miles per hour

Most are less than 4 mph

This is obviously very erratic

And seasonal!

Velocity is VERY important (see above)

Directly related to the stream's energy and its ability to transport sediments

In general terms, the faster the water is moving, the more energy it has

REVIEW: the basic equation for kinetic energy (Ek = 1/2 mv2)

Average velocity: where is it?

Friction is important here

Contact with the sides and bottom slows down the flow

Also contact with the atmosphere!

In most cases, the fastest velocity is near the center of the channel, just below the surface (Monroe: Fig. 15-6, pg. 463)

But this is not necessarily the average velocity

May be impossible to measure due to the turbulent nature of the flow

Therefore, how can we ever trust discharge calculations?

Many factors affect stream velocity


Gradient (Monroe: Fig. 15-5, pg. 462)

Channel shape and geometry (Monroe: Fig. 15-6, pg. 463)

Roughness of channel

Discharge of the stream

Sediment load

Each of these is highly variable, both regionally and locally

And are definitely inter-related, each working with the others to regulate stream flow

Gradient - the down-valley slope (Monroe: Fig. 15-5, pg. 462)

In general, increased gradient increases the velocity

Gradient is usually measured in feet per mile

Can range from waterfalls to very flat sections

Gradient usually decreases as the stream descends to lower elevations

But can be highly variable locally

Directly affected by lithology

What type of bedrock the stream is flowing over

EXAMPLE: the Rogue River

Describe general gradient from Crater Lake to Gold Beach

DEFINE: Base level (Monroe: Fig. 15-24, pg. 482), Temp. (local) base level (Monroe: Fig. 15-25, pg. 483)

Channel shape and geometry

Sides and bottom cause friction

Wide, shallow channels tend to slow the flow

Semi-circular channels allow the fastest flow

Roughness of channel

Smooth channel results in relatively non-turbulent and laminar flow

Rough channel results in relatively turbulent flow

Also tends to increase the total surface area of the channel

Therefore increases friction and decreases velocity

DIGRESS TO: Hippo's Teeth along the Grapevine (I-5)


"The quantity of water which passes a point in a given interval of time"

Rarely constant due to seasonal fluctuations

Also daily fluctuations

High latitudes - daytime highs increase snowmelt and discharge in the afternoon and evening

Global discharge varies greatly

Amazon clearly the largest

Fresh water up to 100 miles out to sea

Definitely affects velocity

In general, increased discharge results in increased velocity

Sounds like a no-brainer, but...

This can be a bit confusing

This is a local variation in a given section of a stream

Usually, increased discharge in the lower portions of a river result in a lower velocity than in the headwaters

Generally related to the decreased gradient

Clearly, the other factors also have an effect

Sediment load

Increased sediment load results in a relative decrease in the amount of water

But, due to density, a relative increase in mass

Therefore, velocity should decrease as sediment load increases

Again, the inter-relationships are beautiful

Increased discharge during flooding results in increased velocity and energy

Results in more sediment in the water, which slows it back down!


Transportation of Sediments

Streams are the conveyor belts which move the weathered and eroded sediments to the ocean

See Strickler's 3rd Law of GeoFantasy

The actual amounts can be rather impressive

Mississippi River: average 1,000,000 metric tons per day

Obviously, can be much higher during flood stage

"Removes 2" of soil every 1000 years"

How (and when) did they get this figure?

Obviously, human interaction in the Mississippi basin has a significant impact on this amount

If this is a recent figure based on recent erosional rates, then the actual lowering of the land is much slower

The reality of the process is relatively simple

But difficult to study

The vast majority of transportation takes place during flood

A dangerous time to sample!

DIGRESS TO: Classification of floods (100 year floods, etc.)

Two general categories

Dissolved load

Solid load (suspended load vs. bed load)

Three types of load are possible

Related to the type and size of the material and velocity of the stream

REVIEW: Chemical vs. mechanical weathering

Click here for more on:

Weathering: an Overview

Mechanical Weathering

Chemical Weathering

The relative proportion of load type can vary locally and regionally within a drainage system

Dissolved load

Material moving downstream "in solution"

Generally produced by chemical weathering processes

Cannot be seen or felt

Can be tasted and smelled

Will not settle out of the water

The dissolved load has a chance of making it to the ocean rather quickly

It's a part of the water and will move with it until it stops

Or a chemical imbalance causes precipitation

Can usually be separated by evaporation of the water

Resulting in the precipitation of the dissolved material

And the formation of chemical sedimentary rocks

Suspended load

"Smaller" solid material moving downstream while suspended in the water

Put dirt in a jug, shake it up, and watch the sediments settle

Pretty much covers the basics of suspended sediments

Generally produced by mechanical weathering processes

Individual particles are called clasts, and result in clastic sedimentary rocks

Can be seen, and will settle out of the water

The size, amount, and length of time clasts remain suspended depends on:

Physical characteristics of the material

Size: clay vs. sand

Shape: clay/mica vs. sand

Specific gravity: gold vs. sand

Velocity of the water

Faster water has greater energy

Can suspend more stuff for a longer time

Turbulence of the water

Keep shaking the jug of dirt and the stuff will never settle

An increase in velocity and/or turbulence will result in more, and larger, clasts in suspension

In general, the bigger stuff (commonly sand) is carried closer to the bottom

And will settle first (a real no-brainer!)

Silt and clay are more evenly distributed throughout the water

Form the majority of the suspended load

Relatively easy to measure

Get some water and let it settle

Weigh the solids

This stuff may make it to the beach on its first try

But no guarantee!

Bed load

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

Generally produced by mechanical weathering processes

Can be seen, and will settle out of the water

Does most of the work in down-cutting and widening stream channels

DIGRESS TO: potholes (Monroe: Fig. 15-8b, pg. 465)

Particles are moved in different ways

Sliding and rolling

The big stuff which is too heavy for a given volume of water to move easily


A "leap-frog" motion of material as it bounces downstream

Transition between true bed load and suspended load

Bed load can be difficult to impossible to measure

Most is moved during floods

REVIEW: Classification of floods (100 year floods, etc.)

The 1964 flood as reported at Agness, Oregon

The bed load has little or no chance of reaching the ocean quickly

The bigger the piece, the longer it will probably take

Additional weathering is required to reduce the particle size

Dissolved vs. solid load (suspended load and bed load)

Ratio between dissolved to solid can vary for many reasons

"Generally, 50% of the total load is in solution"

Basically, the land is dissolving (Strickler's 4th Law of GeoFantasy)

This will obviously vary both locally and regionally

Increased chemical weathering results in increased dissolved load

Tropical climates

Warm, humid, lots of chemical weathering

Increased vegetation to hold clasts (soil) in place

Also greater precipitation so larger river systems

More water reaching the ocean which will skew the global average in favor of the dissolved load

DIGRESS TO: efficiency of chemical weathering in various climates

Increased mechanical weathering results in increased solid load

Colder climates

Mostly mechanical weathering

Sparse vegetation so solid clasts are relatively free to move


Deposition of Sediments

Accumulations of sediments are common along the sides of all rivers and streams

Local accumulations of sand, gravel, and boulders to immense "floodplains" covering thousands of square miles

Monroe: Fig. 15-7, pg. 464

Monroe: Fig. 15-29, pg. 486


The debris which forms the banks of any and all streams

By definition, floodplains are very active depositional/erosional environments

Prone to floods and shifting materials

Not a good place to build!

Floodplains store excess sediments at times of low water

And excess water at times of high water

In reality, the edges of rivers are common sites of human activity

But, prone to flooding at all levels of intensity

Both good and bad effects

Bad are relatively obvious

Good: EXAMPLE - the Nile and the impact of the Aswan Dam

Lots of features associated with floodplains and the deposition of sediments

Meanders - probably the most obvious and recognizable feature

Large, curving bends in a river (Monroe: Fig. 15-11, pg. 468)

Common in areas where the river has cut almost to base level (REVIEW)

Generally form when a river has too much energy, and it needs to slow down

Meandering lengthens the channel and reduces the gradient

Generally results in a lower velocity

Energy drops, and the river stops down-cutting into its bed

Meanders do migrate and cut side-to-side

Water velocity is fastest on the outside of the bend

Results in differences in energy across the channel

Cut bank vs. point bar (Monroe: Fig. 15-14, pg. 446)

Cross-over: where fast water crosses the channel between meanders

Also the best place to ford a river

Relatively shallow depths compared to the cut bank / point bar

The river uses up its energy moving sediments from side to side

The meander will migrate in the direction of the cut bank

Oxbow lakes (Monroe: Fig. 15-12, pg. 469)

Meanders tend to form in areas with strong and cohesive materials

Clay and silt rich deposits

Common in temperate and tropical climates where there is a large amount of chemical weathering

Braided streams (Monroe: Fig. 15-10, pg. 467)

A lot like meanders

Form in low gradient streams as a means of using up excess energy

Without cutting deeper into bed (that base level thing again!)

Common in areas where the stream deposits are loose and non-cohesive

Cannot maintain resistant banks

Sand and gravel are the common floodplain deposits

Arid lands and cold climates where mechanical weathering predominates

SUMMARIZE: meanders vs. braids

Deltas (Monroe: Fig. 15-15/16, pg. 471/472)

Because Q=AV streams drop their load when they enter still water (ocean or lake)

Alluvial fans (Monroe: Fig. 15-17, pg. 473)

Basically dry land deltas

Common to arid regions where there is insufficient flow to completely remove the sediments

Stream terraces (Monroe: Fig. 15-29, pg. 486)

Usually uplifted floodplains

Underlain by floodplain deposits

Can also be cut into bedrock

DIGRESS TO: terrace vs. peneplain

Represent times when the stream was at a higher level

Stabilized long enough to create a relatively flat surface

Results from a change in relative base level

Either the land goes up, or the base level goes down

Stream rejuvenation occurs and the stream begins to cut deeper into its bed

Until equilibrium with the new base level is achieved

Can result in incised meanders (Monroe: Fig. 15-30, pg. 487)

Misfit streams

A big valley with a small stream

Common in cases of stream piracy (Monroe: Fig. 15-28, pg. 486)


Graded Streams

The earth tries for a balance in all things, but...

Rapidly evolving local stream conditions make it tough for nature to keep up

Several factors are relevant here

And, as usual, they are inter-related

Sediment yield

How much material is being transported

Rapidly changing as humans are causing increased weathering and erosion


The size of material that a stream can transport

Depends primarily on velocity

Get the water moving fast enough, and some pretty big things can move

Island Mountain bridge

Crescent City jetty core


The potential load a stream can carry

Again, velocity is important

Also discharge - more water can move more stuff

In general:

Capacity: what a stream theoretically can do

Load: what a stream is actually doing

Why are these different?

Many factors contribute to increasing the yield beyond capacity

As well as reducing the amount of material available to the stream

Put it all together, and we end up with streams which are usually out of balance

Aggrading streams

Too much load, so deposition will occur

Degrading streams

Too little load, so erosion will occur

At grade streams

Equilibrium, where the sediment load is balanced to the stream's capacity

Being "at grade" is the goal and natural end result of stream dynamics

Any disruption or change in local/regional conditions will force the stream to re-adjust in an attempt to restore equilibrium

These re-adjustments result in deposition or removal of sediments

Both are tough on human developments in the vicinity of the stream

Lots of possible disruptions to a balanced stream

Too much load (load greater than capacity)

Landslides, logging, hydraulic mining

An interesting sequence of events as the stream attempts to deal with the additional material

Dump it

Increase the local gradient

Increase velocity and energy

Erode the sediments again

Sooner or later (hopefully) reach equilibrium

Too little load (load less than capacity)

Rapid increase in discharge, loss of sediments (like below a dam)

In this case the erosional ability increases, and the stream picks up additional material

DIGRESS TO: dams in rivers (ultimately self-defeating)

Axiom: all dams are short term features because the lakes fill in and/or the dam is under-cut at the base


Evolution of Drainage Systems

Obviously, streams have a lot to do with the shape of the land

Cut the valleys they flow within

Work with weathering and erosion to reduce relief (DEFINE)

The ultimate goal of all rivers is the creation of Kansas!

In most cases this is not a short-term process

See Strickler's 2nd Law of GeoFantasy

Drainage basin (Monroe: Fig. 15-22, pg. 480)

Land surface included in the area a stream drains

Drainage patterns (Monroe: Fig. 15-23, pg. 481)

Often controlled by lithology and tectonics

The 3 stages of stream development

As usual - lots of intermediate steps and shades of gray

Involve two basic processes (Monroe: Fig. 15-32, pg. 488)

Down-cutting into stream bed

Deepens the valley

Back-wasting of the sides of the valley

Widens the valley

Youthful : generally in the mountains

V-shaped valleys with steep canyon walls

Steeper gradient

Higher velocity

Relatively lower discharge

Down-cutting the dominant process


Rounded hills and valleys

Moderate gradient

Moderate velocity

Moderate discharge

Old age: generally found near the mouths of river systems

Very low relief topography (essentially flat)

Minimal gradient

Very slow velocity

Increased discharge

Meanders and oxbow lakes common

Back-wasting the dominant process


Linkage to Groundwater

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

EXAMPLE: the Colorado River and the Nile


Humans and the Rational Use of Streams

Where do we start? Or stop!

The earth doesn't need or want our help! (Strickler's 5th Law of GeoFantasy?)

The reality is that we will continue to use (and abuse) running water

We need water and it's so damn useful

Back to that energy thing again

Tough to balance development with preservation

Economic, political, and cultural needs and desires at work here

Examples of irrational use abound

Dams in general

DIGRESS TO: I hate to trash the Army Corp. of Engineers, but...

Economic, political, and cultural needs and desires at work here, too

Dams are ultimately self-defeating

Q=AV so their impoundments (and the dams themselves) are temporary

Disruption of sediment transport

Fill in lake

Now clean water below dam with all this extra energy

Increased erosion

Levees and flood control

Levees work great when you don't need them

But, next time the water rises...

Restrict floodplains at times of flood

Increased velocity erodes levee

River gets behind the levee

Levee is now a dam!

Development in urban areas

Make the streams straight and pave them to reduce erosion

Both tend to upset the natural balance

Next time the water rises...

Fishermen are a real problem

Not to mention the mess...

In many parts of the world, the best fishing streams are cleaned of debris

So the fishermen can save money on snagged tackle

But the removed debris was a part of the balance

Next time the water rises...

Used as depositories for many (ultimately ALL) waste products

Open, running sewers in many parts of the world

Would YOU like to swim in the lower Gangees?

In combination with groundwater this can be a real long-term problem

Hanford nuclear facility

How many in the Pacific Northwest depend upon a clean Columbia?


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