Our dependence on Earth’s Rivers:
Understanding Fluvial Systems

 

 

 

I. How Rivers Work (aka Fluvial Processes)

What’s ahead:


A. Carrying the Load (how/what a stream carries)
B. Stream flow regimes
C. Channel Types (shape) and how they change
D. Irony of streams
E. Equations to impress your friends
F. Why Velocity is King
G. How do high velocity streams erode?

 

A. Carrying the Load (bedload, suspended, dissolved): the work of rivers
 

1. Bedload: sediment transported along the bed of a river by sliding & rolling (traction) or hopping (saltation)

o   Occurs where steep gradients can move particles: bedload in alpine streams; desert washes; steep areas

o   Large clasts or smaller clasts, but at least sand sized

o   Saltation of sand makes ripples


2. Suspended load: Particulate sediment that is carried in the body of the flow and that it moves at the same velocity. A much higher velocity is required to entrain clay and fine silt than coarse sand.  Why?

Once the fine sediment is in suspension, a much lower velocity is required to maintain it in the flow (A small particle like clay and fine silt, with a large relative surface area, is held in suspension more easily because of the electrostatic attraction between the unsatisfied charges on grain’s surface and the water molecules. This force, tending to keep the particle in the flow, is large compared to the weight of the particle.)

o   Little Colorado River

o   Muddy Water in a Southwestern Stream

o   Along Peruvian Amazon...

o   Mt St Helens

o   Can you see the suspended load? (QTVR)

o   Clouds lakes, such as the Caspian Sea

o   See evidence in deserts once water dries...

 

3. Dissolved load: Material that is chemically carried in the water, you see clear but has dissolved load

o   Rio Negro & Amazon - 1 - 2 - 3 - 4

o   Kentucky

o   U.K.

o   Desert confluence

o   Blue and White Nile

 

Applying what you know: So...you go hiking, and you see this along the river bank...what type of stream load?

 

B. Stream Flow Regimes

 

Regime

Basics

Examples

Perennial

Flows year round

o   Key is Baseflow

o   Think wet regions 

o   Rivers you kayak, or where have mountains supplying water...

Intermittent

Flows during the wet season

o   Sacramento River Wet Season - Dry Season

o   Little Colorado Dry - Wet Season

o   Interstate 5 in Calif

o   East Africa

Ephemeral

Flows when it rains

(what’s the difference with intermittent?)

o   Example Joseph City July 98

o   Example Nazca

o   Detrital Wash

 

C. Stream channels and how they change

1. Types of Channels

 

Expect them:

Braided: multiple channels across broad floodplain (always changing) - braids can be dry without water

Glacier meltwater streams: QTVR Athabasca River, Ex1, Ex2, Ex3, Ex4, Ex5, Ex6

 

Desert Streams: Nazca, Peru; Mesquite, NV

Meandering: single channel wiggles across floodplain

Wet areas where you have perennial streams (diagram)
 

General Elements: point bar, cut bank, thalweg (Diagram)

 

Channel designed to handle flooding efficiently

Straight: made by people 

Indian Bend Wash, Nazca Peru  (Ground View), LA, Rio Grande (aerial)

 

2. Channel Change in Large Rivers - big consequences for small shifts

 

More examples:

Are you playing in contaminated waste?

o   Mapping change (undergraduate research project)

 

Policy Question for us, lawyers, journalists, politicians:

 

Should we have policies that encourage rebuilding?

 

Big rivers change all the time (Mississippi, Miss1, Miss2, Movie)

 

3. Big Question: How does this happen?


Process 1: Meander amplitude (wiggling) increases:

o   Deposition at point bars and erosion at cut banks

o   High velocity zone of deep water swings from cutbank to cutbank (thalweg, Ex1, Ex2)

o   Cut-Bank erodes, such as this example...more bank collapses: Ex1, Ex2, Ex3, Ex4, Ex5, Ex6, Ex7

o   Point bar (photo) deposits even as cut bank moves over time

o   Keeps going until very wiggly: making goosenecks

 

Process 2: Small floods build a levee:

o   Bankfull reached every 1-2 years (example of bankfull discharge; Susquehanna River example; other example1, other example2)

o   Levees build-up after each flood (oblique diagram): Yellow River in China, what you might see, at New Madrid (QTVR)

 

Process 3: Big floods breaks levees (aerial):

o   Produces a floodplain (example arid region, example wet area)

o   1991 China Flood example; Topeka example, Fort Worth, San Antonio, your neighborhood stream, just look up

 

Process 4: Cut-offs occur when stream gets out of its banks (animation):

o   Small Cut-off

o   When abandoned, create Oxbow lake: Ground View: Close aerial, High aerial

o   So when Isildur died in Lord of the Rings, and the Ring slipped from his finger to be in bedload, how was it possible for Deagol to find it?

 

Summary: what you should see in a meandering river? Diagram and great animation for review...

 

Sidebar Question for Scientists: how do you explain “entrenched meanders”?

 

o   Used by Hollywood: MI2, Maverick

o   “Goosenecks of the San Juan River” great Place for road trip, QTVR, Satellite, Ex1, Ex2

o   Throughout the Colorado Plateau, there are great hikes such as Havasupai

 

So, how were these meanders entrenched (cut deep into the earth)? 

 

D. Streams can have sense of irony:
 

1. Biggest rivers carry smallest particles (small mountain tributaries carry biggest particles)

o   Competence vs. Capacity: definitions

o   Example of small stream that has very high competence (Aptos Creek, Santa Cruz Mtns), much higher than Mississippi River’s great capacity

o   Understanding idea of competence explains the “Opportunity” Mars site mystery of billions of blueberry-sized load

 

2. Biggest floods occur the least often (small floods happen more often)

o   Magnitude vs. Frequency are inversely related

o   Example of rare event in Italy: movie 1, movie 2, movie 3, movie 4

o   Affects where we can live: “100 year” floodplain

o   Large floods along the Salt River near Phoenix, AZ: 1978, 1983

o   But how soon we forget...now they’re building million-dollar apartments on its banks! (sure there’s a “dam” now, but one 100-year flood and...whoosh!)

 

E. Equations to impress your friends, family, and folks who think Geography is just states and capitals and countries...

 

1. Q = wdv (discharge = width x depth x velocity)

o   Channel Geometry Diagram, Q can be fun!!

 

2. Chezy-Manning Equation:


 

o   S - Slope

o   R - An increase in R results in a decrease in frictional resistance exerted by the channel walls (reason why speeds are slower next to banks)

o   n = empirically derived roughness coefficient

o   Vegetation increases n:- Ground, Aerial, Australia

 

F. Velocity is King in understanding Types of Flow (Laminar or Turbulent) (movie example)

o   Look at your sink (movie)

o   Look at someone’s habit (movie)

o   See normal turbulence in a river you might raft (movie)

o   Transition at a “standing wave”: kayakers like turbulence, and knickpoints, but beware when you see standing wave (movie)

More Impressive Equations...

 

  • Reynold’s Number (Re#)

 

(Re#) = pvd/u (for streams d=R):

p = density
v = velocity
d = avg depth of flow or R = hyraulic radius (A/Pw)
u = viscosity (.009 cm2/sec for water)

  • Re# for laminar flow in streams is around 500
  • Most stream have Re#s >2000 and are therefore characterized by turbulent flow

o   Even when you see quiet stream, likely is turbulent.

  • Flow regime:

o   Defined by the Froude Number [F# = v2 /(gd)]

o   See sudden change from streaming (regular turbulence) to shooting (very erosive)

 

 

G. How do high velocity streams erode Earth’s surface?

 

II. Integrating Forms and Processes seen in Fluvial Landscapes

 

A. Basic forms in an individual river or stream

 

B. Graded Streams: streams want to reach a state of equilibrium

o   Graded stream (steady state; balanced; introductory movie): a stream which has regulated its various parameters (depth, width, slope, velocity, etc.) to obtain the most efficient conditions for flow and sediment transport

o   So, how do streams change in the downstream direction?

o   Parameters that might increase:

o   discharge

o   net forward velocity

o   drainage area

o   load (capacity)

o   Parameters that might decrease

o   slope (gradient)

o   n (manning roughness)

o   particle size (competence)

o   Characteristics of a graded stream: (Rhine River, Europe)

 

Dams create artificial base level

Delta deposits above the dam & erodes below the dam (Hoover Dam, Create local base level, with new adjustment upstream and downstream incision)

o   Dams have multiple uses, such as storing water, recreation

o   Critical for summer peaking power (also think about the sediment collecting in behind  Glen Canyon Dam in Lake Powell, not going downstream)

o   Dam failures an increasing concern: Hawaii winter 2006; Malibu 30s small dams ready to fail, another example, just imagine what would happen...

o   Dams important in international issues, because changes control of water (e.g. Ataturk Dam) and cultures, history, and land use

o   Three Gorges Dam (Yangtze River [Chang Jiang], China): location, from air, oblique, on ground

o   Serious discussion on the removal of dams such as those that interfere with biota (e.g., salmon spawning, endangered species), culture, and specific landuse

 

C. Streams trying to reach equilibrium – what do you see?

1. Knickpoints are removed

o   Knickpoint (diagram): rapids, any break in slope (Zion NP), but best associated with waterfalls: Grand Falls, with Case...and break in slope before falls...(more break in slope)

 

2. KNICKPOINTS ARE TEMPORARY:

o   Undercutting (movie showing how undercutting works, and in a QTVR)

o   Contains falls and basins

o   Eroding bed

 

3. Eventually, waterfalls are removed...Examples:

o   Niagara Falls will disappear (movie) causing issues for tourism (QTVR; aerial; space)

o   Grand Canyon tributary

o   Yellowstone River (QTVR)

o   Little Colorado River

o   Yosemite Falls

o   Iguazu Falls (just for fun) (QTVR)

o   Havasupai Falls

o   Great Falls of the Potomac

o   Rapids

o   Mars?

4. If base level rises, valley fills in (aggradation)

o   Example Mississippi Valley, with sea level rise and drop in discharge

o   Example of Napa Valley, where rising sea level led to filling of valleys (QTVR)

o   Glacial valleys tend to be aggrading during the Holocene: Example within Alpine

5. If base level falls, valley erodes deeply (incision)

 

6. Some forms: If the total stream energy is greater than that required to transport the sediment provided it, then the stream will erode

o   Slot Canyons: 1, 2

o   V-shaped valleys

o   Stream terraces

o   Some processes involved: total energy available for erosion is increased by:

o   Increase in gradient; accomplished by aggradation

o   Decreasing in base level; results from drop in sea level or degradation of higher order stream

o   Uplift of mountains, such as in New Zealand, or the Andes

o   Overflow of a lake, such as when lake in Long Valley Caldera overflowed cutting Owens River gorge

o   Increase in Q (discharge) like “scablands” in WA; results from climatic changes or extreme storm

 

Stream Terraces: (Two Towers)

o   Floodplain abandoned as the stream degrades

o   2 Types of Terraces: 

o   Fill (depositional; former floodplain; paired terraces): Rio Grande, Ex1, Ex2, Ex3, Ex4, Ex5, Ex6

o   Strath (Scottish: “wide valley”; erosional remnants of older valley floor; unpaired terraces): Ex1, Ex2, Ex3, Ex4

What you might see on any given trip:

o   Crooked River, Oregon - QTVR

o   Arroyo Seco California - QTVR

o   Along the Snake River - QTVR

 

D. Major Floods

o   Mississippi River Basin

o   Keep in mind situation of rising base level

o   Atmosphere supply moisture

o   Map

o   Hydrograph

o   Aerial photo

o   Satellite view1, Animate; Change in ‘93...Again in 2002

o   2002 Summer Flooding in Europe

o   Elbe River: Dessau, Germany (SW of Berlin)

o   Oder: bank collapse; Bedload Transport (Poland)

o   Wroclaw, Poland Floodwave (NW of Krakow; NE of Prague)