Our dependence on
Appreciating & Understanding Fluvial Systems
I. How Rivers Work (aka Fluvial Processes)
1. Bedload: sediment transported along the bed of a river by sliding & rolling (traction) or hopping (saltation)
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 Along Peruvian Amazon...
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
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
Flows year round
o Think wet regions
o Rivers you kayak, or where have mountains supplying water...
Flows during the wet season
Flows when it rains
C. Stream channels and how they change
Meandering: single channel wiggles across floodplain
Wet areas where you have perennial
Straight: made by people
2. Channel Change in Large Rivers - big consequences for small shifts
Are you playing in contaminated waste?
o Mapping change (undergraduate research project)
o The Rio Puerco
Policy Question for us, lawyers, journalists, politicians:
Should we have policies that encourage rebuilding?
3. Big Question: How does this happen?
Process 1: Meander amplitude (wiggling) increases:
o Deposition at point bars and erosion at cut banks
Process 2: Small floods build a levee:
Process 4: Cut-offs occur when stream gets out of its banks (animation):
Sidebar Question for Those that like Fluvial Stuff: how do you explain “entrenched meanders”?
So, how were these meanders entrenched (cut deep into the earth)?
Streams can have sense of irony:
1. Biggest rivers carry smallest particles (small mountain tributaries carry biggest particles)
2. Biggest floods occur the least often (small floods happen more often)
o Magnitude vs. Frequency are inversely related
o Affects where we can live: “100 year” floodplain
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!)
o Look at your sink (movie)
o Look at someone’s habit (movie)
o See normal turbulence in a river you might raft (movie)
F. 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)
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)
More Impressive Equations...
(Re#) = pvd/u (for streams d=R):
p = density
o Even when you see quiet stream, likely is turbulent.
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
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 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 Dams have multiple uses, such as storing water, recreation
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
3. If base level rises, valley fills in (aggradation)
o Example Mississippi Valley, with sea level rise and drop in discharge
o Glacial valleys tend to be aggrading during the Holocene: Example within Alpine
4. If base level falls, valley erodes deeply (incision)
5. Some forms: If the total stream energy is greater than that required to transport the sediment provided it, then the stream will erode
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 Overflow of a lake, such as when lake in Long Valley Caldera overflowed cutting Owens River gorge
D. When floodplains are abandoned, the result is stream terraces
o Two Types of Terraces:
o So...strath or fill terrace?
o What you might see on any given trip:
o Crooked River, Oregon - QTVR
o Arroyo Seco California - QTVR
o Along the Snake River - QTVR