Geology 230- GW/SW Interaction
Geology 230- GW/SW Interaction Lecture 1
Stream Classification
I) Sedimentologist’s Classification of streams:
- Classically trained sedimentologists speak of braided, meandering and anastomosing streams.
- This is really a gradation, and lines between different types of streams may not be perfectly clear.
- Based on the work of Andrew Mial (Canadian sedimentologist)
A) Braided streams:
I) Channel characteristics of braided vs. meandering streams:
A) Sinuosity:
- Is defined as ratio of channel length to down-valley distance
Draw picture on board
- See Rust Classification of channels (based on sinuosity)
See Table 22 from Reineck and Singh, p. 259 and on handout
- We will only deal with meandering and braided streams in detail
- Should be aware that other channel forms exist (although they are less common)
See Figure 372 from Reineck and Singh, p. 259
- Shows map-view examples of each channel type
B) Braiding parameter
- Refers to number of bar forms / meander wavelength
- Generally:
- Braided streams have > 1 bar form per meander
- Meandering streams have < 1 bar form per meander
II) Braided streams:
A) General characteristics:
- Also called a “bedload” stream: has a high percentage of material
transported as bedload rather than suspended load
- High gradient
- Longitudinal bars
- Bed load dominates (coarse grain sizes: gravel to sand, minor clay)
- Sandy, non-cohesive bank material
- Channel divides and rejoins frequently
- Produces divergent paleocurrents
- Sinuosity < 1.5
- > 1 bar per meander
B) Common Bar forms:
- 3 common bar forms: Longitudinal, linguoid/transverse, lateral
See Fig. 10.7 in book, p. 309
- This is a bit more complex than the general characteristics listed above
1) Longitudinal bar forms:
- Form in mid-channel
- Form during waning stage of floods: contain coarsest material
- Are oriented parallel to flow
- Fine upward through the bar and downstream (2 directions)
See Fig. 10.7 in book, p. 309 again
- Internal structure: massive or crudely bedded
- May have some planar stratification or weakly developed cross bedding
- Probably indicate upper flow regime
2) Linguoid/transverse bars:
- Oriented transverse (at an angle) to the current
- Usually build out from the side
- Most common in sandy braided streams
- Probably represent: large dunes that form under high flow
- Cross bedding common, well developed (planar and trough)
- Surface: may be rippled (lowest waning flow that crosses bar)
- Have 2 different shapes:
a) Linguiod: lobate shape, steep foreset beds
b) Transverse: straight crested
3) Lateral bars:
- Form in lower energy conditions
- Build out from the bank (attached to bank)
- Again: common in sandy streams/conditions
- Structures: cross bedding common, well developed (planar and trough)
- Surface: may be rippled (lowest waning flow that crosses bar)
C) Types (variability) of braided streams:
- 4 types have been defined
- Depend on sediment load, gradient (sediment in = sediment out)
See Fig. 10.8 in textbook, p. 311 (my Fig. 11.7)
1) Scott type:
- Coarsest
- Abundant horizontally bedded gravels
- Minor ss (volumetrically)
- Are proximal, high discharge
- Has some fining upward cycles
2) Donjek type:
- Wider range of sediment types present
- More pronounced fining upward cycles
- Represent deposition in different parts of the stream system or channel shifting: some coarse, high energy, some finer, lower energy
3) Platte type:
- Named after the Platte River
- Have sandier load
- No distinct sequences (minor fining upward)
4) Bijou Creek:
- Finest sediment of the braided streams
- Individual flood events are preserved as fining upward cycles
- Less erosional scour
- Probably a low gradient system
D) Summary:
- General pattern:
- Leave large, sheet-like coarse deposits
- Paleocurrents are divergent
See Fig. 10.9 from textbook, p. 312 (my Fig. 11.8)
- Rare, discontinous clay drapes
- Deposited by episodic high flow conditions
III) See Slides of Braided vs. Meandering streams
I) Meandering streams:
A) General characteristics:
- Fining upward cycles
- Low gradient
- Abundant transverse bars
- Suspended load dominates
- Fine-grained (cohesive) bank material
- Channel is well-defined, stable
- Sinousity > 1.5
- 1 bar per meander
B) Depositional processes:
- Affect bar forms present
- Flow is different:
- Generally more consistent
- Within channel: hugs outer bank of meanders
- In cross section: has a helical pattern (like DNA): corkscrews
See overhead Fig. 7-18 from Davis
- Results in different bar forms
- Fastest line of flow = thalweg
- Position of thalweg varies depending on stage
See Fig. 10.10 from text, p. 313
- Flood events:
- Deposit material in flood plain
- Build levees
- “Normal” flow: stream stays within channel
- Environments are more varied, complex
- Range to very quiet water deposition (swamps, floodplain)
- Generally: more fine sediments are preserved (compared to braided stream systems)
See Fig. 379 from Reineck and Singh (on handout)
C) Common bar forms/deposits:
1) Crevasse splay:
- Forms during high flow when channel breaks levee
- Results in momentary flow perpendicular to main channel
- Is definable by flow indicators, sedimentary patterns:
- Flow is 90°
- Thins, has smaller scale structures outward from main channel
2) Channel deposits:
- Coarsest, bedload
- Usually a minor component
3) Point bar:
- Sandy to silty (finer)
- Attached to bank
- Builds laterally
- Called a lateral accretion surface
- A very distinctive/diagnostic feature
- Deposits are called “Epsilon cross beds”
- Help distinguish braided from meandering stream environments
- Fine upward and downstream
- Indicated by grain size
- Flow structures also indicate lower flow upward and downstream
4) (Natural) levee:
- Is different from channel levee (see book)
- Marks the edge omf floodplain
- Cohesive, slightly sandy
- May be vegetated
5) Floodplain:
- Finest material
- Rippled
- Abundant organic (plant) debris
- Rooted
6) Ox-bow lake
- Often filled by silt
- Choked, shut off from main flow
D) Vertical succession:
!!!! Fines upward
- As channel migrates laterally
See Fig. 10.13 From text, p. 317 (my Fig. 7-36)
See typical ancient example on handout: Fig. 7-36 (Similar to Fig. 10.3 in textbook)
II) Anastomosing:
- A hybrid stream form
- We won’t spend much time on this
- Falls mid-way in the spectrum between braided and meandering
A) Has cohesive banks (like a meandering stream)
- Banks contain the channel, prevent high sinuosity
B) Has sandy bed load (like a braided stream)
- Sandy bed load results in many bar forms per meander wavelength (high braided parameter)
- Channel bifurcates and joins frequently
- Paleocurrents diverge
C) Identified by:
- The high percentage of floodplain deposits (sand/silt/shale ratios)
- Bar forms that are more typical of bed load systems:
Abundant longitudinal bars
Rare or absent lateral accretion surfaces, epsilon cross beds
III) Summary: see handout (from last time) Fig. 7-37
Braided vs. Meandering:
Look for abundant stacked channels vs. Channels encased in floodplain material
Look at bar morphology and presence/absence of lateral accretion surfaces, epsilon cross beds and crevasse splays
II) Rosgen’s stream classification
Specific objectives of the Rosgen stream classification include:
· Predict a river's behavior from its appearance;
· Develop specific hydraulic and sediment relationships for a given stream type and its state;
· Provide a mechanism to extrapolate site-specific data to stream reaches having similar characteristics; and
· Provide a consistent frame of reference for communicating stream morphology and condition among a variety of disciplines.
Criticisms: Isn’t process based
Doesn’t have any implications for sediment transport or changes in stage
Often used by inexperienced people
III) Montgomery and Buffington stream classification
Is more process-based