Important Fluvial Terms
Drainage basin/catchment/watershed:
•defined by the ridges that control the direction of precipitation
drainage. Every stream has a drainage basin.
•A drainage basin collects water, which is delivered to a
larger basin, creating larger streams
Continental Divide:
•The line separating subcontinental-scale drainage basins
Water and sediment usually terminate in oceans
Internal drainage basins are areas in which water does not
terminate in an ocean. (leaves via evaporation or subsurface
gravitational drainage
http://atlas.gc.ca/site/english/maps/freshwater/distribution/drainage
Drainage Basins
Red: selected
drainage
basins for first
order streams
(collection of
red areas should
fill the yellow
area but some
streams not
represented)
Yellow:
larger drainage
basins for river
Drainage density is determined by dividing the total
length of all streams by the area of the basin
For a given surface, a higher drainage density is generally
found in a humid area than in a dry area
Drainage Pattern is the arrangement of channels in an
area. Drainage pattern is determined by:
•Slope of the catchment
•Rock resistance to weathering
•Climate
•Underlying bedrock
•Subsurface hydrology
Drainage Patterns
1.
Dendritic
Tree-like pattern [Fig. 11-6 (a)]
Efficient –branch length minimized
2.
Rectangular
A faulted and jointed landscape directs streams
along right angle turns [Fig. 11-6 (e)]
3.
Trellis
Forms where resistance of bedrock varies
or along a folded landscape
Folds create parallel large streams, capturing
runoff from smaller streams and joining into
larger rivers at right angles [Fig. 11-6 (b)]
4.
Radial Drainage
Streams flow from central peak or dome
5.
Annular Drainage
Occurs in dome structures with
concentric patterns of rock strata
6.
Parallel drainage [Fig. 11-6 (e)]
Steep slopes - similar to dendritic,
but steep slopes cause branches to
appear almost parallel to one another
7.
Deranged Drainage
In areas with disrupted surface patterns
there is often no clear drainage geometry
(common in glaciated areas)
Flow velocity: A measure of how fast a stream
moves downstream (v in m/s). It depends on the
discharge, slope, size and shape of the channel.
Discharge: The amount of water flowing through
a cross section of a stream (Q in m3/s). Fluctuates
seasonally and diurnally (eg. melting greatest in
spring/summer and during the day) Q=wdv
Capacity: The amount of sediment that can be
carried by a stream (m3/s or kg/s). Capacity
increases with discharge.
Competence: The maximum particle size that can
be carried by the stream (related to flow velocity)
Sedimentary load is the total amount of sediment carried
by a stream. Sedimentary load is carried by bedload,
suspended load and washload.
Bedload: Coarse particles (eg. sand) which have high
settling velocity. Sediments are transported near the
streambed, kept loose by turbulence and particle interaction.
Suspended load: Particles are in the water column, sorted
by weight (larger particles near the bottom). The higher
the discharge, the higher the suspended load.
Washload: Fine particles with low settling velocity, which
travel at the same speed as the flow. Almost independent of
discharge.
Solution load: The total amount of dissolved ions
transported by a stream. Determined by nature of source
material and the physical and chemical characteristics of
the stream.
Sediment bulking:
Sediment is added to a stream in excess of its capacity.
In normal, turbulent flow, the critical point of the
dynamic equilibrium between erosion and deposition is
exceeded with sediment addition, thus, deposition
(aggradation) occurs.
Aggradation may lead to the development of a braided
stream.
Braided stream, S. Alaska
Photo: M. Miller (U. Oregon)
Flow Regimes:
Understanding of different flow regimes helps us to understand
resulting sediment bedforms and structures.
Reynolds Number (R):
R=
inertial forces_____ = lv
internal frictional forces

where  = fluid density
l = flow depth
v = overall mean velocity
 = viscosity of the fluid
Laminar flow:
Quasi-laminar flow occurs in slow moving
(R<500)
streams with smooth bottoms. Frictional
forces exceed inertial forces (500<R<2000)
Turbulent flow:
Higher flow velocity and bed roughness, with
(R>2000)
eddies. Friction with bed reduces v. Velocity
constant with depth except basal laminar layer.
Flow Regimes
A. Laminar flow
B. Quasi-laminar
flow
C. Turbulent
flow
D. Obstruction
in turbulent
flow
Relative velocity profiles
2.
Froude Number (F):
F
=
inertial force
gravitational force
=
v
(gl)-2
The ratio between the average velocity of the entire liquid mass
and the rate of downflow wave migration (celerity).
F<1
Inertial forces are less than gravitational forces. Therefore,
celerity of surface waves exceeds stream velocity (ie.,
waveforms faster than river, and break downstream)
F=1
Critical flow (no whitewater)
F>1
Main body of water moves faster than surface waves.
Surface waves break upstream.
Turbulent flow
A. Subcritical
(waves break
downstream)
B. Critical
(standing waves
at water surface)
C. Supercritical
(waves break
upstream)
CELERITY
FLOW VELOCITY
Quasi-laminar flow:
Sand ripples with ripple crosslamination form, unless the sand
is very coarse.
Turbulent, subcritical flow:
Sand dunes showing cross-beds
in section form
Turbulent, critical flow:
Horizontal beds with plane
laminations and minute strings of
particles aligned parallel to flow
direction at surface
Turbulent, supercritical flow:
Poorly-developed dunes or other
massive beds form with steeper
side facing upstream
How can we tell which type of flow was occurring by looking at
a cross section ? In the following slides, we will look at the
sediment facies resulting from (a) meandering streams, (b)
decelerating streams and (c) shallow streams with highly variable
water and sediment discharge.
Meandering streams
Streams take on a sinuous form where slope is shallow.
Water flows faster around the bends. The varying current
causes distinct erosional and depositional surfaces
1.
Outer bend
Erosional, supercritical flows
- undercut bank
2.
Middle-outer
Deposition of coarse-grained
or parallel, laminated sand
(critical flow)
3.
Middle-inner
Depositional, subcritical flow
Cross-bedded sand dunes
4.
Inner bend
Fine-grained, cross-laminated
sand ripples (point bars located here)
As the meander bend migrates, the lateral sedimentary facies stack
vertically. In other words, as the bend increases, coarse-grained
material becomes covered with progressively finer-grained sediment.
Meandering Stream
As this stream meanders, the bend seen in cross-section A extends
further to the left. As this occurs, there is a transition from erosional,
supercritical flows to critical and subcritical flow, with the associated
depositional features superimposed as in cross-section B.
*Note: Plane beds and antidunes are only
formed when flows are charged to capacity.
Bar Formation
A.
In high-competence flows in shallow streams with variable
water and sediment discharge, coarse grained sediment forms
upstream from finer-grained sediment (sheet bar).
Such bars are parallel to current with no flow separation
bubble (B)
C.
Finer-grained sediment tends to form transverse bars of various
shapes and do contain inclined depositional surfaces
downstream (D)
Bar formation
Hydraulic conditions
change during floods,
eroding bars to form
Scours
Coarser material is
deposited during the high
flow, with progressively
smaller particles deposited
thereafter
New bars form and finer
sediment is again found
above the gravel, resulting
in cut and fills
Cut and fills
Oxbow lake
A lake that was formerly a channel of a meandering stream
Formed when a meandering stream erodes back upon
itself, straightening the main channel. The old river channel
is still filled with water until sedimentation fills it again
Oxbow Lake,
Milk River, MT
(just south of
Canadian border)
Stream gradient
A stream usually has a steeper slope upstream and a
gentler slope downstream, resulting in an uneven, concave
shape
Nickpoint
A nickpoint is the location at which an abrupt change in
stream gradient occurs
Waterfall
At a nickpoint, the water falls to softer, more easily
erosive rock strata, leading to undercutting
Floodplains
Flat, low-lying areas near a river that are repeatedly flooded.
Rivers overflow during high flow and deposit sediment
upon the floodplain.
Rivers of a floodplain are generally embedded within the
sediment of the floodplain itself.
Levees develop along the banks of rivers as a result of
flooding. When a river floods, the velocity is reduced
beyond the bank, leading to sedimentation. The larger
particles fall first, leading to the creation of a natural levee.
The river may rise relative to the floodplain, leading to
backswamp areas.
Terraces
Uplift may allow a stream to cut deeper into its own
floodplain (rejuvenation), leading to alluvial terraces.
Such terraces look like steps above the river (Fig. 11-19)
River Deltas
The velocity of a river rapidly decelerates as it reaches
a large body of water. This leads to deposition, of
progressively smaller particles (large ones first). A
characteristic triangular shape forms (hence the term delta)
The river channels divide into smaller ones in all the
sediment, leading to what appears to be a reversed
dendritic drainage pattern (braided delta).
Nile River Delta