Suspended Sediment,
Bedload, and Substrate
Composition
Stream Ecology
1/22/2009
Stream Load Classifications
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Bed load : Sediments moving predominantly in contact with or
close to the stream bed.
Suspended load : Sediments that move primarily suspended in fluid
flow but may interact with bed load and is comprised primarily
of silts and clays and sometimes sand. Suspended load has a
continual exchange between sediment in fluid flow and on the
bed as it is constantly being entrained from the bed and
suspended while heavier particles settle out from the flow to the
bed.
Solute load refers to the total amount of dissolved material (ions)
carried in suspension and can only be quantified by laboratory
analytical techniques.
Total load is the total amount of sediment in motion and is the
sum of bed load + suspended load.
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Relatively artificial categories.
The sediment load carrying capacity of a stream
constantly changes both spatially and temporally
as flow changes.
Flow in any river or stream is never
homogenous so the resulting sediment
movement varies greatly.
 The majority of material transported by rivers is in suspension.
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Erosion rates and average suspended sediment
load of rivers depends, in lieu of human
activities, on the following factors:
Age and morphology of the surfaces.
Composition of the bedrock material.
Tectonic uplifting.
Climate and vegetation cover .
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Milliman and Meade (1983) estimated the annual
suspended load of the world rivers to be about
15 x 109 tons.
The distribution of the sediment loads of
different areas is very heterogeneous, and high
sediment transport rates are concentrated in
regions near the Equator and in Arctic regions.
For every cubic meter of water reaching the ocean, there is
(on average), 0.5 kilogram of sediment carried away from
the continents.
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There is a relationship between climate and
erosional rates and, therefore suspended
sediment loads in streams.
Relatively modest shifts in average climate
conditions have large impacts on the behavior of
a streams flood response and thus sediment
yield.
A few classic examples of anthropogenic
activity known to increase sedimentation are:
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Logging, deforestation, wildfire, vegetative change.
Specific types of logging activity can increase sediment
yield by 100X for short periods. Fire suppression and
drought can combine to create catastrophic wildfire
which can have devastating impacts on receiving waters
from these areas.
Overgrazing by domestic animals. Sedimentation
can increase not only due to decreased vegetative
interception of precipitation enhancing erosion, but
also through direct trampling of the streambed and
channel.
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Urbanization and road construction. Road
construction commonly results in a 5 - 20 fold increase
in suspended sediment yield. Impervious materials such
as pavement, parking lots, rooftops, etc. will increase
the velocity of storm water runoff which will increase
erosion once this water comes in contact with soil.
Mining operations. Mines, particularly strip mines,
can lead to extraordinarily high levels of erosion and
subsequent sedimentation in rivers. Some coaling strip
mines in Kentucky have shown 1000X increases in
suspended sediment yields.
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While rivers may be transporters of pollution,
suspended sediment is the “package” many
pollutants are carried in.
Heavy metals, organic pollutants, pathogens, and
nutrients responsible for eutrophication can all
be found attached to sediments in flowing water.
The “quality” or overall pollutant load of
suspended sediment depends on the degree of
pollution in the watershed.
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The load carrying capacity of a stream or river
constantly changes both spatially and temporally as
flow changes.
Flow in any river or stream is never homogenous so the
resulting sediment movement in any section of stream
or river varies greatly.
Sediment Transport Kinetics
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The quantification and predicted rates of
transport of sediment are based upon the
assumption that for any given flow and
sediment, there is a unique transport rate.
Estimates of sediment transport rates are based
upon measures of flow (including velocity,
depth, shear velocity, viscosity, and fluid density)
and both sediment size and density.
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Particle stability depends upon individual particle
weight and cohesiveness versus drag and lift
forces imposed by flow.
Variables interfering with this simplistic
relationship include fluctuations in current
speed, variation in particle shape, density, and
degree of exposure to flow.
The equilibrium that exists between the sheer
stress promoting entrainment and forces
resisting entrainment such as density and gravity
is referred to as the Shields Parameter (Ø)
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Variation in Ø changes with a particles Reynolds
Number.
Reynolds number is the ratio of inertial forces (vsρ) to
viscous forces (μ/L).
The Shields Parameter predicts when a particle with a
given Reynolds Number (Re#) will be transported into
either bed or suspended load.
The Shields Parameter can be visualized as:
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θ = Tc / (ρsed - ρfluid ) Dsed) where Tc = critical sheer
stress, ρsed and ρfluid are the densities of sediment and fluids
respectively, and Dsed is the diameter of the particle.
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Particles that are too heavy to be fully suspended
may roll or slide along the bed (traction load) or
hop as they rebound on impact with the bed.
In the latter case, ballistic trajectories occur and
the particle is said to move by saltation.
Stream competence refers to the heaviest particles a
stream can carry.
Stream competence depends on stream velocity;
the faster the current, the heavier the particle
that can be carried.
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Stream Capacity refers to the maximum amount
of total load (bed and suspended) a stream can
carry.
It depends on both discharge and velocity since
velocity affects the competence and therefore
the range of particle sizes that can be
transported.
Note that as stream volume and discharge
increase so do competence and capacity.
This is not a linear relationship and doubling
discharge and velocity does not automatically
double competence and capacity.
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Stream competence varies as approximately the sixth
power of velocity.
For example, doubling velocity usually results in a 64X
increase in competence.
Tripling the discharge usually results in a 9 – 27 times
increase in capacity.
Most of the work of streams is accomplished during
floods when stream velocity and discharge (and
therefore competence and capacity) are many times
their level compared to periods of quiescent flow.
This work is in the form of bed scouring (erosion),
sediment transport (bed and suspended loads), and
sediment deposition.
Total Suspended Solids and
Turbidity
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Total suspended solids are defined as all solids
suspended in water that will not pass through a
2.0 μm glass-fiber filter (dissolved solids would
be the fraction that does pass through the same
size filter).
The filter is then dried in an oven between 103
and 105o C and weighed.
The increase in weight of the filter represents
the amount of TSS.
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Generally, water with less than 20 mg/L is considered relatively
“clear” , levels between 40 and 80 mg/L tends to be “cloudy”,
while levels over 150 mg/L appears to be “dirty” or “muddy”.
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Related to, but not a proxy for, suspended sediments is
turbidity.
Specifically, turbidity is the quantification of the light
that is scattered or absorbed rather than transmitted
through a water sample.
Turbidity is another measure of water clarity but is not
a measure of dissolved substances that can add color to
water.
Particulates are what add turbidity to water and can
include such things as silt, clay, organic matter, algae
and other microorganisms, or any other particulate
matter that can scatter or absorb light.
The amount of light scattered or absorbed is
proportional to the concentration of particulates in the
sample.
Sediment Sampling
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Suspended sediment concentration in a stream
varies from the water surface to the streambed
and laterally across the stream.
Concentration generally increases from a
minimum at the water surface to a maximum at
or near the streambed.
Depth Integrating Samplers
Point Integrated Samples
Bedload Samples
Bedload Traps
Substrate
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The bed material of a stream determines
roughness and subsequent hydraulics.
Of primary importance to aquatic life and
provides refuge for many different species of
aquatic macroinvertebrates and is one of the
most important aspects of fish habitat.
Substrate Size-Class Composition
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Relatively simplistic methods to determine the
dominant types of bottom material.
The Modified Wentworth Classification is an easy
field technique using marked (every 1 – 2
meters) lead-core ropes to determine dominant
substrate.
Embeddedness
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A field technique to determine the degree to
which larger substrate is surrounded by fine
material.
High levels of embeddedness is often associated
with impairment and benthic anoxia.
Modified Wolman Pebble Count
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Rapid, repeatable technique superior to visual
assessments.
Often combined with embeddedness techniques
for a good representation of how land-based
activities affect stream substrate quality.
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Streams and rivers, over geologic time scales, have shaped
continents.
Anthropogenic causes have caused both excess and
limitations of sediment delivered to streams.