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`CADDIS Volume 2
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`
`Sediments
`
`Overview When to List Ways to Measure
`Literature Reviews
`References
`
`Conceptual Diagrams
`
`Overview
`
`Suspended Sediment
`Deposited and Bedded Sediment
`Insuicient Sediment
`
`This module considers the physical eects of inorganic and organic particles as
`candidate causes. It considers these particles in terms of both excessive suspended,
`deposited and bedded sediment and insuicient sediment. When sediments are
`contaminated, their physical and toxic eects are evaluated as distinct (but related)
`candidate causes. For more information on these non-physical eects, see the Metals,
`Nutrients and Unspecified Toxic Chemicals modules.
`
`

`

`Figure 2. Example of excessive
`suspended sediment in the
`Little Miami River, OH.
`
`Sediment is a natural part of aquatic habitats. However, its quantity and characteristics
`can aect the physical, chemical and biological integrity of aquatic ecosystems (U.S.
`EPA 2006a).
`
`Figure 3. Example of excessive
`deposited & bedded sediment.
`
`We oen categorize stream sediments in terms of particle size, which influences
`mobility, substrate quality and other characteristics.
`Particle sizes can be roughly sorted as follows:
`
`< 0.063 mm: fine inorganic clay and silt and well-decomposed organic matter
`particles that are typically suspended in water column; may settle in pools or other
`low-velocity waters
`0.063–0.250 mm: fine sands that are suspended under high water velocity
`conditions but typically settle as velocities decrease
`0.250–2 mm: small bedload, medium to coarse sands
`> 2 mm: coarse bedload, mainly gravels and small cobbles
`
`

`

`Figure 4. Example of
`insuicient sediment, resulting
`from retention of sediment
`behind dam and periods of
`high flow downstream of
`Skelton Dam, ME.
`
`Figure 1 shows how sources and activities can lead to excessive or insuicient sediment
`and subsequently to biological eects. Excessive suspended sediment (Figure 2),
`excessive deposited and bedded sediment (Figure 3), and insuicient sediment (Figure
`4) have dierent modes of action that cause dierent biological eects. We have
`divided this module into these three subsections, and they should be considered as
`separate candidate causes.
`
`Suspended Sediment
`
`Suspended sediment (SS) is primarily fine inorganic particles of clay and silt (typically <
`0.063 mm. It also may include fine sand (0.63-0.250 mm) and particulate organic matter
`suspended in the water column.
`
`Checklist of Sources, Site Evidence and Biological Eects
`
`Excessive SS should be included as a candidate cause when potential human sources
`and activities, site evidence, or observed eects support portions of the source-to-
`impairment pathways (Figure 1). This diagram and other supporting information also
`may be useful in Step 3: Evaluate Data from the Case.
`
`The checklist below will help you identify key data and information useful for
`determining whether to include excessive SS among your candidate causes. It can help
`guide you in collecting evidence to support, weaken or eliminate SS as a candidate
`cause. For more information on specific entries, go to the When to List tab.
`
`

`

`<https://epa.gov/sites/default/files/2015-12/sed-cd_sim_1000.jpg>
`
`Figure 1. A simple conceptual diagram illustrating causal pathways, from sources to impairments, related
`to sediments. Click on the diagram to view a larger version.
`
`Consider including SS as a candidate cause when the following sources and activities,
`site evidence or biological eects are present:
`
`Sources and Activities
`
`Exposed soil
`
`Autumn plowing
`Livestock grazing
`Devegetated banks or shores
`Logging roads and trails
`Construction
`Road maintenance
`Landslides
`Burned forests
`Erosional rills and gullies
`Stored soil or waste
`
`

`

`In-stream processes
`
`In-stream gravel mining
`Vehicle or boat traic
`Dredging and trawling
`Breached impoundments
`Incised channels
`Channel modification
`Eroding and collapsing stream banks
`Shallow or poorly developed root systems
`Fish activity that resuspends sediments
`Altered flow
`
`Impoundments
`Upstream scoured streambeds
`Impervious surfaces
`Lack of connectivity with floodplain
`
`Site Evidence
`
`Muddy or turbid water
`Visible plume of discolored water
`Spectral imagery of plumes or suspended material
`Deposited sediment
`Embedded substrate
`
`Biological Eects
`
`Changes in fish assemblages, such as fewer fishes that depend on sight for feeding
`(e.g., salmonids, cyprinids, centrarchids)
`Changes in invertebrate assemblages, such as fewer invertebrates with gills (e.g.,
`mayflies) and more filter feeders
`Changes in submerged aquatic vegetation, such as loss of eel grass or reduced
`primary productivity
`
`

`

`Consider contributing, modifying and related factors as candidate causes when SS
`is included as a candidate cause:
`
`Deposited and bedded sediment
`Flow alteration
`Pathogens
`Insuicient light for photosynthesis
`Nutrient enrichment
`Altered physical habitat
`Low dissolved oxygen
`Excess plant growth
`Specified and unspecified toxic chemicals
`
`Consider deferring SS as a candidate cause when, at the site of the impairment:
`
`Water is clear and elevated SS has not been observed based on continuous, long
`term monitoring.
`
`Deposited and Bedded Sediment
`
`Deposited and bedded sediment (DBS) refers to mineral and organic particles that
`settle out of the water column and either collect on the streambed (Figure 3) or travel
`by rolling along the streambed. DBS includes surficial and deeper deposits and bedded
`layers within the depths used by organisms.
`
`Other terms commonly used to describe DBS include: bedded sediment, clean
`sediment, bedload, fines, deposits, soils and eroded materials. The organic
`components include organic solids such as soil organic matter, algal cells, particulate
`detritus and anthropogenic materials (e.g., organic flocs).
`
`Changes in DBS composition, distribution or quantity can alter the behavior, health and
`survival of biota by altering benthic habitat quality and availability.
`
`Checklist of Sources, Site Evidence and Biological Eects
`
`

`

`Excessive DBS should be included as a candidate cause when potential human sources
`and activities, site evidence or observed eects support portions of the source-to-
`impairment pathways (Figure 1). This diagram and supporting information also may be
`useful in Step 3: Evaluate Data from the Case <https://epa.gov/node/79643/>.
`
`The checklist below will help you identify key data and information useful for
`determining whether to include excessive DBS as a candidate cause. The list can help
`you collect evidence to support, weaken or eliminate DBS as a candidate cause. For
`more information on specific entries, go to the When to List tab.
`
`Consider including DBS as a candidate cause when the following sources and
`activities, site evidence or biological eects are present:
`
`Sources and Activities
`
`Also see sources for suspended sediments (SS)
`
`Downstream dams
`Channel modification
`Water withdrawal
`Incised channel
`Widened stream channel
`
`Site Evidence
`
`Slow-moving or stagnant water
`High fish density
`Presence of organic waste
`Foul odor
`Presence of organic suspended solids or floc
`Alkaline, anoxic or warm water
`High plant production (e.g., algal blooms)
`
`Biological Eects
`
`Reduction or absence of ammonia-sensitive species
`
`

`

`Physiological eects (e.g., decreased nitrogen excretion, decreased oxygen binding
`to hemoglobin)
`Behavioral eects (e.g., loss of equilibrium, hyperexcitability, increased breathing)
`Morphological eects (e.g., proliferation of gill lamellae, reduction of lymphoid
`tissue in the spleen, lesions in blood vessels, mucus secretion)
`Organismal and population eects (e.g., decreased growth and abundance, mass
`mortality)
`
`Consider contributing, modifying and related factors as candidate causes when
`DBS is included as a candidate cause:
`
`Excess suspended sediment
`Insuicient light for photosynthesis
`Nutrient enrichment
`Invasive macrophytes
`Low dissolved oxygen
`Pathogens
`Ammonia
`Specified and unspecified toxic chemicals
`Altered physical habitat
`Flow alteration
`
`Consider deferring (or eliminating) DBS as a candidate cause when, at the site of the
`impairment:
`
`Water is clear and geologic substrate is bare and not embedded, based on more
`than one year's monitoring.
`Substrate is composed of only boulders or bedrock.
`
`Insucient Sediment
`
`Insuicient sediment (IS) refers to a reduction of the amount of sediment relative to
`similar streams.
`
`

`

`When sediment resuspension exceeds deposition through an entire stream reach,
`streambed scour and downcutting can occur (Figure 4). Streambed scour can be
`observed downstream of many dams. Many organisms require sediment or substrates
`to spawn, avoid being displaced by streamflow, avoid predators or capture prey.
`Insuicient habitat can lead to an environment that supports few organisms.
`
`Checklist of Sources, Site Evidence and Biological Eects
`
`IS should be included as a candidate cause when potential human sources and
`activities, site evidence, or observed eects support portions of the source-to-
`impairment pathways (Figure 1). This diagram and other supporting information also
`may be useful in Step 3: Evaluate Data from the Case.
`
`The checklist below will help you identify key data and information useful for
`determining whether to include IS as a candidate cause. The list can guide you in
`collecting evidence to support, weaken or eliminate IS as a candidate cause. For more
`information on specific entries, go to the When to List tab.
`
`Consider including IS as a candidate cause when the following sources and activities,
`site evidence or biological eects are present:
`
`Sources and Activities
`
`Upstream dam
`Impervious surfaces
`Channel modification
`Incised channel
`
`Site Evidence
`
`Substrate composed of only large boulders or bedrock
`Concrete or riprap streambed
`
`Biological Eects
`
`

`

`Low abundance or diversity of
`
`Fish species
`Invertebrate species
`Submerged aquatic vegetation
`
`Consider contributing, modifying and related factors as potential candidate causes
`when including IS as a candidate cause:
`
`Flow alteration
`Physical habitat alteration
`
`Consider deferring or eliminating IS as a candidate cause at the impaired site when:
`
`The stream bed is embedded or substrate is composed of mostly small-sized
`particles.
`When to List
`
`Suspended Sediment
`Insuicient Sediment
`
`Deposited and Bedded Sediment
`
`Suspended Sediment
`
`Sources and Activities that Suggest Listing Suspended Sediment as a Candidate
`Cause
`Site Evidence that Suggests Listing Suspended Sediment as a Candidate Cause
`Biological Eects that Suggest Listing Suspended Sediment as a Candidate Cause
`Site Evidence that Supports Excluding Suspended Sediment as a Candidate Cause
`
`Sources and Activities that Suggest Listing SS as a Candidate Cause
`
`Observations of increased supply or delivery of suspended particles indicate that SS
`should be included as a candidate cause. These sources and activities may occur at or
`upstream of the impaired site. Sources that increase SS also influence other stressors.
`Therefore, when considering SS also consider:
`
`

`

`Excess deposited and bedded sediment
`Insuicient light for photosynthesis
`Nutrient enrichment
`Altered physical habitat
`Flow alteration
`
`If nutrients or organic matter are parts of the causal pathway leading to SS, the
`following may also be of concern:
`
`Excess plant growth
`Ammonia
`Pathogens
`Low dissolved oxygen
`
`Increased SS can result from any activity or land use that:
`
`Loosens soil or exposes bare ground, especially during rainy or windy seasons,
`thereby increasing sediment supply
`Increases the force of flowing water over the landscape or in the stream, thus
`increasing delivery of particles into waterbodies and maintaining them in
`suspension
`Increases sediment resuspension or erosion of the beds, banks or shores of
`waterbodies
`
`Figure 5. Aerial photo of extensive
`gully formation on a hog farm
`built on steep slopes.
`
`

`

`EXPOSED SOIL – Bare ground is susceptible to erosion. Specific activities that
`expose soil and make it more erodible include:
`Autumn plowing: Tilling soil in autumn tends to leave fields bare longer and during
`seasons with greater rainfall, thus increasing soil erosion.
`Livestock grazing: Livestock trample banks, shores and beds and can remove or
`disturb vegetation (see Figure 5).
`Devegetated banks or shores: Devegetated banks or shores expose erodible
`materials that can enter the waterbody (see Figure 5).
`Logging roads and trails: Exposed erodible materials can enter streams with runo.
`Logging can make soil susceptible to erosion.
`Construction: Erodible materials that can enter streams with runo are exposed
`during construction. Compacted earth is also less permeable, increasing delivery of
`stormwater (see Figure 6).
`
`Figure 6. Drain pipe half filled
`with sediment from a construction
`site.
`
`Road maintenance: Grading of dirt roads, application of crushed limestone, and
`application of ash and sand to icy roads provide material that may wash into
`streams during snowmelt or storms.
`Land slides: Landslides can deliver sediment directly to streams. Also, erosion can
`carry material from landslides to waterbodies.
`Burned forest: Fires destroy trees, undergrowth and leaf litter that normally
`stabilize soils.
`Gullies: Gullies indicate removal of soil by erosion that may carry sediment to a
`water body.
`
`

`

`Stored soil or waste: Soil or organic wastes stored in outdoor piles may be washed
`into water bodies.
`
`IN-STREAM PROCESSES – Processes or conditions in streams or other waterbodies
`that generate or resuspend sediment include:
`
`Figure 7. Erosional and
`depositional zones near an
`unfenced grazing area and access
`for o-road recreational vehicles
`in Ground House River, MN.
`
`In-stream gravel mining: Gravel mining exposes and suspends sediment.
`Vehicle traic in stream: Vehicles fording streams erode banks and suspend
`sediment (see Figure 7).
`Boat traic: The force from props or dragging of anchors can resuspend sediment
`from the river or lake bottom. Boat wakes erode banks and shores.
`Dredging: The process of dredging resuspends some of the material being dredged.
`Trawling: Dragging equipment and nets along waterbody bottoms resuspends
`sediment.
`Breached impoundment: Upstream impoundments normally enhance settling and
`decrease sediment delivered downstream. However, if the impoundment is
`breached, large amounts of sediment can be rapidly released downstream.
`Incised or widened stream channels: This is indicative of flow conditions that are
`eroding the streambed and transporting sediment from the streambed or bank
`downstream.
`
`

`

`Figure 8. Eroding stream bank
`with no connection to floodplain.
`
`Channel modification: Installation of culverts or bridge pilings, as well as
`deepening, straightening or redirecting channels, can disturb deposited and
`bedded sediment, disrupt stream banks and alter flow regimes.
`Eroding and collapsing streambanks: These are sources of sediment which may
`arise due to undercut or unstable banks (see Figure 8).
`Shallow or poorly developed root systems: Streambanks erode due to many
`factors, but the lack of stabilizing structures (e.g., complex, interwoven roots)
`commonly contributes to bank erosion and increased sediment supply (Figures 7
`and 8).
`Impoundments: Downstream dams create impoundments that increase deposition
`but also can favor production of algae that act as suspended particles.
`Fish activity: Some bottom-feeding fishes (e.g., carp) stir up sediments to find food,
`thereby resuspending fine materials.
`
`ALTERED FLOW – Specific situations that may alter flow and thus increase sediment
`delivery include:
`
`

`

`Upstream scoured streambeds: If the diverse sediment sizes normally present are
`absent from upstream locations, then the material has been suspended and carried
`downstream.
`
`Figure 9. Stream bank eroding
`from impervious parking lot run-
`o.
`
`
`
`This also suggests that hydrologic forces are strong and that there may be other
`erosive forces acting as sediment sources.
`Impervious surfaces: Impervious surfaces in the watershed may increase the
`magnitude and frequency of high flow events, thereby increasing resuspension and
`bed and bank erosion (see Figure 9).
`Lack of floodplain connectivity: Suspended material remains in suspension rather
`than being deposited on floodplains where velocity is slowed compared to the main
`channel. Furthermore, deepened channels or intermittent barriers to floodplains
`can cause streams to cut into the streambed, eroding bed material and deepening
`the channel. Ultimately, banks are undercut and collapse, hillsides collapse and the
`overall stream is widened. During these processes, new sediment is suspended in
`the stream.
`
`Site Evidence that Suggests Listing SS as a Candidate Cause
`
`

`

`Figure 10. Iron River, WI, near its
`confluence with Lake Superior,
`discolored with iron-rich silt.
`
`In addition to observations of sources discussed above, direct observation of increased
`SS (especially at impaired sites) may be used as evidence of spatial or temporal co-
`occurrence. Site observations suggesting that conditions are conducive for elevated SS
`may be used as evidence of a complete causal pathway. Some examples include:
`Muddy or turbid water: Unlike chemical contaminants, we can see SS. If water is cloudy
`or muddy, there is an excess of some sort of material suspended in the water (see
`Figure 10). If water is tinged with green, golden or brown coloration, there may be
`phytoplankton that may act as particles similar to clay. One caution, colored
`substances dissolved in water (e.g., humic acid) can be mistaken for SS (see Figure 11).
`
`Figure 11. Naturally occuring
`humic substances staining water
`in a stream in Maine.
`
`

`

`Visible plumes: Discontinuous color between water in the channel and another
`source (e.g., tributary, outfall, ditch) suggests that SS should be considered. Spectral
`imagery of plumes or suspended material from aerial photography (see Figure 12)
`and hyperspectral imagery can document plume location and suspended material
`concentrations in deeper waterbodies.
`Deposited sediment: Sediment on substrates or aquatic plants, or a muddy
`bottom, suggest that SS was present but has since been deposited.
`Embedded substrate: Embedded substrate suggests that SS was present but has
`since been deposited.
`
`Biological Eects that Suggest Listing SS as a Candidate Cause
`
`In general, biological eects due to sediment and related stress are not suiciently
`specific to be considered symptomatic of sediment.
`
`Figure 12. Aerial image of
`deposited sediment in the center
`of a channel and suspended
`sediment reflecting a tan color in
`Little Miami River, OH.
`
`Because biological eects depend on the natural history of the species in your region,
`our advice is general and is intended to encourage consideration of local fauna and
`flora.
`
`

`

`Changes in fish assemblages: Fishes that rely on sight to locate and pursue prey
`(e.g., salmonids, cyprinids and centrarchids) may be less successful in turbid water.
`Others, like catfish and suckers, are tolerant because they hunt using olfactory or
`tactile sensations. Reduced feeding may result in reduced growth and local
`extirpation. Suspended particles can damage gills, resulting in increased
`susceptibility to low dissolved oxygen or to pathogens, and in extreme cases death.
`However, relative sensitivities to this mechanism are unknown.
`Changes in invertebrate assemblages: Filter-feeding invertebrates (e.g., net-
`spinning caddisfly larvae) and species with gills (e.g., mayflies) are particularly
`sensitive to SS. However, if the suspended sediments consist primarily of algae or
`other organic particles, filter feeders may thrive.
`Changes in submerged aquatic vegetation: Submerged aquatic vegetation may
`die out due to reduced light penetration and scouring from suspended sediments.
`
`Site Evidence that Supports Excluding SS as a Candidate Cause
`
`Additional site observations can support deferring analysis of SS as a candidate cause.
`High quality in-stream measurements and the absence of sources or activities that may
`increase SS may suggest that SS should not be listed as a candidate cause. When water
`is clear, rocky substrates are consistently bare and not more embedded than
`unimpaired sites, and these conditions have persisted for more than a year, you may
`choose to defer analyzing SS as a candidate cause.
`
`General advice on excluding candidate causes from the list is provided in Step 2.2 of the
`Step-by-Guide and in Tips for Listing Candidate Causes.
`
`Deposited and Bedded Sediment
`
`Sources and Activities that Suggest Listing Deposited and Bedded Sediment as a
`Candidate Cause
`Site Evidence that Suggests Listing Deposited and Bedded Sediment as a Candidate
`Cause
`Biological Eects that Suggest Listing Deposited and Bedded Sediment as a
`Candidate Cause
`
`

`

`Site Evidence that Supports Excluding Deposited and Bedded Sediment as a
`Candidate Cause
`
`Sources and Activities that Suggest Including DBS as a Candidate
`Cause
`
`Nearly all DBS begins as suspended sediment. Therefore, all sources of suspended
`sediment and activities that suspend sediment are indirect sources of deposited and
`bedded sediment. They are listed in Sources and Activities that Suggest Including SS as
`a Candidate Cause <https://epa.gov/node/124889/>.
`
`Figure 13. Channel modification
`with silted culvert.
`
`This section is limited to sources of deposition.
`Downstream dams: Impoundments can reduce water velocity in upstream reaches,
`resulting in settling and accumulation of sediment.
`Channel modification: Installation of culverts or bridge pilings, or straightening
`and redirection of channels can increase local deposition of sediment (see Figure
`13).
`Water withdrawal: Reduction of water volume in a flowing system reduces the
`velocity, thus increasing settling of sediment.
`
`Site Evidence that Suggests Including DBS as a Candidate Cause
`
`Since suspended sediment may be deposited, Site Evidence that Suggests Including SS
`as a Candidate Cause are also suggestive of DBS <https://epa.gov/node/124889/>.
`
`

`

`Figure 14. Chesapeake Bay
`grasses with silt deposition.
`
`Observations at the site suggesting that conditions are conducive for the occurrence of
`DBS may be used as evidence of a complete causal pathway. In addition, direct
`observation of increased DBS, especially at the impaired site, may be used as evidence
`of spatial or temporal co-occurrence. Some examples include:
`Silt: Fine sediment on aquatic plants and rocks suggests that sediment has been
`deposited (see Figure 14).
`Embedded substrate: This suggests that sediment has been deposited, bedded,
`and retained at the site.
`Discolored underside of rocks: Cobble or rocks with a black rim indicates that the
`substrate is suiciently embedded to cause anoxic conditions.
`Deposits of sediment: Sediment bars of mud, muck, or sand, and filling of pools
`with sediment are direct evidence of DBS (see Figure 15).
`Slow-moving water: When the force of water is slight, particles settle and
`substrates may not be periodically cleared of excess DBS.
`
`Biological Eects that Suggest Including DBS as a Candidate Cause
`
`Figure 15. Sand bar deposited
`below bridge, as well as undercut
`
`

`

`and collapsed stream bank.
`
`Biological eects of DBS depend on the natural history of species in your region.
`Therefore, our advice is general and is intended to encourage consideration of local
`fauna and flora. However, some biological eects may suggest DBS as a candidate
`cause.
`Biological eects of DBS are in general not suiciently specific to be considered
`symptomatic. Therefore, when considering DBS also consider:
`
`SS
`Other types of habitat alterations
`Flow alteration
`Insuicient light for photosynthesis
`Nutrients
`Pathogens and contaminants that migrate with particles
`Excess growth of plants that root in the deposited and bedded sediments
`Ammonia that forms in anoxic sediments
`Low dissolved oxygen in poorly aerated sediments
`
`
`

`

`Figure 16. Spawning salmon are
`intolerant of turbid and silt laden
`gravels.
`
`Changes in composition of fish assemblages: Populations of salmonids and other
`lithophilic species may decline when spawning substrates are embedded or buried
`in sediment (see Figure 16). Filled interstitial spaces of gravel can prevent gas
`exchange and asphyxiate embryos or trap sac fry. Other species, such as darters
`require coarse gravels as habitat for all stages of life.
`Changes in composition of invertebrate assemblages: Organisms that prefer
`coarse gravel are reduced [e.g., EPT (Ephemeroptera, Plecoptera, Tricoptera) (see
`Figure 17)], whereas some burrowing species increase in response to sediment
`deposits. Other burrowing organisms, such as threatened and endangered unionid
`mussels, live in deeper gravels and cannot survive in embedded substrates.
`Changes in submerged aquatic vegetation: Submerged aquatic vegetation may
`die out due to reduced light when they are covered with sediment. Alternatively,
`DBS may provide a suitable habitat for rooting of pioneering species that would not
`normally occur in a particular aquatic system.
`
`Site Evidence that Supports Excluding DBS as a Candidate Cause
`
`Figure 17. Caddisfly larvae,
`shown in their protective cases,
`
`

`

`shown in their protective cases,
`are intolerant of sediment
`deposition.
`
`Additional site observations can support excluding DBS as a candidate cause. When the
`water is clear, the rocky substrates are consistently bare, and these conditions have
`persisted for more than a year, you may choose to defer analysis DBS. Further, when the
`substrate is composed of boulders or bedrock that suggests that insuicient sediment
`is a candidate cause, rather than excess, you may choose not to list DBS.
`Insuicient sediment
`
`General advice on excluding candidate causes from your initial list of candidate causes
`is provided in Step 2.2 of the Step-by-Guide and in Tips for Listing Candidate Causes,
`and includes the use of high quality in-stream measurements and the absence of
`evidence of sources or activities that may increase DBS.
`
`Step 2: List Candidate Causes <https://epa.gov/caddis-vol1/caddis-volume-1-stressor-
`identification-step-2-list-candidate-causes#tab-2>
`Tips for Listing Candidate Causes <https://epa.gov/node/151513/>
`
`Insucient Sediment
`
`Sources and Activities that Suggest Listing Insuicient Sediment as a Candidate
`Cause
`Site Evidence that Suggests Listing Insuicient Sediment as a Candidate Cause
`Biological Eects that Suggest Listing Insuicient Sediment as a Candidate Cause
`Site Evidence that Supports Excluding Insuicient Sediment as a Candidate Cause
`
`Sources and Activities that Suggest Including IS as a Candidate Cause
`
`Recorded or direct observations of decreased supply, delivery, or retention of sediment
`indicate that IS should be included as a potential candidate cause. These sources and
`activities may occur at or upstream from the impaired site thereby removing or
`interfering with sources that would normally supply sediment. Situations that restrict
`settling of sediment or remove sediment by forceful scouring also can lead to IS,
`sometimes referred to as “sediment starvation.” When IS is suspected, the flow of water
`
`

`

`is usually strong and may be a direct cause or may modify the habitat in other ways. So,
`when including IS, also consider including flow alteration and physical habitat structure
`as candidate causes.
`
`Upstream dam: Sediment settles out of suspension in the low velocity pool
`upstream of dams and is not transported downstream creating a sediment deficit.
`Forceful release from dams can also scour sediments.
`Impervious surfaces: During storms, water is delivered to streams more quickly
`and with greater force when it runs o impervious surfaces. The force of the water
`may be suicient to scour sections of stream bed and interfere with settling of
`particles.
`Channel modification: Some channel modifications, particularly channel
`straightening, can alter hydrology and geomorphology of streams resulting in areas
`of scour.
`Incised stream channels: This is indicative of flow conditions that are eroding the
`streambed.
`
`Site Evidence that Suggests Including IS as a Candidate Cause
`
`In addition to observations of sources discussed above, direct observation of IS at the
`impaired site may be used as evidence of either spatial or temporal co-occurrence.
`
`Figure 18. Scoured bedrock
`devoid of deposited sediment.
`
`Insuicient sediment should be listed as a candidate cause if:
`The substrate is composed of only boulders or bedrock (see Figure 18).
`The streambed is lined with bare concrete or riprap.
`
`Biological Eects that Suggest Including IS as a Candidate Cause
`
`

`

`In general, biological eects due to insuicient sediment and related stress are not
`suiciently specific to be considered either symptomatic or more suggestive of
`sediment over other causes. Therefore, when there are very low abundance and
`diversity of species, you should also consider exposures to toxic substances, extremes
`of temperature, low dissolved oxygen, or flows that dislodge organisms.
`
`Low abundance or diversity of fish species, invertebrate species, and submerged
`aquatic vegetation may occur when IS is a cause, because there is no suitable substrate
`habitat for shelter or reproduction, few prey, or no rooting substrate.
`
`Because biological eects depend on the natural history of species from a geographical
`region, our advice is general and is intended to encourage consideration of local fauna
`and flora.
`
`Site Evidence that Supports Excluding IS as a Candidate Cause
`
`Insuicient sediment is a relatively rare cause and is more likely to be overlooked than
`to be included without reason.
`
`The presence of abundant sediment over a long period of time would support deferring
`insuicient sediment as a candidate cause. Additional discussion of excluding
`candidate causes is provided in:
`
`Step 2 of the Step-by-Step Guide <https://epa.gov/node/83171/>
`Tips for Listing Candidate Causes <https://epa.gov/node/151513/>
`Ways to Measure
`
`Ways to Measure Suspended Sediment
`Ways to Measure Deposited and Bedded Sediment
`Ways to Measure Insuicient Sediment
`
`You may encounter a variety of existing sediment-related data expressed in units or
`types of measurements listed below [see Edwards and Glysson (1998) for field methods
`for measurement of fluvial sediment]. In addition, measures of channel structure are
`included here because they can be informative regarding sources and mechanisms that
`alter sediment supply.
`
`Ways to Measure Suspended Sediment
`
`

`

`Turbidity: The amount of light transmission due to absorption and scattering as
`aected by suspended sediments [nephelometric turbidity units (NTU)].
`Total suspended solids (TSS, also termed total filterable solids): Suspended organic
`and inorganic solids that are not in solution and which can be removed by filtration
`(mg/L).
`Suspended sediment concentration (SSC): Dry weight of sediment from a known
`volume of water-sediment mixture (clay, silt, sand and organic matter) (mg/L).
`Light penetration: Amount of light that can reach various depths of water due to
`attenuation [Secchi depth (m) or extinction coeicient].
`Water clarity: Qualitatively reported observations of transparency of water.
`
`Ways to Measure Deposited and Bedded Sediment
`
`Measures of DBS include:
`
`Bedload sediment/bedload transport: Proportion of total sediment rolling,
`sliding, and bouncing along the stream bottom and being transported downstream.
`The proportion of bottom sediments moving as bedload will depend on, at a
`minimum, stream power, the sizes and size distribution of the available bottom
`sediment particles, the size and quantity of woody debris in the streambed, and the
`rile/pool structure of the stream. Bedload is measured several ways and is usually
`expressed in kg/day (see U.S. EPA 2006b).
`Percent fine sediment at surface: Proportion of fine sediment on substrate surface
`(percent fines at surface, % fines).
`Percent fine sediment at depth: Proportion of fine sediment to a certain depth of
`substrate (percent fines at X cm of depth).
`Sedimentation rate: Amount of suspended sediment that settles onto substrate per
`unit time, typically reported as grams per square meter of substrate per day
`(g/m /d).
`2
`Embeddedness: Degree to which interstitial spaces between particles in coarse
`substrates are filled by finer particles (% embeddedness).
`Suspendable solids: Amount of fine sediment re-suspended upon disturbance of
`streambed, usually described in grams per square meter of substrate (g/m ).
`2
`
`

`

`Settleable solids: Essentially the same as suspendable solids, equal to amount of
`fine sediment suspended during disturbance of streambed (typically during period

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