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CADDIS Volume 2: Sources, Stressors & Responses

Sediments

Suspended sediment

Sources and activities that suggest Including SS as a candidate cause

Recorded or direct observations of increased supply or delivery of suspended particles indicate that SS should be included as a potential candidate cause. These sources and activities may occur at or upstream from the impaired site. Sources that increase SS also influence other stressors. Therefore, when considering SS also consider: excess deposited and bedded sediment, insufficient light for photosynthesis, candidate causes associated with nutrient enrichment, altered physical habitat, and flow alteration. If nutrients or organic matter are parts of the causal pathway leading to SS, excess plant growth, ammonia, pathogens, and low dissolved oxygen also may be of concern.

Increased SS can result from any activity or land use that:

  • Loosens soil or leaves bare ground, especially during rainy or windy seasons, thereby increasing the readily available supply of sediment,
  • Increases the force of flowing water over the landscape or in the stream, thus increasing the delivery of particles into water bodies or maintaining them in suspension, or
  • Increases sediment resuspension or erosion of the beds, banks, or shores of water bodies

EXPOSED SOILBare ground is susceptible to erosion. Specific activities that expose soil and make it more erodible include:

Gully formation on a hog farm built on steep slopes.  Photo by Susan Cormier, USEPA.
Figure 5. Aerial photo of extensive gully formation on a hog farm built on steep slopes.
Courtesy of S.M. Cormier, U.S. EPA.
Drain pipe half filled with sediment from construction site.  Photo by Susan Cormier, USEPA.
Figure 6. Drain pipe half filled with sediment from a construction site.
Courtesy of S.M. Cormier, U.S. EPA.

Autumn plowing: Tilling soil in the 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 (Figure 5).

Devegetated banks or shores: Devegetated banks or shores expose erodible materials that can enter the water body (Figure 5).

Logging roads and trails: Exposed erodible materials can enter streams with runoff. Logging can make soil susceptible to erosion.

Construction: Erodible materials that can enter streams with runoff are exposed during construction. In addition, compacted earth is less permeable, increasing delivery of storm water (Figure 6).

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: Land slides can deliver sediment directly to streams. Also, erosion can carry material from land slides to water bodies.

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 PROCESSESProcesses or conditions in streams or other water bodies that generate or resuspend sediment include:

In-stream gravel mining: Gravel mining exposes and suspends sediment.

Off road vehicle tracks entering stream.  Photo by Susan Cormier, USEPA.
Figure 7. Erosional and depositional zones near an unfenced grazing area and access for off-road recreational vehicles in Ground House River, MN.
Courtesy of S.M. Cormier, U.S. EPA.

Vehicle traffic in stream: Vehicles fording streams erode banks and suspend sediment (FIgure 7).

Boat traffic: The force from props or dragging of anchors can resuspend sediment from the river or lake bottom and wakes erode banks and shores.

Dredging: The process of dredging resuspends some of the material being dredged.

Trawling: Dragging equipment and nets along the bottom of water bodies resuspends sediment.

Breached impoundment: Upstream impoundments normally enhance settling and thus 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 stream bed and transporting sediment from the streambed or bank downstream.

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 bank with no connection to flood plain.  Photo by Susan Cormier, USEPA.
Figure 8. Eroding stream bank with no connection to flood plain.
Courtesy of S.M. Cormier, U.S. EPA.

Eroding and collapsing stream banks: These are sources of sediment which may arise due to undercut or unstable banks (Figure 8).

Shallow or poorly developed root systems: Stream banks erode due to a number of factors, but the lack of stabilizing structures such as complex, interwoven roots commonly contribute 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 which act as suspended particles.

Fish activity: Some bottom-feeding fish such as carp stir up sediments to find food, resuspending fine materials in the process.

ALTERED FLOW – Specific situations which may alter flow, and thus increase the delivery of sediment, include:

Stream bank eroding from impervious parking lot run-off.  Photo by Susan Cormier, USEPA.
Figure 9. Stream bank eroding from impervious parking lot run-off.
Courtesy of S.M. Cormier, U.S. EPA.

Upstream scoured stream beds: If the diverse sizes of sediment that would normally be present are absent from upstream locations, then the material has been suspended and carried downstream. It also suggests that hydrologic forces are strong and that there may be other erosive forces that would act as sources of sediment.

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 (Figure 9).

Lack of connectivity with flood plain: Suspended material remains longer in suspension rather than being deposited in flood plains where velocity is slowed compared to the torrent of the flooding waterway. Furthermore, deepened channels or intermittent barriers to flood plains can cause streams to cut down into the stream bed, 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.

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Site evidence that suggests including SS as a candidate cause

In addition to observations of sources discussed above, direct observation of increased SS (especially at impaired sites) may be used as evidence of either spatial or temporal co-occurrence. Observations at the site suggesting that conditions are conducive for the occurrence of 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 the water is cloudy or muddy, that is, not transparent for any reason, there is an excess of some sort of material suspended in the water (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 (Figure 11).

River discolored with iron-rich silt.  Photo by Anett Trebitz and John Morrice, USEPA, 1999.
Figure 10. Iron River, WI, near its confluence with Lake Superior, discolored with iron-rich silt.
Courtesy of A. Trebitz and J. Morrice, U.S. EPA.
Naturally occuring substance staining water in a stream.  Photo by Susan Cormier, USEPA.
Figure 11. Naturally occuring humic substances staining water in a stream in Maine.
Courtesy of S.M. Cormier, U.S. EPA.
Deposited sediment in the center of a channel and suspended sediment reflecting a tan color.  Photo by Susan Cormier, USEPA.
Figure 12. Aerial image of deposited sediment in the center of a channel and suspended sediment reflecting a tan color in Little Miami River.
Courtesy of S.M. Cormier, U.S. EPA.

Visible plumes: Discontinuous color between the 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 (Figure 12) and hyperspectral imagery can document location of plumes and concentrations of suspended material in deeper waterbodies.

Deposited sediment: Sediment on substrates or aquatic plants, or a muddy bottom suggests that SS was present but has since been deposited.

Embedded substrate: Embedded substrate suggests that SS was present but has since been deposited.

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Biological effects that suggest including SS as a candidate cause

In general, biological effects due to sediment and related stress are not sufficiently specific to be considered symptomatic of sediment. Because biological effects 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 composition of fish assemblages: Fish that rely on sight to locate and pursue prey may be less successful in turbid water (e.g., salmonids, cyprinids, and centrarchids). 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 composition of 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. When the water is clear, the 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, and includes the use of high quality in-stream measurements and the absence of sources or activities that may increase SS.

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Deposited and bedded sediment

Sources and activities that suggest including DBS as a candidate cause

Channel modification with silted culvert.  Photo by Susan Cormier, USEPA.
Figure 13. Channel modification with silted culvert.
Courtesy of S.M. Cormier, U.S. EPA.

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. 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 (Figure 13).

Water withdrawal: Reduction of water volume in a flowing system reduces the velocity, thus increasing settling of sediment.

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 Bay grasses with silt deposition.  Photo by Rich Batiuk, USEPA.
Figure 14. Chesapeake Bay grasses with silt deposition.
Courtesy of R. Batiuk, U.S. EPA.
Sand bar deposited below bridge, as well as undercut and collapsed stream bank.  Photo by Susan Cormier, USEPA.
Figure 15. Sand bar deposited below bridge, as well as undercut and collapsed stream bank.
Courtesy of S.M. Cormier, U.S. EPA.

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. 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 (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 sufficiently 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 (Figure 15).

Slow-moving water: When the force of water is slight, particles settle and substrates may not be periodically cleared of excess DBS.

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Biological effects that suggest including DBS as a candidate cause

Salmon.  Photo by Robert T. Lackey, USEPA.
Figure 16. Spawning salmon are intolerant of turbid and silt laden gravels.
Courtesy of R.T. Lackey, U.S. EPA.
Caddisfly cases in stream channel.  Photo by D.J. Norton.
Figure 17. Caddisfly larvae, shown in their protective cases, are intolerant of sediment deposition.
Courtesy of D.J. Norton, U.S. EPA.

Biological effects 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 effects may suggest DBS as a candidate cause.

Biological effects of DBS are in general not sufficiently specific to be considered symptomatic. Therefore, when considering DBS also consider: SS, other types of habitat alterations, flow alteration, insufficient 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, and low dissolved oxygen in poorly aerated sediments.

Changes in composition of fish assemblages: Populations of salmonids and other lithophilic species may decline when spawning substrates are embedded or buried in sediment (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)  (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

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 insufficient sediment is a candidate cause, rather than excess, you may choose not to list DBS.

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.

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Insufficient sediment

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 off impervious surfaces. The force of the water may be sufficient 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 stream bed.

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Scoured bedrock devoid of deposited sediment.  Photo by Susan Cormier, USEPA.
Figure 18. Scoured bedrock devoid of deposited sediment.
Courtesy of S.M. Cormier, U.S. EPA.

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. Insufficient sediment should be listed as a candidate cause if:

  • The substrate is composed of only boulders or bedrock (Figure 18), or
  • The streambed is lined with bare concrete or riprap.

Biological effects that suggest including IS as a candidate cause

In general, biological effects due to insufficient sediment and related stress are not sufficiently 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 effects 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

Insufficient 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 insufficient sediment as a candidate cause. Additional discussion of excluding candidate causes is provided in Step 2 of the Step-by-Step Guide and in Tips for Listing Candidate Causes.

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