This module deals with the physical effects of both inorganic and organic particles as candidate causes, in terms of excessive levels of suspended deposited & bedded sediment as well as insufficent sediment levels. When sediments are contaminated, the physical effects of sediment and the effects of associated nutrients or contaminants are both evaluated as distinct (but related) candidate causes. For more information on these non-physical effects, see the metals, unspecified toxic chemicals, and nutrients modules.

Although sediment is a natural part of aquatic habitats, the quantity and characteristics of sediments can affect the physical, chemical, and biological integrity of aquatic ecosystems (U.S. EPA 2006a, Wood and Armitage 1997, Waters 1995).

We often categorize stream sediments in terms of particle size, as this influences mobility, substrate quality, and other characteristics. Particle sizes can be roughly sorted as follows:

• < 0.063 mm: fine particles of inorganic clay and silt as well as well-decomposed organic matter typically found in suspended sediments, but may settle in pools or other low-velocity waters;
• 0.063–0.250 mm: fine sands that are suspended during times of high velocity but typically settle as water velocities decrease;
• 0.250–2 mm: small bedload, medium to coarse sands; and
• > 2 mm: coarse bedload, mainly gravels and small cobbles.

Excessive suspended sediment (Figure 2), excessive deposited and bedded sediment (Figure 3), and insufficient sediment (Figure 4) have different modes of action that cause different biological effects. We encourage you to consider these as separate candidate causes, and we have divided most of the material in this module into these three subsections (denoted by links at the top of each page).

The generic conceptual diagram in Figure 1 shows how sources and activities lead to excessive or insufficient sediment and subsequently to biological effects. If you are still deciding what aspects of sediments are of greatest interest, we recommend reading through the conceptual diagram and this entire module. If you already know the attribute of interest, you can use the tabs and links above to skip immediately to the relevant subsection.

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Suspended Sediment

Suspended sediment (SS) is primarily fine inorganic particles of clay and silt (typically < 0.063mm), but 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 effects

Excessive SS should be included as a candidate cause when potential human sources and activities, site evidence, or observed effects support portions of the source-to-impairment pathways in the conceptual diagram for sediment (Figure 1). This diagram and some of the other 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. The list is intended to guide you in collecting evidence to support, weaken, or eliminate SS as a candidate cause. For more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.

Consider including SS as a candidate cause when the following sources and activities, site evidence, or biological effects are present:

Consider contributing, modifying, and related factors as candidate causes when SS is included as a candidate cause:

• Deposited and bedded sediment
• Flow alteration
• Pathogens
• Insufficient 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:

• The water is clear and periods of SS have not occurred based on continuous, long term monitoring.

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Deposited and Bedded Sediment

Deposited and bedded sediments (DBS) are mineral and organic particles that settle out of the water column and collect on the bed of a water body (Figure 3), or that travel primarily by rolling along a stream bed rather than moving in the water column. It 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 such as organic flocs.

Changes in the composition, distribution, or quantity of DBS can alter the behavior, health, or survival of biota by altering benthic habitat quality or availability.

Checklist of sources, site evidence and biological effects

Excessive DBS should be included as a candidate cause when potential human sources and activities, site evidence, or observed effects support portions of the source-to-impairment pathways in the conceptual diagram for sediment (Figure 1). This diagram and some of the other 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 DBS among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate DBS as a candidate cause. For more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.

Consider including DBS as a candidate cause when the following sources and activities, site evidence, or biological effects are present:

Consider contributing, modifying, and related factors as candidate causes when DBS is included as a candidate cause:

• Excess suspended sediment
• Insufficient 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:

• The water is clear, the geologic substrate is bare and not embedded, and it has been this way for more than a year.
• The substrate is composed of only boulders or bedrock.

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Insufficient Sediment

Insufficient 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 a mixture of substrates as habitat to spawn, to avoid being displaced by the force of the water, to position themselves, to avoid predators, or to capture prey. Insufficient habitat can lead to an environment which supports few organisms.

Checklist of sources, site evidence and biological effects

IS should be included as a candidate cause when potential human sources and activities, site evidence, or observed effects support portions of the source-to-impairment pathways in the conceptual diagram for sediment (Figure 1). This diagram and some of the other 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 among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate IS as a candidate cause. For more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.

Consider including IS as a candidate cause when the following sources and activities, site evidence, and biological effects are present:

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 the substrate is composed of mostly small-sized particles.

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