CADDIS Volume 2: Sources, Stressors & Responses
Sediments
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Authors: R.L. Spehar, S.M. Cormier, D.L. Taylor
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.
![River with opaque water due to suspended sediment. Photo by Susan Cormier, USEPA.](images/cc2-1.jpg)
Courtesy of S.M. Cormier, U.S. EPA.
![Embedded stream bed. Photo by Susan Cormier, USEPA.](images/cc2-2.jpg)
Courtesy of S.M. Cormier, U.S. EPA.
![Retention of sediment behind dam and periods of high flow result in large diameter substrate and no fine substrate. Photo by Susan Cormier, USEPA.](images/cc2-3.jpg)
Courtesy of S.M. Cormier, U.S. EPA.
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:
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 gravel mining
- Vehicle or boat traffic
- 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
- Impoundments
- Upstream scoured stream beds
- Impervious surfaces
- Lack of connectivity with flood plain
Site Evidence
- Muddy or turbid water
- Visible plume of discolored water
- Spectral imagery of plumes or suspended material
- Deposited sediment
- Embedded substrate
Biological Effects
- Changes in composition of fish assemblages (e.g., reductions in taxa that depend on sight for feeding such as salmonids, cyprinids, centrarchids)
- Changes in composition of invertebrate assemblages (e.g., reductions in taxa with gills such as mayflies, increases in filter feeders)
- Changes in composition and abundance of submerged aquatic vegetation (e.g., loss of eel grass, 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
- 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.
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:
Sources and Activities
Also see sources for suspended sediments (SS)
- Downstream dams
- Channel modification
- Water withdrawal
- Incised channel
- Widened stream channel
Site Evidence
- Silt on plants or rocks
- Embedded substrate
- Discolored underside of rocks
- Deposits of sediment
- Lack of pools
- Sand bars
- Slow moving water
Biological Effects
- Changes in composition of fish assemblages (e.g., reductions in species that depend on coarse substrate for reproduction or feeding activities such as salmonids and darters)
- Changes in composition of invertebrate assemblages (e.g., reductions in species that prefer coarse substrate such as some Unionid mussels)
- Changes in abundance of macrophytes (e.g., reductions in submerged aquatic vegetation and increases in pioneering species)
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.
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:
Sources and Activities
- Upstream dam
- Impervious surfaces
- Channel modification
- Incised channel
Site Evidence
- Substrate composed of only large boulders or bedrock
- Streambed is concrete or riprap
Biological Effects
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:
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.