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Understanding Significance - Full EAF (Part 3)

Full Environmental Assessment Form (FEAF) Workbook

The key characteristics of possible impacts that should be considered in determining significance are "magnitude", "duration", and likelihood (probability). Magnitude assesses factors such as severity, size, or extent of an impact. Duration looks at how long an impact may last. Importance relates to how many people are going to be impacted or affected by the project or what role a resource, including natural resources, may have in the community. Likelihood measures how probable it is that the impact may occur. Generally, bigger impact (larger "magnitude") projects are more likely to need more detailed analysis. Taken together, magnitude, duration, and likelihood; along with the context and scale of the proposal; define an impact's importance."Importance" requires us to look at an impact in relation to the whole action. The short or long term or cumulative nature of the impacts also needs to be considered. Each impact of an action must be judged by these characteristics within the context of the site and community.

Magnitude, duration, and likelihood of an impact occurring are described as follows:

Magnitude:

For each potential impact being evaluated in Part 3, decide how large of an impact there might be. Magnitude reflects both the area of land as well as the amount of a particular resource or the number of people being impacted. Magnitude is conveyed as 'moderate' or 'large'. It is assumed that all impacts that would be considered 'small' in magnitude were identified as such in Part 2 and therefore not considered in Part 3.

  • Moderate Impact: These are impacts that are of a size that will likely result in more impacts on one or more environmental resources but are more localized, and not regional in nature. Moderate impacts can occur when the project affects a portion of a parcel or even a larger area extending to a small area just beyond the parcel. Moderate environmental impacts may be either isolated (only in one location), or of neighborhood concern. An impact of moderate magnitude would likely affect a moderate number of people. Size in acreage or people affected is not the only aspect of magnitude, however. If a project affects a small area of land, but the resource being impacted is locally rare, for example, then the actual impact may be large. When reviewing an impact's magnitude, the reviewing agency should consider the size of the impact and resource, as well as the scope and context of the project. A proposed project that impacts a small number of people may also be considered a moderate impact. The resources affected by a moderated impact may often have broad local concern and often are activities or resources that are regulated or protected by some local, state, or national agency.
  • Large Impact: These are impacts that may cover larger areas beyond the parcel in the neighborhood or community or impact larger numbers of people. As described above related to a moderately sized impact, size in acres is not the only aspect of this either. Impacts on large areas of land, or on a large number of people however, would usually be classified as a 'large' impact. The resources affected by a large impact often have broad local or regional concern and often are activities or resources that are regulated or protected by some local, state, or national agency.
Duration:

For each potential impact being evaluated in Part 3, decide if it will be short-term, medium-term, long-term, or irreversible in its duration. Duration refers to how long an impact is likely to last.

  • Short-term Impact: Some actions may have short-term impacts. These are often due to the initial land disturbance or construction phase. Short-term impacts can occur for a few days, weeks or several months, and then improve quickly. In this case, short-term impacts may be of minor or negligible importance in a long time frame. It is very important to evaluate the duration of an impact in the context and scope of a project. A short-term impact in one situation may not be significant, but in other cases, may be very significant.
    • An example of a short-term impact would be stock-piling topsoil and placement of erosion control methods in one location during construction of a structure. After construction, the topsoil would be graded and re-seeded or landscaped. Short-term impacts would occur due to the initial disturbance of soil and vegetation, but within several weeks, it would be replaced.
  • Medium-term Impact: Some actions may have impacts that last longer but that are still not permanent or irreversible. Medium-term impacts can be measured in months, over several seasons, or perhaps a few years, but have an end-point where the conditions improve and adverse impacts dissipate. Depending on the context and scale of the project, as well as the other features evaluated in Part 3, medium-term impacts could have minor or large significance.
    • An example of a medium-term impact might be construction of an access way using a single culvert over a small, non-regulated stream that has wooded stream banks. Construction of the culvert and driveway will require removal of some additional stream-side vegetation and disturbance to the water flow. Thus it could affect water temperature (by removal of the trees), increase turbidity, change water flow, and reduce habitats for fish and invertebrates. In this example, there could be both short-term and medium-term impacts. After construction, the water flow and turbidity issues would dissipate, but the changes to the stream bank and stream bottom habitats could last months or seasons before the vegetation returns and habitats re-formed. If the applicant included stream bank and stream bottom restoration, use of best management practices for stream corridors, and re-planting of deciduous trees, then the adverse impacts could be moderated in duration.
  • Long-term Impact: These are impacts that last for years, or last as long as the activity that generates the impact continues to take place. Some projects continually impact the environment in an adverse way while the activity takes place, but then the environment improves if the operation ceases. Other actions may occur only for a short period of time, but the impacts last a very long time and it takes years for the environment to recover. Examples might be:
    • Adverse changes in air quality while a manufacturing use operates, or continual production of noise levels above ambient levels while the use operates. Should the manufacturing cease operations, the air pollution and noise impacts end. Removal of large acreages of forest lands on a portion of a parcel to be planted in grass would likely be considered long term impact, even though the forest might regenerate if maintenance of the lawn stopped and trees were allowed to re-grow.
    • A chemical spill that pollutes water or soils that would take decades before the natural resources are recovered.
    • A large residential construction project with multiple phases could last a decade once built, actual construction sequences might be deemed moderate, but the long lasting effect of the constructed property may be viewed as long term.
  • Irreversible Impact: These are impacts that occur where the environment can't return to its original state at any time or in any way. Use of nonrenewable resources may be irreversible since it is unlikely that the resource can be used again. Impacts that generally commit future generations to similar uses may also be considered irreversible impacts. Projects where there is no potential for future restoration are also considered irreversible. In some cases, there may be difficulty distinguishing between a long-term impact and one that is irreversible, but generally, irreversible impacts are those that permanently result in an adverse change.
    • Examples of irreversible impacts include:
      • The extinction of an animal or plant species
      • Conversion of prime farmland soils to residential use
      • Construction of a structure that permanently alters a scenic view in a negative way
  • Other impacts may not fit neatly in the short, medium or long term categories because they may be continuous, or intermittent. The reviewing agency should use their best judgment to determine the category that fits the duration of the potential impact.
Likelihood:

For each potential impact being evaluated in Part 3, the reviewing agency will need to decide if the impact will be unlikely to occur, will possibly occur, or will probably occur. Given the nature of the project, some impacts may be very likely to occur while others may possibly occur, and others are unlikely to occur. The reviewing agency may decide that unlikely impacts may be of large magnitude or long duration but are ultimately not significant because they are so unlikely to actually occur. In other cases, an unlikely impact may carry such a high risk that the reviewing agency may decide it is very significant.

  • Unlikely to Occur: These are impacts that have a very low chance of occurring now or in the future.
    • An example of an impact that is unlikely to occur could be a spillage of a toxic chemical used in a manufacturing process. There is an extremely low probability of this occurring, in part because of protocols used in handling such materials.
  • Possibly will Occur: These are impacts that are possible, but not likely occur.
    • An example of an impact that possibly could occur would be the growth inducing aspects of a new 100-lot subdivision development in a city that has had very slow growth and is not near an urbanized area. The residential development may create consumer demands that will influence and promote development in another location in the community. There is the potential for impacts to the community long-term, but may possibly occur given the character and economy of the area.
  • Probably will Occur: These are impacts that are very likely to occur
    • An example of an impact that probably will occur would be loss of fisheries due to a dredging operation throughout a water body that supports warm water fish species that require shallow water to survive.

The importance of an impact is more subjective and is based on the combination of magnitude, duration, likelihood, the specific environmental setting where the activity is proposed, and on the values, history, and preferences of the community. After evaluating all of these characteristics of a potential impact, a community may decide that it is not important and therefore, may determine that the impact is not significant. For example, a project that adds many lighting fixtures and parking lots in an urban neighborhood having many existing lights and parking areas may be deemed to be not important. Conversely, An example of an impact that is very important could be a project in a community whose economy is based upon tourism related to fishing cold-water species. The impact of a project that directly discharges warm water to that cold water stream would be very important.

Beginning the evaluation

Once the reviewing agency understands these concepts, it is time to begin evaluating each of the moderate to large impacts identified in Part 2.


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