Penobscot River

Classification of Maine Waters

Purpose of Classification

Maine has had a water classification system since the 1950's. This classification system establishes water quality goals for the State. The classification system is used to direct the State in the management of its surface waters, protect the quality of those waters for their intended management purposes, and where standards are not achieved, direct the State to enhance the quality to achieve those purposes. The classification standards establish designated uses, related characteristics of those uses, and criteria necessary to protect the uses, and establish specific conditions for certain activities such as the discharge of wastewater.

While it is desirable for the actual quality of a water to achieve the standards in any proposal to upgrade a classification, upgrades may be proposed where there is a reasonable expectation for higher uses and quality to be attained. Upgrades to classification may be appropriate where it is socially or ecologically desirable to attain higher standards and where the technological and financial capacity exists to achieve those higher standards within a reasonable time. Once a classification assignment is made, and the uses and criteria are achieved, that goal is protected by the antidegradation provisions of the water quality statute (38 M.R.S. §464(4)(F)). Thus, the law provides a mechanism for the State to continually move forward in the improvement and protection of water quality. Downgrades to classification have been infrequent and, as directed in State and federal law, are limited to situations where existing conditions do not afford the possibility to achieve the assigned class.

Water Quality Classes

The State has four classes for freshwater rivers (AA, A, B, C), three classes for marine and estuarine waters (SA, SB, SC), and one class for lakes and ponds (GPA). A close comparison of the standards will show that there is actually not much difference between the uses or the qualities of the various classes. All attain the minimum fishable-swimmable standards established in the federal Clean Water Act. Most support the same set of designated uses with some modest variations in their description.

The classification system should be viewed as a hierarchy of risk, more than one of use or quality, the risk being the possibility of a breakdown of the ecosystem and loss of use due to either natural or human-caused events. Ecosystems that are more natural in their structure and function can be expected to be more resilient to a new stress and to show more rapid recovery. Classes AA, GPA and SA involve little risk since activities such as waste discharge and impoundment are prohibited. The expectation to achieve natural conditions is high and degradation is unlikely. Class A waters allow impoundments and very restricted discharges, so the risk of degradation while quite small, does increase since there is some small human intervention in the maintenance of the ecosystem. Classes B and SB have fewer restrictions on activities but still maintain high water quality criteria. Finally, Classes C and SC have the least restrictions on use and the lowest (but not low) water quality criteria. Classes C and SC waters are still good quality, but the margin for error before significant degradation might occur in these waters in the event of an additional stress being introduced (such as a spill or a drought) is the least.

Standards and Classification. The reclassification of waters of the State is governed by 38 M.R.S.§§ 464(2), 464(2-A) and 464(3). This statute requires the Department of Environmental Protection to conduct water quality studies, and the Board of Environmental Protection to hold hearings and propose changes to the water classification system to the Legislature for final approval. This is to be conducted from time to time, but at least every three years. In 2019, the Department concluded a re-classification initiative; please see here for further details. In January 2020, the Department initiated a Triennial Review of Maine's water quality standards to evaluate whether changes are needed. For further information, please visit the Triennial Review webpage.

The links in the list below for specific sections in the classification law go to statutes on the Maine Legislature's website. Statute text is usually updated a few months after the end of a session.

    • 464 Classification of Maine waters
    • 465 Standards for classification of fresh surface waters
    • 465-A Standards for classification of lakes and ponds
    • 465-B Standards for classification of estuarine and marine waters
    • 465-C Standards of classification of ground water
    • 466 Definitions
    • 467 Classification of major river basins
    • 468 Classification of minor drainages
    • 469 Classification of estuarine and marine waters
    • 470 Classification of ground water   

Related pages:

GIS Maps and Other Data Files- contains water quality classification ArcGIS Online web maps

Maine's water quality standards

Water Quality Re-Classification Initiative

Integrated Water Quality Monitoring and Assessment Reports

Total Maximum Daily Load (TMDL) reports approved by the U.S. Environmental Protection Agency


Meagan Sims 207-530-2518