Inland Water Quality

Drivers and Stressors

Most Lakes and Streams Meet Water Quality Standards

Urban Waters More Likely to Have Poor Water Quality


The Casco Bay watershed includes all lands and waters that drain to Casco Bay. The watershed is linked hydrologically and ecologically, from headwaters to the Bay. Flowing waters transport wood, sediment, and other materials downstream, carving the valleys and shaping the stream channels that provide habitat for aquatic organisms. If rivers and streams are healthy (and unblocked by dams or other barriers) they allow fish, aquatic insects, and other animals to move from bay to river to lake and back again.

If water quality is poor, however, not only can pollutants be transported downstream to the Bay, but those long-distance ecological linkages can be disrupted, lessening the ecological integrity of our waters, including the Bay. Both direct and indirect effects of poor water quality in the watershed make our lakes, rivers, and the Bay more vulnerable to other stressors, including climate change.

The fresh waters of the Casco Bay watershed are a major economic asset. Our lakes, rivers and streams support boating and recreational fisheries. Our region’s healthy waters underpin a robust tourism economy. Sebago Lake provides drinking water to more than 200,000 people in Portland and the surrounding region.


Lakes, Rivers, and Streams

Under Maine law, every body of water must meet water quality criteria, specific to the “designated uses” associated with the waterbody’s assigned water quality class. For example, lakes, rivers, and streams must have sufficient dissolved oxygen to support healthy insect and fish communities. Waters that do not meet related standards are labelled as “impaired”.


The Maine Department of Environmental Protection (DEP) has developed statistical tools to evaluate the health of rivers and streams based on the composition of stream biota, especially invertebrates like insects, snails and worms. DEP uses invertebrate data to determine whether a stream meets Class A (best), B, or C requirements, or is in “non-attainment” (not meeting even Class C standards). We looked at the most recent biomonitoring results available from sites monitored over a ten-year period (2009 through 2018).

Photo: Jerry Monkman,
Most streams in our region meet water quality standards (blue streams on map). Streams that do not meet the standards (yellow and orange streams) are predominantly associated with urbanization and suburbanization. Darker gray areas indicate higher relative urbanization based amount of roads, roofs, and parking areas, known collectively as “impervious surfaces”. A few streams outside of urban areas fail to meet water quality criteria because dams or agricultural practices affect dissolved oxygen, bacteria levels, or fish communities.

Percent Impervious Surfaces in Watershed

Biomonitoring shows that streams in urban areas (darker gray on map) have less complex communities of insects. Streams with healthy insect communities are most likely to be found in rural areas with lower amounts of impervious surfaces (lighter gray).

Observed ClassMeanStandard ErrorSample Size
A 5.2 1.28 15
B 9.9 2.96 9
C 13.0 3.52 4
Non-Attaining 26.9 2.89 27
Indeterminate 8.5 1.11 13

Presumpscot Region River Monitoring

A dedicated group of volunteers, led by Presumpscot Regional Land Trust (PRLT), monitors water quality in the Presumpscot River and its tributaries. Data on dissolved oxygen and bacteria levels were collected regularly from over thirty sites from 2015 to 2019.

The PRLT data shows that instances of poor levels of dissolved oxygen (first map) are uncommon, but elevated bacteria levels (second map) are widespread. When compared to State water quality criteria, conditions at most sites along the main stem of the Presumpscot and Pleasant Rivers usually have acceptable levels of dissolved oxygen and meet bacteria criteria. Several tributaries have persistent problems with low dissolved oxygen, and most show elevated levels of E. coli bacteria. Elevated bacteria levels pose a potential health risk, especially to swimmers. (Excellent: Meets class A/B standards almost always. Good: Meets Class C standards almost always).

Lakes and Ponds

Maine has a long history of monitoring lake water quality, with data for some lakes extending more than 40 years. Much of the data has been collected by volunteers working with lake associations or regional monitoring networks. DEP aggregates and curates lake data, releasing it to the public. (Data on recent conditions in Sebago Lake were provided directly by Portland Water District.)

Monitoring practices vary from lake to lake, and have changed over time, so not all water quality indicators are available for all lakes, or for long enough to evaluate trends. Most monitoring programs, however, have long collected Secchi depth data. Secchi depth measures relative water clarity based on how deep an observer can see a dinner plate-sized disk—the greater the Secchi depth, the more transparent the water. Tens of thousands of observations have been collected from waterbodies in the region. The length of the record varies for each lake, with only a few recent samples for Duck and Bog Ponds, and thousands collected over decades from our most heavily studied lakes.

Of 36 ponds and lakes, only four had a median Secchi depth less than four meters: Papoose Pond, Holt Pond, Duck Pond, and Bog Pond.
Lakes and ponds in the Casco Bay watershed generally have clear waters. Over the past ten years, median Secchi depths for all but four lakes in the region for which data are available were over four meters (about 13 feet). (Blue shapes show relative amount of observations at different Secchi depths for each lake.)
Water clarity is related to lake size, especially depth. Deeper lakes tend to have clearer water. The larger volume of deep lakes can absorb a greater load of nutrients, without showing water quality problems, making the lake more resistant to pollution. Larger lakes also tend to have larger watersheds with a high percentage of intact forest, which naturally filters water as it flows to the lakes, thus reducing nutrient inputs to the lake.

Table: Water clarity in most lakes in our region has been stable or improving slowly. Secchi depths in more than half the lakes in our region have improved over the past ten years. Longer-term (more than fifteen years) conditions have improved in just under half. Only five lakes show evidence of declining water clarity.
Water Quality Trend
LakeShort Term
(<10 Year)
Long Term
(>15 Year)
Adams PondImprovingNo Change
Bay of Naples LakeNo ChangeNo Change
Bear PondImprovingNo Change
Bog PondNo ChangeNo Data
Coffee PondNo ChangeImproving
Cold Rain PondNo ChangeNo Change
Collins PondImprovingNo Change
Crescent LakeNo ChangeNo Change
Crystal LakeImprovingImproving
Crystal LakeImprovingNo Change
Duck PondNo ChangeNo Data
Forest LakeNo ChangeImproving
Foster PondDecliningNo Change
Highland (Duck) LakeImprovingDeclining
Highland LakeImprovingImproving
Holt PondNo ChangeNo Change
Island PondImprovingNo Change
Keoka LakeImprovingImproving
Little Moose PondNo ChangeNo Change
Little Sebago LakeImprovingImproving
Long LakeImprovingImproving
Long PondImprovingImproving
Notched PondImprovingImproving
Otter PondNo ChangeImproving
Panther PondImprovingNo Change
Papoose PondNo ChangeImproving
Parker PondDecliningNo Change
Peabody PondImprovingNo Change
Pleasant LakeImprovingNo Change
Raymond PondNo ChangeDeclining
Sabbathday LakeImprovingImproving
Sebago LakeImprovingNo Change
Songo PondImprovingImproving
Stearns PondImprovingImproving
Thomas PondImprovingImproving
Tricky PondNo ChangeDeclining
Woods PondImprovingImproving

successes & challenges

  • Our region has a large, diverse constituency for clean water, which supports efforts by organizations and state agencies to protect water quality. Lakes are safeguarded by lake or watershed associations and regional lake organizations. Boaters, anglers, hunters, hikers, and residents recognize the importance of our lakes, rivers, and streams. Businesses from tackle shops to hotels and real estate agencies benefit from clean water.
  • All of Maine’s fresh waters are polluted by trace levels of mercury, principally transported from coal-fired power plants in the Midwest. A national transition away from fossil fuels as our economy’s primary energy source would not only reduce greenhouse gas emissions, but also mercury pollution in Maine.
  • Forests and wetlands protect water quality. Close to two-thirds of the Casco Bay watershed remains forested, and the proportion of forested area inland is even higher. Water quality in many of our inland waters remains excellent.
  • Replacement of forest with suburban and urban land degrades water quality. Thus expansion of Portland’s suburban and exurban communities threatens water quality in our lakes and streams. That is true even if local communities follow regulations and policies designed to reduce the impact of suburbanization. Investing in forest conservation can help safeguard water quality for future generations.
  • Natural wetlands and floodplains reduce flooding, protect water quality, and support stream ecosystems. Native floodplain trees and shrubs shade streams, cool the water and ensure a healthy supply of dissolved oxygen. They slow flood waters, protect the structural integrity of stream channels, and build stream habitat. Floodplain vegetation supports aquatic food webs by contributing food for aquatic insects, and also protects habitat for the terrestrial adult forms of many aquatic insects.
Photo: Jerry Monkman,