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Interconnectivity Between Sebago Lake and Its Upland Waters

posted Feb 25, 2016, 6:14 AM by Joseph Bickard

February 15, 2016 - Sebago-A plan just released by the Cumberland County Soil and Water Conservation District offers a unique perspective on how the waterbodies within the Sebago Lake watershed affect the quality of the largest water supply in the State of Maine. The document, “Sebago Lake Subwatershed Assessment and Prioritization,” was compiled by the Conservation District with numerous partners including LEA, the Portland Water District, the University of Southern Maine, two state agencies, the Town of Standish and the U.S. Environmental Protection Agency.


Because the study looked at the entire Sebago watershed, many of the lakes and ponds examined are within LEA’s service area and our strong, decades-old testing program provided needed background data for the evaluation. While the ultimate goal of the project is to determine how to protect Sebago Lake by prioritizing where to apply limited resources, the document also provides an interesting assessment of the lakes and ponds in the area and changing land use patterns over time.

Unlike many other water supplies across the country, drinking water from Sebago Lake is not filtered before it is distributed to greater Portland. Because the water is extremely clean to begin with, it is instead treated only with ozone and ultra-violet light to make it potable. The reason the water is so clean is because the vast majority of the watershed is still naturally forested. In fact, according to the study, 82% is forested. This number is important because past studies have shown that when watersheds fall below 75-80% forested, water quality noticeably declines.

The report also states that from 1987-2009, green space and forested land in the watershed decreased by 3.5%. If this trend continues, Sebago Lake could be dangerously close to the 75-80% threshold within 25 years. Although water quality of Sebago is currently excellent, the document also states that transparency in the lake has declined since 1990 and phosphorus, the nutrient that controls algae growth, has been increasing since 2006. The study determined the majority of nutrients are coming in from Sebago’s immediate watershed, including the Crooked River. The second largest contribution, which was still only 10%, comes from Long Lake. The remaining 27 lakes and ponds in the greater watershed individually contributed small amounts to Sebago but cumulatively, these waterbodies added almost a quarter of the nutrient load.

Upper watershed lakes were also assessed for a variety of parameters such as land cover change, quality of partnerships around each waterbody, existing water quality conditions and trends over time. In LEA’s service area, Adams Pond, Brandy Pond, Crystal Lake, Otter Pond, Trickey Pond and Woods Pond all were ranked highest in land cover change during the study period of 1987 to 2013. In these watersheds, the amount of developed area increased between 5 and 12% in this time period. These numbers are concerning and emphasize the importance of properly installed and maintained stormwater controls.

Because it takes the whole community working together to keep a lake clean, the commitment level of local partners was evaluated within each watershed. This ranking primarily took into account whether there have been successful partnerships in the past and how likely it is that a whole watershed project would be undertaken in the future. While there are several small ponds at the very bottom rank within our service area, most fell in the middle ground. Keoka Lake stood out as having very active partners, an ability to provide matching funds or services, and readiness for an updated survey and watershed work.

Using an advanced statistical analysis, the report also assessed trends in each of the sub-watersheds. In LEA’s service area, Trickey Pond, Bear Pond, Little Moose Ponds and Brandy Pond all showed signs of declining water quality during the study period. While these four ponds are currently still in good shape, new and existing development within these watersheds should be examined for appropriate stormwater controls to prevent the trend from continuing.

As part of this project, individual, four-page summary reports were compiled for each of the lakes and ponds that drain to Sebago Lake. These fact sheets summarize all the assessed criteria in one easily understandable publication and are available on the LEA’s Website, www.mainelakes.org.

Colin Howe, LEA Lake News, Winter 2016

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