On Thompson Lake, the monitors take readings of water clarity at two-week intervals during the spring, summer, and fall months. Typically the monitor tries to select a consistent time of day, say mid-afternoon, when wind and sunlight conditions replicate those of previous readings. By use of a hand-held GPS unit and a depth/fish finder, the monitor is able to return to the same point in the lake with considerable accuracy. In our case, the reading is always taken slightly south of Hayes Point in Oxford, in 100 feet of water. After dropping an anchor to stabilize the boat, the monitor releases the Secchi disk on the shady side of the boat. The Secchi disk is an 8-inch disk marked with alternating black and white quarters which is tethered to a tape measure. The monitor uses a viewing scope to watch the disk descend in the water. Just when the disk is no longer visible, the monitor reads the marking on the tape measure at the water surface. That marking represents the maximum depth of the day's water clarity. The reading is noted on a form, along with other data, such as time of day, sun conditions, and estimated wind velocity and direction. All these variables can affect the reliability of the reading. By season's end, some 7 or 8 readings will have been made, and the form will be sent to MVLMP. There, professional staff, headed by biologist Scott Williams, will compile the data along with other, more sophisticated samplings, including Phosphorus, Chlorophyll a, color, conductivity, pH, and living organisms. All these readings result in a comprehensive picture of the overall condition of the lake. By comparing such studies over a period of years, we can observe changes in conditions and better understand how people and the environment affect our lake.

Table 1, below, offers a comparison of water clarity observations on Thompson in May and June, through the use of the tools and techniques described above, over the past 5 years.

Please notice that the 2008 readings suggest that Thompson Lake's water in those months were significantly above average in clarity. In his 2007 MVLMP publication, The Water Column, Scott Williams remarked that on several lakes he monitored that year in Western Maine, readings were considerably lower, or poorer, in May 2007 than in most years, a fact he did not find surprising given the heavy rains and unusual fluctuations of that spring, marked also by the consequent erosion of shore land areas and road washouts.

Such disturbances introduce suspended matter into the lake and provide nutrients for algae and zooplankton. The density of these organisms, in turn, affects the transparency of the water column being tested. Concentrations of algae and other materials fluctuate throughout the year. Typically, as the water warms and the rains subside in the summer months, water clarity improves. Another factor is the heavy pine pollen "blooms" that will cover many water bodies in Maine during early and mid June. This condition, along with a host of other complex natural phenomena, also contrives to affect water clarity, making it difficult to define just what's "normal" for a lake. Again, water testing needs to be ongoing and based on good science.

Currently rated by MVLMP as one of the 10 best lakes in Maine for water quality, Thompson is also recognized as an "at risk" lake because of the extent of shoreline development and the lake's slow flushing rate, or the time required for all the water in the lake to be replaced, estimated to be about 3 years. That is why TLEA continues to work so hard in the watershed to protect this beloved lake.


    2015      2014            2013          2012       2011     

2012 Statewide Perspective on the Health of Maine Lakes,  CLICK HERE

For more info on lake data click on,   http://mainelakesdata.org/ 

2015 Secchi Disk Readings:

May 16 - 6.9 meters, May 29- 8.38 meters

June 10 - 9.18 meters, June 26 - 9.19 meters

July 3 - 9.31 meters, July 29 - 10.49 meters

August 5 - 11.40 meters, August 12 - 10.4 meters

2016 Secchi Disk Readings

May 14 - 10.92 meters, May 21 - 9.54 meters, May 28 10.14 meters

2017 Secchi Disk Readings 

May 17 - 6.90 meters, June 7 - 8.76 meters, June 28 - 8.23 meters

July 12 - 8.71 meters, July 16 - 8.41 meters,

August 1 - 8.22 meters, August 21 - 8.90 meters, September 28 - 8.61 meters

  Late May Mid-June Late June
2004 N/A 9.72 (27.3) 9.78 (27.5)
2005 10.6 (34.7) 9.9 (32.5) 10.1 (33.1)
2006 9.5 (31.7) 9.0 (29.5) 11.0 (36.1)
2007 6.3 (20.6) 7.5 (24.6) N/A
2008 N/A 11.1 (31.2) 11.7 (32.8)


2012 Secchi Disk reading: At the start of summer, the reading was 5.3 meters

(less than 17.5 ft.) which is bad for Thompson Lake. Note: The summer was off to a bad

startwith a 8-9 inch rainfall. With a slow recovery, the end of the summer reading was

recorded at 9.6 meters(31.4 ft.). The average reading for Thompson Lake in 2012 was

9.1 meters (29.8 ft.) which falls within the normal range for this lake. 



"Examples of Water Sampling Methods"

Surface Grab

Epilimnetic Core

Profile Grab


           Ron Armontrout taking a secchi disk reading.

Dissolved Oxygen Reading at VLMP Workshop

Maine Lakes with Certified Volunteer Water Quality Monitors

                                  Secchi Disk



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