Water level plays a crucial role in the prevention of shoreline erosion, protection of our native fisheries, as well as the recreational use and economic value of this lake and its surrounding property.

The dam was built in 1863 with the construction of the Robinson Mill. The dam has three gates leading to sluices. Prior to renovations in 2020, the east and west gates were a system of stacked wooded planks or “stop logs” and the center gate had a metal sliding gate that could only be raised or lowered with a motorized hand drill.

Construction of the dam raised the level of the lake approximately 10 feet and converted the outlet river in Oxford to a channel that is now adjacent to Pismo beach. The sole purpose of the dam was to turn the water wheel that, in turn, powered the looms of this woolen textile mill; so little consideration was given to lake levels. This changed as production at the mill declined and seasonal residences increased along the shoreline of the lake. By the late 20th century there was general discontent with fluctuations of the water level and concerns that the level was too low. The Robinson Mill leadership hired an engineering consultant in 1993 who, with input from the Maine DEP and a resident survey, came up with a “Rule Curve” that established guidelines for the water level of the lake that balanced the needs of the mill and the desires of the residents.

The town of Oxford took over the operation of the dam when the defunct mill became delinquent on property taxes. As the dam was no longer needed to power the mill presented an opportunity to reassess the water level guidelines to maximize the recreational and environmental benefits. In 2012 the town requested that TLEA study this issue and provide recommendations. The Dam Committee of TLEA was created to establish optimal seasonal water levels that would: 1) reduce shoreline erosion, 2) protect spawning grounds for the fishery and 3) optimize the summer recreational use of the lake.

After researching dam operations of surrounding lake and ponds, reviewing the historical levels of Thompson lake and the effects of rain storms and spring runoff, as well as consulting with the Maine Department of Inland Fisheries and Wildlife, the following recommendations were made: The fall draw down of the lake should start the week after Columbus Day, to a winter level of 32 inches below the top of the dam by the first week in November. In the spring, the water should be drawn down following ice out (to prevent scouring of the shoreline by ice), with the goal of 14 inches from the top of the dam by Memorial Day weekend. This would remain the summer level until the fall draw down. The dam would be operated to maintain these levels to within plus or minus two inches daily. These proposals were presented at an Oxford town meeting in the summer of 2012 and were accepted without objections. TLEA reviews the water levels annually to see if these guidelines are followed and, if necessary, provide suggestions to protect the environmental quality of the lake.

Dam coffer TLEA Thompson Lake Environmental Association Maine

In the spring of 2018, there were concerns raised regarding the condition of the dam and erratic water levels. The town of Oxford hired the engineering firm MBP to perform an inspection of the dam and to make recommendations regarding possible renovations. The firm assessed the overall condition of the dam to be “fair to poor”. This poor rating was due to the “very limited hydraulic capacity, numerous leaks and voids in the masonry, extensively leaking stop logs, inoperable gate, disintegration of the apron, formation of the scour pool at the dam toe and potential instability at the left retaining wall at the discharge channel.”

The report pointed out that the dam was at risk for “over topping”. This is especially true with climate change and the likelihood of increasing frequency and severity of storms. Overtopping could cause “deterioration of the structure and abutments, erosion, scour, base undermining, instability and failure of the dam.” It was pointed out that overtopping events constitutes about 80% of dam failures.

The engineer’s report in 2018 also stated that the hydraulic capacity of the west and east sluices is not considered sufficient to safely pass even a “small storm event”. There was extensive leakage at the masonry around the east gate and the inlet gate to the mill. The foundation and face of the dam showed signs of deterioration, especially at the eastern side. It was noted that if the center gate were fully functional the hydraulic capacity of the dam could handle a “10-year flood” but this is much less than the 100-year flood spillway design required. They recommended remedial measures.

This situation threatened our water quality, property values and the tax base of the 4 towns that border Thompson: Oxford, Poland, Casco and Otisfield. Erratic water levels could destabilize our shore land resulting in increased deposits of phosphorous and sediment. This would stimulate algae growth and de-oxygenate the water, which would affect the biology of the lake and diminish its recreational and economic value. Failure of the dam could lead to precipitous lowering of water level that would adversely affect boating, recreational activities and property values throughout the lake.

Based on the recommendations of the MBP engineering firm, the town of Oxford  developed a proposal to renovate the east gate of the dam and reface the eastern segment of the upstream portion of the dam with a concrete overlay (37 feet of width and 16 feet of height). This would result in the replacement of the stop log east gate with a new 70 in. by 70 in. steel slide gate with a fish screen. The proposed concrete work is designed to seal the upstream face of the dam, eliminate leakage through the masonry, and protect the east sluice masonry from erosive deterioration. The renovations of the east gate should reduce the risk of sudden failure of the dam and uncontrollable release of water downstream. This plan was approved by the state Department of Environmental Protection.

On June 4, 2020, the Oxford Board of Selectman voted to proceed with this renovation plan. The renovations to the east gate began in July of that year and were completed in early 2021. This included a screen to prevent the loss of fish through the sluice. The town then partnered with the Maine Department of Inland Fisheries and Wildlife (MDIF&W) to apply for grants for screens for the other two gates.

James Pellerin of MDIF&W conducted a review of the operation of the dam and historical water levels of Thompson Lake in 2021. The conclusions were that the water levels recommended by TLEA in 2012 had not been consistently followed and that with the dam renovations the former management scheme for water levels may not be possible or beneficial. He also found that the fish screens were not effective, as they clogged easily with leaves during the fall draw down and lack the ability to be easily removed for maintenance.  MDIF&W is working with the town on modifications for the screens.

Recommendations were given to the town of Oxford regarding the draw downs, including starting them earlier and recognizing the limitations of outflow with the present dam design. Cleaning of the debris and leaves from the screens must be done multiple times a day during heavy leaf falls. Anticipation of rain events and snow melt should be considered, with pre-emptive draw downs. A management plan should be developed that clearly defines the lake level schedule and correlates the measured outflow at the outlet stream to the dam discharge, to develop standard operating procedures.

TLEA will continue to work with the town of Oxford in an advisory capacity regarding the maintenance and future renovations of the dam. Further infrastructure improvements of the dam may be necessary to prevent further deterioration of the outlet retaining walls, expansion of the scour pools, and improve the water capacity at the center and western gates. Routine maintenance of the dam and monitoring of water levels will be necessary in the future. We would like to engage all the towns surrounding the lake on this issue. The maintenance of the dam and the control of water levels on the lake affects everyone that visits the lake or has property on it. It is essential in protecting the water quality of the lake and the economy of the surrounding communities.

The following is TLEA’s position on the Oxford dam:

To properly protect the lake and its environment, TLEA asks that the Town of Oxford develop a long range maintenance plan, as well as a management plan for controlling water levels of the lake.  The recommendations of the Maine Department of Inland Fisheries and Wildlife should be taken into consideration. TLEA believes that if Oxford wants the other towns to contribute towards the dam’s repairs and maintenance costs, the Dam Committee should have representation from each town. The committee should be enabled to make decisions regarding which repairs are to be done and when; and should ensure that routine maintenance is being done properly and on a timely basis. Lastly, TLEA supports all TLEA members encouraging their respective towns’ officials from Casco, Otisfield and Poland to partner with the Town of Oxford so that the concerns of all lake property owners are heard and addressed.