Water Level Changes – Thompson Lake Dam History
From the Original Document submitted by the TLEA dated
August 30th, 2012
The Thompson Lake Environmental Association examined the issue of lake water level draw down in the fall and the target level for the lake for winter. After extensive review of the historic records and conversations with biologists and managers of dams in the area, targets were established.
Dam control is a means to significantly reduce the threat of soil erosion dumping phosphates into the water. Maintaining shoreline integrity helps minimize undermining large trees that would fall into the lake as well. According to lake biologists, the largest soil erosion comes from erratic water levels, especially those above or below what is called the “cobble zone” ( the zone is where the rock and pebble composition is at the highest).
Between 1966 and 1986 the average winter water level was 37.1 inches below the top of the dam. In the following years winter water levels were left significantly higher, resulting in spring flooding and erosion. In order to keep the water levels in the cobble zone it was decided to target the winter level at 32 inches.
The water draw down will start around October 1st and finish by October 20th to the winter level of 32 inches.
The schedule has multiple considerations:
- To preserve boat navigation in the shallow areas until Columbus Day Weekend.
- To protect the lake trout spawn which starts in mid October and finishes in early November.
- Working with the capacity of rapid water draw downs by using just the spillways and not the center gate
Several Important Facts:
- In heavy rainfall conditions, 1 inch of rain results in roughly 2 inches of lake level rise.
- The maximum water release of 2.5 inches a day is very feasible utilizing the spillways.
- With all of the planks in place the spillway lake level is approximately 16 inches which is the maximum acceptable level. (Prior to 2012 and the TLEA research, the lake level was approximately 14 inches with all planks in place)
- When a storm is forecasted to drop 2 or more inches of rain while the lake is at 16 inches or closer to the top of the dam, a preventative water release should be done the day prior.