Sedimentation has had little or no effect on the storage capacity or surface area of Long Lake. The lake is a natural lake that formed in unconsolidated glacial deposits. The lake is essentially two lakes: a small southern part and a large northern part. The lake receives drainage from Thumma Ditch. At the outlet of the lake, the drainage area of Thumma Ditch is 12.0 square miles. Locations where the largest amount of sediment has accumulated for the 29-year period 1959-88 are in the southern part of Long Lake where Thumma Ditch enters the lake, and in the narrow channel that connects the southern and northern parts of the lake.
In general, there has been little or no sediment accumulation in most of Long Lake. The surface-area of the lake was 1,738,000 square feet (39.9 acres) in 1959 and was 1,733,400 square feet (39.8 acres) in 1988. In 1959, the volume of water in Long Lake was 27,555,000 cubic feet; in 1988, the volume was 27,511,000 cubic feet. The amount of sediment accumulation in the lake during 1959-88 was 44,000 cubic feet. In 1988, the volume of water remaining in the lake was 99.8 percent of the 1959 volume; 0.2 percent of the 1959 lake volume had filled with sediment. The average annual rate of sediment accumulation in the lake during 1959-88 was 1,520 cubic feet per year.
Potential decreases in the storage capacity of Long Lake for the 29-year period 1989-2018 were estimated assuming steady-state conditions. The volume of water in the lake in 2018 is estimated to be 99.6 percent of the 1959 volume; 0.4 percent of the lake is estimated to be filled with sediment.
