PATTERN OF SEDIMENTATION IN A SMALL LAKE

Mirror Lake is a 5.7 hectare lowland lake located in northwestern Washington, near Bellingham. Since 1962, this lake has been used as a settling pond for sediment-laden water that has been diverted into the lake from the South Fork of the Nooksack River. The survey of 2000 was designed to determine the pattern of infilling in the lake since a 1991 survey. The depth of the lake was surveyed at approximately 440 locations, using a sonar depth gauge, which had an precision of 0.3 meters, and a theodolite, which was used to establish the locations of depth measurements and to survey the shore of the lake. A surface was then fitted to these measurements using the kriging method in the program Surfer.

The surface computed in 2000 could not be accurately compared to the surface computed in 1991 except in an area near the main sediment input, because the changes were not significantly greater than the precision of the depth measurements. Where sediment deposits are thinner, coring will be performed to determine the thickness. However, in an area of 3900 square meters where the deposits have the greatest thickness, about 3000 cubic meters of sediment have been deposited. The deposits are over three meters thick in a 10 meter by 15 meter area centered about 15 meters downstream from the end of the input channel. Water velocity slows significantly from input velocity in this area, which has a steep bottom slope.