Changes in Mirror Lake, Northwestern Washington, as a Result of the Diversion of Water from the Nooksack River

Karel Tracy, Western Washington University


Mirror Lake, a small lake in northwest Washington, has been used as a settling pond for water diverted from the Middle Fork of the Nooksack River since 1962. In this thesis, I combine bathymetric data and sediment sampling to document the changes in sedimentation that have resulted from this diversion, and compare these results to a previous study conducted in 1991.

To document the change in the bathymetry of Mirror Lake since 1991, I surveyed the lake in the summer of 2000 using a theodolite and sonar depth gauge. I compared a contour map generated from this survey to the map from the survey of 1991. However, inconsistency in measurement of x-y position between the two surveys prevented accurate comparison of the bathymetry except in an area near the delta.

Four cores collected from three locations in the lake, and 14 grab samples from the uppermost bottom sediments allowed analysis of the post-diversion sediments and comparison to the pre-diversion sediments. The deposits since 1962 were analyzed for the thickness of the strata, grain size, organic content and magnetic susceptibility, and compared to the pre-diversion sediments. The sediments from the diverted water have a thickness of about 1.3 meters near the middle of the lake. The sediments are characterized by very fine to medium sand (diameter 0.063 to 0.5 mm) at the delta and by medium to coarse silt (0.016 to 0.063 mm) near the center of the lake. In contrast, the pre-diversion sediment is primarily organic sediment that has a median grain size equivalent to very fine sand (0.063 to 0.125 mm).

Between 1991 and 2000, about 15,000 +/- 2000 cubic meters of sediment have been deposited; this estimate translates to about 1700 +/- 200 cubic meters per year, or about 0.6% of the lake volume. The estimate is based on the change in bathymetry from 1991 to 2000 in the places where this change was significantly greater than measurement error. For the majority of the lake, I estimated sediment thicknesses from water depth and the sediment thickness at nearby coring sites. My estimate of error for this rate combines errors in estimating the stratigraphic location of the 1991 strata and in estimating sediment thickness where neither coring nor bathymetric comparison provided this information. Five radiocarbon dates from three cores, as well as the presence of Mazama tephra, suggest that organic sediment accumulated at a rate on the order of 40 cubic meters per year prior to the diversion.