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(Excerpt from "1994 Statewide Water Quality Assessment Report-Lakes Chapter" by J. Rector and D. Hallock)
Seventeen lakes were monitored by volunteers every year from 1989 to 1993. Secchi depth data from these lakes were statistically analyzed using the seasonal Kendall test for trend. Slopes were also calculated to determine annual changes in Secchi depths from 1989 to 1993. The statistical software WQHYDRO (WQHYDRO Consulting, Portland) was used for data analysis.
Three lakes exhibited statistically significant trends. Big Meadow Lake and Lake Thomas showed significant improving trends in water clarity from 1989 to 1993 (significant at the 99% and 80% levels, respectively). Long Lake in Thurston County showed a decreasing trend in water clarity (significant at the 80% level).
Seasonal slopes, percent annual change of Secchi depths, and probability of trend for lakes monitored by volunteers during 1989-1993.
Lake
Seasonal Slope *
Average Percent Annual Change
Probability Big Meadow (Pend Oreille) 1.2691
12.9
0.0001 *** Black (Stevens) 0.3083
1.8
0.6663 Curlew (Ferry) 0.1667
0.9
0.6427 Eloika (Spokane) 0.3056
4.1
0.2418 Killarney (King) -0.2475
3.1
0.4395 Lacamas (Clark) 0.2486
4.3
0.7861 Long (Thurston) -0.4982
6.2
0.1824 ** Mason (Mason) -0.1241
0.5
0.8197 Osoyoos (Okanogan) 0.1673
1.9
0.7105 Phillips (Mason) -0.4762
3.8
0.4884 E. Samish (Whatcom) -0.2492
1.9
0.4785 W. Samish (Whatcom) -0.0418
1.3
0.8166 St. Clair (Thurston) -0.0313
0.4
1.000 Thomas (Stevens) 0.4786
3.2
0.1378 ** Wenatchee (Chelan) -0.0376
0.2
0.8844 Williams (Spokane) -0.7921
6.7
0.2998 Wooten (Mason) -0.5192
2.4
0.3631
* Positive slop values (Sen's slope estimator) indicate increasing Secchi depths (i.e., greater clarity) and negative values indicate decreasing Secchi depths.
** Significant at 80 perent level
*** Significant at 99 percent level
Please report errors to Dave Hallock.
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.
