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Lakes that are 20 acres or larger in size meet the criteria for designation and protection under the jurisdiction of Washington’s Shoreline Management Act. However, since the original list of designated lakes was established in 1972, Washington Department of Ecology biologists have found that some lakes that meet the size criteria were erroneously not designated as Shorelines because mapping methods were generally limited to coarse measurements of open water areas or did not include the area of wetlands within the ordinary high water mark (OHWM) of the lake.
The objectives of this project were to estimate the error rate in current lake designation in the state and develop a reliable and cost-effective method that could be used by local governments to identify lakes that meet the Shoreline criteria. We used GIS data in Phase I of the assessment to classify 8,888 lakes in Washington based on their size and potential to meet Shoreline criteria. Lake size included two components –acres of open water and acres of adjoining wetland. In Phase II we measured lake size using aerial photo interpretation for a random stratified sample of 108 lakes in western Washington. In Phase III, we field verified a random sub-sample of 12 lakes of these lakes.
We found that, the accuracy of the GIS analysis in identifying lakes that meet Shoreline criteria varied among lake classes, partly because the portion of mapped wetlands that was actually within the OHWM was highly variable. Ninety percent of the lakes with mapped open water areas of 20 acres or greater appeared to meet Shoreline criteria upon further investigation, while only about 30 percent of lakes with a open water areas between 10 and 19 acres and adjacent wetlands (for a total combined size of 20 acres or more), were found to meet Shoreline criteria. Lakes with smaller open water areas or limited wetlands were even less likely to meet Shoreline criteria. In summary, we estimate that in addition to the 765 lakes are currently designated as Shorelines in Washington, approximately 253 lakes meet Shoreline criteria but are not designated. In other words, an estimated 25 percent of the lakes that meet Shoreline criteria in Washington State are not designated or protected under the Shoreline Management Act.
We recommend that local governments conduct a survey of lakes within their jurisdiction to identify and designate lakes that meet Shoreline criteria .We have prepared GIS shape files that summarize the Phase I analysis and including a map of each lake in the state greater than 1 acre in open water area, with its classification and other pertinent data (click here to download this file) . We recommend that these data be used to identify lakes in each local jurisdiction that have a moderate to high probability of meeting Shoreline criteria; namely those lakes classified as “Open Water 10-19 Acres, Possible Shoreline” and “Open Water 20+ Acres, Possible Shoreline”. If time and funding permit, lakes within lake classes that are less likely to meet Shoreline criteria should also be included, particularly lakes within “Open Water 1-9 acres, Possible Shoreline”. For each lake identified, a biologist trained in OHWM determination by Washington Department of Ecology should first estimate the lake area using aerial photos and a dot matrix grid or GIS tools as was done in Phase II of this project. Because of the potential error in using aerial photo interpretation alone, all lakes should also be field verified unless aerial analysis unequivocally shows that the lake is less than 20 acres in size, including wetlands. Biologists should conduct the field verification using the methods we developed for Phase III.