Inversely 84% of Shoreline is Not Eroding and
Some of the Erosion is Indeed Quite Slow
In the recently re-released erosion study, it was noted that there were areas of observed erosion. These included areas where with differing bank stabilization measures created a discontinuity that led to erosion near the interface, as well as areas that are dominated by reed canary grass, where undercutting was commonly observed. While vegetated, this was also considered to be eroding. It was stated that differentiating “eroding” from “non-eroding” parcels proved to be more challenging than expected because the degree of erosion was highly variable.
Based on their observations, about 16% of the shoreline, or approximately 11,000 linear feet, is exhibiting at least some level of erosion, most of which (~72%) occurs in unarmored areas, although some (~28%) also occurs in areas where existing armoring has been damaged or is of limited vertical extent. Inversely 84% of the shoreline, or approximately
In an attempt to chart the advance erosion on a historical basis the District asked that an animation be made of the shoreline from aerial photographs. Observing these it struck us that there really was not much movement of the shoreline from 1939 to 2007; especially in the natural areas owned by the State. We noticed that the northern shoreline designated as eroding did not change whatsoever; this is clearly observable in the animation without enhancement. 58,000 linear feet, is exhibiting no signs of erosion. Over half, nearly 6,000 feet of the unarmored areas identified as eroding are located in the western end of the lake in the land owned by the State; a section of the shoreline adjoining the campground as well as the marsh on the southern shore where Rock Creek enters the lake.
In an attempt to verify that the shoreline did not see much change, we extracted images from each year and drew a line along the shore. In doing so, we discovered that there were two distinct water level groups; those primarily prior to 1992 where the resting height was somewhat lower than today and those primarily after 1992 where the height was about where it is today. Of course, the water levels vary at the time of the photo each year impacting the location of the shoreline. If one looks at each year individually you will see each line vary slightly toward and away from the lake but generally they stay about the same within the two groups.
To simplify this we have shown a comparison of the two sets; low water years and years with water about the same as now with just the oldest and most recent lines. The low water years include 1939 and 1992; in this image, one can see that the water level is lower in 1992 but the shoreline shelf representing the higher watermark is in the same location as in the 1939 image. It does however appear that a lobe of shoreline near the entrance of Rock Creek disappears by 1992 when the last meander of Rock Creek breaks through the shoreline. Perhaps more important to today’s discussion is the second image because it encompasses the years the dam has been in operation. This image displays a shoreline in 1977 that is almost a perfect overlay of the shoreline in 2007 indicating the shore did not move in a significant way during the 30-year period.
Many variables would prevent us from making absolute claims related to shoreline advance. The angle of the image, the height of the lake, the quality of the image, all make this inexact; but from all appearances better than half the shoreline the District claims to be eroding is not doing it in a way significantly observable way over a 68 year history. Take a close look at the animation video at each section of the lake and while you will see some man-made changes in the shoreline, you will not see major sections of shoreline disappearing from erosion. The study does not quantify the rate of erosion so this video is all we have in the place of scientific measurements. There are locations that are eroding but this does not seem to be an issue of epic proportions that justify the corrective measures being proposed by the District.