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The 24-Foot Secchi Depth of Legend: A 32-Year Record of Water Clarity

Clarity of the water in Lake Glenville is its prime hedonic value. Its converse, turbidity, is typically associated with various measures of water pollution. Water clarity can be diminished by sediment-laden streams entering the lake, by shoreline erosion and resuspension of unconsolidated bottom sediments in shallow areas by motorboat wakes, by faulty waste disposal practice, and by growth of microorganisms in the lake. Growth of algae and other organisms is promoted by nutrient elements, primarily phosphorus, released into the lake or its watershed. For these reasons turbidity has long been a major focus of the FLG water quality monitoring program. There have been occasional laments that things are not what they used to be; spoken or written allusions to a lost pristine condition (whatever “pristine” means with reference to a reservoir) when Secchi depths were 24 feet. That depth is 2 to four times as great as those measured in recent years.

One winter evening I pondered that historic time, and undertook to learn the source of that information. I found the origin easily enough, and, to paraphrase Pogo Possum, “I have found the culprit, and I am it”. I had forgotten reporting it in my first contribution to the FLG Newsletter (see Archives: Fall, 2005). Therein lies a story~ and some good news for lake lovers.

We acquired our cabin on the Mill Creek arm of the lake in early 1992. I brought a canoe and began exploring the lake. That craft was fun, but a tad slow and small for the size of the lake and of the wakes of the larger boats shredding it. Next I trailed my 20- foot sailboat up from Miami, but found that, beautiful as it is, Lake Glenville doesn’t match the sailing conditions on Biscayne Bay. Finally, by 1995 I had chosen a lowpowered skiff as adequate for my interests and environmentally unobtrusive.

During this learning period I had formed a subjective impression that, year by year, the lake water was becoming noticeably more turbid. To document the change with at least semi-quantitative measurements, I made a Secchi disc, and began measuring and recording Secchi depths at two sites in 1996. Named for Angelo Secchi, the Italian astronomer and polymath who conceived and employed it in studies of the Mediterranean Sea in 1865 (my grandfather’s birth year), the Secchi disc (Figure 1) is an eight-inch disc usually painted in black and white quadrants for limnological use, attached to a metered line. It is lowered into the lake and the depth at which it just disappears is recorded. Secchi disc measurements are confounded by effects other than turbidity, such as sky condition, time of day, roughness of the water surface, side of the boat from which the measurement is made (sunny or shaded), visual acuity of the observer and more, but can provide informative results if carefully and consistently used.

FLG Water 24FootSeccheDepthOfLegend Figure1 NoCaption

Figure 1: The Secchi Disc

One of my measurement sites was just off my dock, about 500 feet from Mill Creek Falls, and the other, which I named Talweg (German for “valley way” or valley bottom) is at the point of deepest water off the entrance to the Mill Creek arm of the lake (Figure 2). This story is focused primarily on the measurements at Talweg. which is less subject to transient effects of intense rain showers or resuspension of bottom sediments by weekend motorboat wakes than are shallow water sites up in the several arms of the lake.

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Figure 2: Secchi disc sampling sites

Figure 3 displays 223 measurements (black crosses) made at the Talweg site over the 24 years from 1996 to 2019 by myself (and now George Dalzell who has taken up the cause in the most recent years). All measurements have been made during the summer season (April-October), mostly at one to two-week intervals. In this report depths are mostly stated in meters (for reference, one meter equals about 3.28 feet). My previous impression led me to expect Secchi depths to decrease with time but, to my surprise, they actually increased from an average of 3.3 meters (~11 feet) in 1996 to 7.5 meters (24.6 feet) early in 1999. Week-to-week variations of Secchi depth during the summer of 1999 also were the most extreme that I have seen. The 7.5 meter depth was observed on 12 June, my first sampling trip of the season. It did not persist however. Secchi depths greater than 5 meters continued for about a month before decreasing to 3 meters by August 1 before deepening again to greater than 4.5 meters by the end of September.

FLG Water 24FootSeccheDepthOfLegend Figure3 NoCaption

Figure 3: Secchi depths measured at Talweb and DWQ-P sites on Lake Glenville
+ denotes FLG data; * denotes DWQ data

Relatively deep Secchi depths of 4 to 6 meters were measured again during 2000 and to a lesser extent 2001, but the latter was a poorly sampled year as I was called away from the lake by terminal illness of my father. Thereafter it diminished further, to less than 3 meters during 2002. An extraordinarily low value of 0.6 meters was observed on September 18, 2004, a day after the lake was savaged by Hurricane Ivan (see Archives, Spring, 2007). Since then Secchi depths have undergone more subtle variations on weekly, seasonal and interannual time scales; Secchi depths as deep as 5 meters have never been observed again.

While out on the lake during 2004, I chanced upon a team obviously engaged in water sampling. They were from the North Carolina DENR, Division of Water Quality (DWQ), which has been monitoring Lake Glenville since 1988. In the early years they made single visits to the lake at 1-to 4-year intervals, sampling a variety of parameters, surface to lake bottom, at four deep water sites. In 1999, their sampling was changed to three times per year at five year intervals. Since 2009 DWQ has been sampling the lake five times a summer at five-year intervals. I had opportunity to accompany them on one of their sampling trips, and Ms. Debra Ow en kindly shared their data with me. Some have been used in issues of the FLG Newsletter (e.g., Spring, 2016). The Secchi depth measurements from DWQ are a valuable contribution to the history of lake water quality and to this article in particular. Their sites, labeled L, N, P, and R from south to north, also are shown in Figure 2. Their Site P is very close to my Talweg site, so I have combined their 27 Secchi depth measurements at this site (red stars in Figure 3) with my own to create a 32-year long record of 250 water clarity measurements at that site.

Several interesting results appear in this combined data set. First, it is reassuring that although observed on different days, by different observers using different equipment and possibly differences of technique, the Secch depth measurements are in mutual agreement when they are close in time. This lends credence to both sets of measurements, and to the Secchi disc as a useful tool. Second, the June, 1999 measurements by DWQ match my extreme measurement and even slightly exceed it in early July. Furthermore. the DWQ data suggest that, although not as well documented, a similar event occurred during the early summer of 1993. It is personally gratifying that the DWQ Secchi data from 1992- 1995 support my subjective impression at the time that water clarity was decreasing during those years. Laboratory instrumental turbidity measurements by DWQ confirm that the near surface water was of very low turbidity (less than 1.0 NTU) in June of both 1993 and 1999, making it unequivocal that near-surface waters of the lake were extraordinarily clear at those times. Finally, the earliest data for the lake show Secchi depths very similar to those measured in recent years.

The years around 1999 also marked a transition in lake level management. Lake levels were raised, and level changes became smaller and more regular. I found no suggestive clues In the lake level history to a connection with large Secchi depth variations observed during 1993 to 2001. An hypothesis has been offered that raising the lake level exposed a higher level of a previously stable shoreline to wave attack, increasing shoreline erosion which led to the increased turbidity reflected in the post- 2001 data shown in Fig 3. Lake level history does not support that hypothesis. During a decade before 2000 the average annual maximum level was 97.5 feet above lake datum, and in a subsequent decade it became 98.1 feet, only seven inches higher. The big change was to the depth of winter drawdowns, which went from 79.4 to only 90 feet, about half as much. I know of no mechanism whereby the depth of drawdown might influence turbidity in a subsequent season.

I looked for clues in other variables that might influence water clarity. Other variables measured by DWQ were supportive but did not suggest an explanation of the mechanism of the anomalous water clarity. I also obtained and studied to no avail weather records (rainfall and temperature) from the Highlands Biological Station and measurements made for many years atop Toxaway Mountain by Mr. and Mrs. R.C. Cole, both of which I take to be more relevant to the Lake Glenville watershed than is the nearest river gauge at Prentis on the Little Tennessee River.

I next turned attention to detailed examination of the 1999 anomaly. Figure 4 demonstrates that the extreme Secchi depths observed at Talweg and DWQ site P were not characteristic of either the entire lake or the entire summer. Rather, there was initially a north-south gradient of Secchi depth values. While the northern sites showed the extreme Secchi depths that both DWQ and I had measured, southerly sites show modest deepening from what has been observed in other years. Secchi depths converged to about 3.5 meters at all sites by early August. In 1993 Secchi depths were less variable among the DWQ sampling sites. Again Secchi depths at northern sites were somewhat greater than southern sites, but all were more than 5 meters deep, Their evolution with time cannot be gauged.

FLG Water 24FootSeccheDepthOfLegend Figure4 NoCaption

Figure 4: Temporal variation of Secchi depths by site

It now is clear that the legendary 24-foot Secchi depth is not the spectre of a vanished pristine condition, but rather of a transient local anomaly of less turbid water. Concern that our water quality may have suffered a precipitous decline can be dismissed.

Although I have not been able to find a satisfying explanation of the transient clear water events of the 1990’s, there is an important message from this 32-year time series of Secchi disc measurements. In my opinion the long term trend of water clarity has in fact been small, less than the variability within any summer season. Some useful statistics can be computed from the observations. Estimates based on the entire data set will be biased by the large transient values of the 1990’s. The mean or average value of Secchi depths will be biased toward greater (desireable) values, but the temporal trend will be biased toward decreasing (undesireable) values. I think that the data subsequent to 2001 provide a better basis for estimations. For this 17-year period the mean, or average, Secchi depth was 2.7 ±0.05 (standard error) meters. The median probably is a more appropriate measure of central tendency because it is resistant to extreme values like those of 1999 or Hurricane Ivan event in 2004, but at 2.6 m that is only slightly less. The secular trend, whether the water clarity is improving or degrading over time is of greater interest. A least square linear fit to the 17-year post 2001 data set yields 1.5 centimeter (a little over a half-inch) per year increase of Secchi depth. This is a trivial rate of change, in the desirable direction, but effectively zero. The FLG and DWQ data separately yeild secular changes of opposite sign, both of trivial magnitude.

These data also demonstrate the value of maintaining the easy and inexpensive Secchi disc measurement series. The longer the data series is maintained, the more informative it becomes for detecting secular change, but that implies a requirement for record keeping. I have given the data used herein and more to three members who expressed an interest in the FLG water quality program, but expect that all have been lost. Since 1996 FLG has been submitting data to the DipIn program now managed by the North American Lake Management Society, but their program has been built around a single annual measurement, which is inadequate for most lakes, including Lake Glenville. In 1996, a particularly well sampled year, my Secchi depths ranged from 3 meters in early June to 4.4 meters later in June and again in August to 1.7 meters in October. Any single measurement would be a poor measure of the lake.

It would be interesting also to make some measurements in winter. I expect winter Secchi depths to be deep, but they have not been measured.