June 24, 2020
The lake level today is 49 3/4″. See chart below for historical numbers.
Black Oak is known limnologically as a groundwater seepage lake. This is a water body with no inflow or outflow streams. Water enters the lake only through precipitation or by underground flow. Before 2007 we knew there were significant springs in the lake by the large holes that remained in the forming ice of early winter long after the majority of the lake had frozen solid. But since the drought started and the surrounding water table went below the lake level, the springs have stopped flowing and the lake now freezes over all at once. Water leaves the lake only through evaporation or by underground flow. The proof of the low water table lies in what happens when we get a good rain but the lake level goes up by a lesser amount. Some of that rain soaked into the dry surrounding ground.
It would be logical that the lake level would correlate with annual precipitation but long term studies on Buffalo Lake in Oneida County do not support that idea. Buffalo Lake is also a groundwater seepage lake and its level has been accurately recorded every Sunday since the 1940s. DNR researchers then assumed that summer precipitation is mostly lost by either surface evaporation or transpiration from trees. Only the precipitation that falls from October through March would really make a lake level difference. When that was considered the correlation with the lake level was quite close. And we note the same thing on Black Oak except for the summer of 2002 when there was a lot of summer rain. See bottom paragraph below for graphs of long time level recordings for this area. Recorded snowfall compared to our spring / fall water levels shows that about a 70 inch snowfall winter results in no change between the previous fall and following spring. The accumulating snow during a winter is shown on page 2 of the weekly Vilas County News Review.
The Ordinary High Water Mark (OHWM) is a DNR determined level used as a basis for building setbacks and many other things. To determine this level they use things like clearly defined watermarks or where the shoreline growth changes from aquatic to terrestrial. The OHWM is also the limit of your property ownership. On Black Oak Lake this level is at approximately 45” above the reference used for the levels shown below – a height rarely reached by the actual water level. This means that there is normally a good bit of land in front of your property that you don’t own. But Wisconsin is one of the few states that grants riparian owners the right to exclusive use of such land. You can’t fence it but you can post two NO TRESPASSING signs on the water line facing away from “your” property.
To create a depth reference a concrete block was buried in the bottom in about 24 inches of water near the Dirk Meyer / Walt Bates property line in spring, 2001. It is assumed that winter ice does not disturb this block though there is a much deeper and larger block we sunk in 1987 in about seven feet of water that we use as a cross check. Depth readings have been taken over this shallower block immediately after ice out and just before freeze up each year since it was placed and are shown below. An accurate water level history has been kept here only since 2001 though we do have a DNR lake bottom contour map from August, 1969. This map gives a water level that would be at 34 ¾” on the chart above, exactly like 2004. Also, some old pictures have happened to record approximate levels. Most pictures from the 1930s and 1940s show the water much lower than present levels while pictures from the 1970s show a much higher level. The Black Oak Lake history book has a photo on page 59 from the early 1930s that shows a water level that would be about zero inches on the reference stick used for the readings below. Note that we reached that again in spring, 2010. A small though interesting factor is that according to DNR data the volume of the lake is 19,956 acre feet or 863,380,710 cubic feet or about 6.5 billion gallons. Approximately 50% of Black Oak’s water is in its epilimnion (the water above the midsummer thermocline at about 30 feet of depth). Given that water density varies 0.2% between the all lake temperature of 4 degrees C of winter and the epilimnion’s 20 degrees C of summer, the lake rises and falls just under one half inch between mid summer and the frozen time due to this warming and cooling. One more factoid: the height of the bottom of the culvert through which water can flow under Highway B towards Lake George is 26 ¾”. This does not mean that water will flow to and through the culvert when it reaches that level since a lot of sand has washed up a bank between the Black Oak water line and the culvert.
|YEAR||ICE OUT||FREEZE UP|
Average depth from ice-out 2001 through ice-out 2020 is 23.4″.
Though Black Oak’s level has been accurately recorded only since 2001 there are two other area lakes where levels have been recorded since the 1940s. Buffalo and Crystal Lakes are near Woodruff and, like Black Oak, are groundwater seepage lakes. Therefore, their levels go up and down in lockstep with Black Oak’s. CLICK HERE to see the graph of all three together. The graphs have been normalized to show what the levels have done for the entire (1942 to present) time span. Look for any regular spacing in the highs and lows – they appear to be totally random. One obvious thing is that most of the peaks are very temporary, two or three years being about the longest before the level reverses.
PIEZOMETER STUDY Summer 2013
You’ve been seeing these things all around the perimeter of the lake all summer. As described in the fall newsletter they measure the difference in water pressure at the lake bottom. A higher pressure below the bottom implies water flow into the lake at that site and vice versa, the amount of flow being roughly proportional to the pressure difference and the porosity of the underlying substrate. In this photo the column of water in the tube coming out of the pipe that’s driven about three feet into the bottom is about 1″ higher than that in the tube open to lake water showing water flow up into the lake at this site.
On this linked spreadsheet are the final results. There were 53 sites and for each you see a reference number. These numbers are in clockwise order starting at the public beach and have no relation to your property number shown in the directory. Following that are the sites’ Latitudes/Longitudes, the nearest owners’ names, the final piezometer results, and the individual readings and dates leading to that final number. The logic here is that when initially placed it takes a while for the two levels to stabilize, up to a week or more in soft mucky areas. So the “final” is not the average of all the respective readings but rather where the two levels were when they finally stopped moving. For the finals greater than one inch I colored the numbers red for outflow and green for inflow. Note that there are four such “significant areas” centering on Flemma, Senechalle, Annin, and northeastern Barber’s Bay.
Initially, I placed 42 piezometers at about a 1,000′ spacing (Black Oak has just over 40,000′ of frontage). Wherever one showed a significant pressure difference, defined as 1 inch or more between the two water column heights, I followed up by placing more piezometers closer to it to get a profile of that area. Those four “areas of significance” are described here:
1) Note how there is no flow at Cuttell/MacDonald, then an inflow of 4 1/2″ at Loader reversing to a minus 6 1/2″ at von Estorff. (I did find a few examples of big changes in short distances). Then it goes positive again at Hinze and then to a 7″ outflow at Allman. Not until Hostetler does it settle near zero.
2) By far the largest area of outflow is from Felton to Hattenhauer finally zeroing out near the Osprey nest. The negative numbers are large and the area affected is wide.
3) A smaller area of outflow is along the north side of Annin’s peninsula.
4) The only area of significant inflow is along the northeast shore of Barber’s Bay.
There seems to be no relation between a piezometer reading and the adjacent shoreline profile. Note that in front of flat swales where you’d think that there would be strong flow one way or the other (Trochlell, McAdams, Osprey Nest, Public Beach) there was no flow either way. And in front of most hilly shorelines there was either significant outflow or no flow. During Dr. Susan Knight’s AIS survey visit in early October she commented on some of these irregularities in the piezometer readings. She said that flow depends more on what the glacial deposit underlying that part of the lake is. A firm clay will not have the flow that areas of porous gravel would have. These are known as glacial eskers and kames…….google that for bedtime reading as a sleep aid! Former geology teacher at Conserve, Paul McLeod, has also reviewed this data and agrees saying, “I think Susan is probably right about the underlying glacial sediments. You can wander to any of the gravel pits and road-cuts in the area and see that in some places the sediments are highly porous and permeable, while in other places the clays make the sediments impermeable or at least low permeability so there’s no reason to think that you wouldn’t have the same variability beneath Black Oak Lake”. And if you are really interested in this you can convert the differential column heights to psi using the relationship that one inch of water depth exerts 0.036 psi. This means that the negative 12″ reading near Hattenhauer equates to just under one half of a psi and that’s a lot of pressure forcing outflow. To test this I took a plastic sheet about 2′ x 3′ to that area and laid it on the bottom in about two feet of water. Sure enough, it stuck to the bottom!
In summary, without considering precipitation the lake is losing more water than it is gaining. I guess this should be expected considering the relatively high elevation (1,711′ above sea level and 1,108′ above Lake Superior) of Black Oak Lake. The “Subcontinental Divide” runs along Black Oak’s north shore meaning that we drain to the south while Big Donahue flows to the north. There is little area around us that has significantly higher elevation and water table than the height of the lake. According to DNR’s Topo maps Black Oak’s “drainage basin” is 2 square miles (including the 1 square mile of lake surface). This is a very small ratio as many southern Wisconsin lakes receive drainage from an area over 100 times the size of the lake. Lakes both to our east and west are lower than we are (the Cisco Chain is over 60 feet lower). The upside is that this is very good for water clarity.
Update: Spring 2014 The abundant snowfall of last winter brought all lakes up some but Dollar Lake went up the most putting it nearly two feet higher than Black Oak. So I drove a piezometer into the bottom just east of the Osprey nest expecting to see some positive inflow coming through the wetlands between Black Oak and Dollar. Instead it showed no flow either way. This points out that piezometers can only measure flow at the precise location where they are placed. In this case flow is almost certainly coming into Black Oak but is likely passing below the piezometer site and emerging in deeper water in Black Oak.