September 22, 2023:
Water level is 45″. Last fall it seemed like it was heading back down but record snowfall this past winter has put it back where it has been for the last five years. See chart below for historical numbers. Water clarity is 27 feet.
The most common question people ask when looking at this depth chart is “inches above what?” We needed a reference that could be counted on to never move and that is a challenge giving the freezing and thawing of the lake. So in the winter of 1987 we slid a large concrete block out onto the ice near the Bates / Meyer property line. The block is about 2 feet by 3 feet by 8 inches thick and landed flat on a gravel bottom in about 7 feet of water where ice would never affect it. It is about 150 feet out from shore and therefore inconvenient to regularly access for readings. So we established a shallower and closer to shore reference whose elevation above the deep block is known and that is the reference used for the recorded depths. That’s the answer to the “inches above what” question.
|YEAR||ICE OUT||FREEZE UP|
Average depth from ice-out 2001 through ice-out 2023 is 26.75″.
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
These devices 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 Croft, 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 Williams 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 Beutel 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.