I have archived the November 2018 Water Level Chart (at left). I’m not sure what to say about it except that the lake level is too dam high! It is ironic that for the last seven years I’ve been wishing for HIGHER water, only to now find myself hoping for a LOWER water level so we can complete repairs to the Clary Lake dam. The lake rose only 6″ in October, and I had hoped that gradual rate of increase of the water level would continue into November, but it was not to be. We started the month at -48.84″ below the high water mark, peaked at -29.88″ on the 15th, and ended the month at -32.64″ below the high water mark. There is currently about 8″ of water flowing over the original log flume, enough to prevent finishing the repairs at this time.
We received 7.10″ of rain in November or a whopping 2.73″ more than the average for the month. This brings us to 39.06″ for the year to date, 0.39″ more than average. I suppose, considering how much rain we received in November, we’re lucky the lake didn’t rise even more. I attribute this to the vastly increased outflows resulting from opening up the original 7′ wide log flume in the middle of the dam on November 9th.
Now here’s something interesting to think about. We all know that water flows downhill so it isn’t too hard to figure out that when water is flowing out of the lake, it is flowing downhill and therefore there must be a gradient (slope) to the water surface in the channel. It should then be obvious that the elevation of the water surface at the dam will be lower than the elevation of the water surface at the lake. Furthermore, it should be obvious that the more water there is going over or through the dam, the larger that gradient will be and conversely, the less water going over or through the dam the smaller that gradient will be. Finally, if there is NO water going over or through the dam, then there is no flow and hence there is no gradient. After all, if there is no flow, there is no slope.
So now that we’ve got a handle on the basics of stream hydrology, how much of a gradient are we talking about? You might be surprised! Now that I have unrestricted access to the dam (without trespassing) I’ve been recording water levels both at the lake and at the dam so I can measure the actual difference in elevation. Currently, with BOTH the 34″ gate and the 7′ flume wide open, the surface of the water at the dam is on average 1.8′ LOWER than at the lake:
GradientThis table shows the difference in elevation of the water surface at the dam and at the lake. Measurements are in feet, elevations are for the NAVD88 datum (the elevation of the Normal High Water Mark of Clary Lake is 151.17')
at the Dam
at the Lake
Yeow! Yes, even I was surprised at how much of a gradient there currently is. I have measured the gradient in the past when the lake was mostly full and the 34″ gate was wide open, and the gradient was more on the order of 2″ to 4″ which is more reasonable. I’ve also seen the water level at the dam HIGHER than at the lake, right after a rain event which introduced a lot of runoff into the great marsh, resulting in water flowing INTO the lake from the channel rather than the other way around. This is because roughly 2/3 of the watershed drains into Clary Lake from Clark’s Meadow Brook and only 1/3 comes in from Three Corner Pond.
Besides outflows, the main factors currently affecting the gradient are inflows (how much water is flowing INTO the lake) and channel friction caused by the water having to flow through 1.5 miles of narrow winding channel to get to the dam. When the lake level is higher and the marsh is underwater, there is still friction caused by the laminar flow of water over shallow ground, but not as much as we’re seeing now with the channelized flows.