As of this writing the lake level is 97.6 feet above sea level, meaning there is still just under a foot to go until full. It may seem like it has taken a very long time to fill these last two feet, and that is not your imagination. There are three factors that come into play when we fill the lake; lake topography, rain and vegetation.
Think of Oswego Lake as a dessert glass where there are three distinct zones; the deep, the transition and the shallow.
The deep zone is below 20 feet, essentially the original lake before Oswego Dam. The transition is from 20 feet up to about three feet. That is the part where we start filling the west end, Lakewood Bay, Blue Heron canal, West Bay and finally the shallowest part of Oswego Canal. Once the entire lake is flooded things seem to stall because there is a large surface area to fill and it takes a lot of water to fill it. When the lake is full it occupies just over 400 acres. At one foot down it takes 400 acre-feet of rain to increase the lake level one foot, or 130 million gallons.
This has been a very dry spring and after a relatively wet January only six inches of rain have fallen. This is only the second time we have filled the lake using rainwater only, so let’s compare this drawdown to the last time we did it in 2006/2007. In 2006 we started refill December 8th and it took just under 16 inches of rain and three months to fill the lake. In fact, the lake rose over five and a half feet in December 2006, twice the amount as December 2017. And as of this writing, almost four months after starting the refill it has rained just under 17 inches and we are still a foot from being full.
How can this be? First, we have to look at the rainfall from 2006/2007 to see how different the weather was then. That year the lake received a little over seven inches of rain in December, the first month of refill. This alone would start refilling the lake pretty well but look at the month before and you can see why the lake really shot up in December. In November 2006 we had a total rainfall of 14.6 inches, and though were not refilling in November all that water saturated the watershed. In December when we closed everything and captured runoff there was still a lot of water leaving the watershed and draining into the lake. Look how steeply the lake recovered in December, and this after a refill that did not start until the eighth of the month. By the end of the month the lake had recovered over five feet from full drawdown depth.
Compare 2006/2007 to this drawdown. The recovery curve is much shallower, but also the rainfall peaks are much lower. We had a fair bit of rain in November, but December was dry, and it did not rain much at all the first two weeks. Rainfall during November and December 2017 was half the amount received the same months in 2006, and as a result the lake only refilled 2.5 feet. One other thing to note, On December 14, 2007 it rained over 1.5 inches after several days of constant rain. Rainfall that intense quickly runs off into the streams and flows into the lake. This year there were few days with heavy rain, with the heaviest back in October when we were actively drawing down.
We talked a lot about how the intensity and amount of rain influences our lake refill. Two other factors that come into play are whether the soil is frozen and if vegetation is actively growing. If it has been really cold and precipitation came as snow or freezing rain, it would not come into the lake until it warmed enough to thaw. On the other hand, if the soil was frozen and heavy rain fell on it almost all that water would drain into the lake since it could not soak in. On the other hand if the refill was delayed into spring budding plants and trees would start taking up water. This spring we are still refilling as trees are putting on leaves and other watershed vegetation are leaving out. As our watershed becomes green a lot of water is transpired up to the canopy and evaporated out the leaves. This reduces the amount of water in the ground that can run off into the lake.
Ironically a green watershed is very good for the lake because it reduces the intensity of runoff entering the lake and intercepts pollutants. It is only during times like this when we are looking forward to a full lake that a green watershed works against us. However, I would not want it any other way.