I've likewise noticed recently that some of the rivers in these parts have been surprisingly low, though I considered that it could have been due to dam work downstream.
Lake McDonough here is down quite a bit, but not all that unusual at this time of year for it to be lower, and I've certainly seen it lower than this in prior, drier years.
There aren't any reservoirs around here that I pass by regularly so I don't have a sense of usual patterns. I had been by the Whately reservoir once back in May, when it was full, one little island, really just a patch of reeds about 10 yards across. This time the reeds were still there but now perched way up above the water and quite a striking image.
I've been to Quabbin several times in the past year, and it's was certainly filling up this spring. Last time there a couple of weeks ago it hadn't dropped much.
Our standard measure of water levels is the Ct River, just now starting the 6-month window when we can see it out from the house. It was running high through early summer, much later in the year than usual (it's always high in April/May), but in the last month or two it's been very low, lots of mudflats showing.
So I guess it has been dry. Certainly haven't had many days when I couldn't get a ride in, and I haven't been riding in the rain.
The dam operators tend to drop the levels in reservoirs at this time of year in anticipation of winter and (theoretical) higher levels of precipitation. It avoids them having to use overflow systems when everything starts to melt and risking flooding downstream. Not sure whether that is the case with the one you were observing is low, but probably.
The CT is really low at the moment - I did a canoe race on it near Holyoke a couple of weeks back and it was shallow everywhere - there is normally a deep channel somewhere in the river!