The Green Acton Water Committee maintains a stream monitoring station on Nagog Brook. This being a low-tech, low-cost installation, our station requires that a volunteer scramble down to the site once a week to read the staff gage, which is like a giant ruler mounted vertically on a pole in the stream.
Last week (January 3), I (Kim) bundled up and trudged down through the snow to the site. The temperature had been below freezing continuously for more than nine days straight, with several nights dipping below 0°F. I was sure I was wasting my time and would find the brook frozen over.
But to my surprise and pleasure, the brook at the gage station was burbling along, as merrily as ever. The water level was a bit lower than it had been in recent weeks, probably because of ice upstream, but the stream was definitely flowing. (Click link below to view video.)
This observation is significant because it confirms that there is substantial groundwater input into the brook. The temperature of groundwater doesn’t vary much over the course of the year, hovering around the average annual air temperature. Here in Massachusetts, that would be approximately 50°F, according to the EPA. During the recent remand hearing on Nagog Pond, Dr. Peter Shanahan, a hydrologist, testified on behalf of OARS that drawing down the water level in Nagog Pond by increasing the water withdrawal into Concord’s proposed water treatment plant could threaten the cold water fishery of Nagog Brook. The threat is two-fold. First, there would be less water flowing over the spillway from the pond into the brook. Second, there would be less hydraulic head to drive groundwater flow laterally into the brook. Groundwater input keeps the brook cool in summer (good for fish) and warm in winter.
Thanks to Dr. Peter Shanahan for confirming (email 3 jan 2018) that groundwater input was the plausible explanation for Nagog Brook’s open water throughout our recent cold snap.