The US Geological Survey maintains a continuously operating stream gauging station on Nashoba Brook, off of Wheeler Lane. You can go to their site and see the amount of water flowing through Nashoba Brook at any time of the day or night.
On the graph below, the horizontal axis shows time and spans one year. The vertical axis shows how much water is flowing through the stream in units of cubic feet per second. Notice that the vertical scale is not linear.
The yellow dots show the average daily stream flow over 44 years of data collection at this site. You can see that in a typical year the lowest stream flow is in the Summer, and the highest stream flow is in the Spring.
The blue line shows the stream flow during 2016. As you may recall, that was a very dry year and the summertime flow at Nashoba Brook was much lower than the historical average.
The red horizontal line marks 2.3 cubic feet per second, which is equivalent to 1.5 million gallons per day (MGD). This is the proposed capacity of the water treatment plant that Concord wants to build at Nagog Pond.