Nagog Brook Streamflow: Torrent in Winter, Trickle in Summer — But Not This Year

by Kim Kastens and Song Leav

The Green Acton Water Committee has monitored the stream level of Nagog Brook since the summer of 2017. The general pattern we have seen is a low and consistent streamflow during the summer and into the fall, followed by a high and variable streamflow during the winter and spring. But the summer of 2021 has broken that pattern.

Nagog Brook is a small brook, in the western reaches of Acton, that flows out of Nagog Pond. Nagog Brook and Nagog Pond came to the attention of Green Acton in 2016–2017, during a series of hearings before the Acton Conservation Commission and Board of Selectmen over the Town of Concord’s desire to enlarge its water treatment facility and increase its water withdrawals from Nagog Pond. One fear was that lowering the level of Nagog Pond could diminish the flow of Nagog Brook, which has been designated as a cold water fish resource by the Massachusetts Department of Environmental Conservation.

As detailed in a previous web post, Nagog Brook flow is monitored at least once a week by a Green Acton volunteer. The volunteer reads the depth of the water flow on a device that looks a giant ruler, recording the water depth to the nearest hundredth of a foot. This value is called “stage height.” When the flow is low, the volunteer also records a qualitative observation of flow state, assigning one of these terms of art: dry stream bed, wet stream bed, isolated pools, standing water, trickling water, and flowing water. The volunteer also checks the status of the automatic water temperature sensor.

Here is a graph of the 4+ years of stage height data:

The dominant pattern in the data (up until this year), has been that summers and early fall have had consistently low flow, in the range of 0.1–0.2 feet (approximately 1–2 inches). Although the stream never stopped flowing, it stayed consistently low. Winters and springs, in contrast, have high and variable flow, typically more than one foot, and ranging up to 1.8 feet. The onset of the summertime, low-flow regime has been in June or early July. The contrast between low-flow season and high-flow season is dramatic, with an increase of more than an order of magnitude. We aren’t set up to measure the volume of flow (called “discharge”), but if we were, that contrast would be even more extreme, because discharge scales up much faster than stage height as the languid flow of summer gives way to the rushing torrent of winter.

The Nagog Brook pattern from 2017–2020 is characteristic of streams and rivers throughout New England: low flow in the summer and fall, and higher flow in the winter and spring. For example, across 58 years of data collection at Nashoba Brook in Acton, the U.S. Geological Survey (USGS) has found that on average the streamflow reaches a maximum in early April and then drops to a minimum in early September.

We do not have a marked wet season and dry season, so seasonal variation in precipitation cannot explain this pattern (see, for example, climate data from Maynard, Bedford, and Lowell). Warmer temperatures are part of the explanation: in the summer heat, a higher fraction of water evaporates from the landscape before it can flow into the local streams. Another part of the explanation is transpiration: the process by which water is taken up by the roots of plants, used for metabolic and physiologic functions, and then eventually released into the atmosphere through the plants’ leaves. New England’s famous seasonal foliage changes — from bare branches in the winter, through budding leaflets in the spring, to full leaf out in summer, to falling leaves of autumn — means the amount of water transpired through the leaves of deciduous plants cycles seasonally. During the growing season, a lot of water is intercepted by plant roots before it can reach Nagog Brook and other streams; during the winter, not so much. Scientists discovered this relationship between streamflow and transpiration for New England streams by cutting down the trees in one small watershed in the Hubbard Brook Experimental Forest in New Hampshire while leaving similar watersheds undisturbed (Hornbeck, Pierce & Federer, 1970).

The 2021 flow pattern at Nagog Brook was different. The summertime, low-flow regime never really happened. June and July came and went and we never observed the 0.1–0.2 foot stream depth characteristic of previous summers. Finally, in August, for one observation session (on August 18, 2021), Nagog Brook exhibited its more-typical summer, low-flow status. That data point is marked by an x on the graph, because silt had piled up around the base of the gage and we had to estimate the depth by comparing a photo from 2021 with photos from previous summers. But just two days later, after a rainfall, the stream depth was back up to 0.62 feet.

Why was 2021 so different from previous years?

One reason could be that the Town of Concord did not draw water from Nagog Pond during summer 2021 because work on the intake pipe was being done. In previous years, Concord has used Nagog as a water source during the high-demand summer months.

However, lack of reservoir withdrawals cannot be the only factor. Other streams and rivers in our area have also run high during the summer of 2021. The graphs below show the discharge of the Nashoba Brook in Acton and the Assabet River in Maynard, as recorded on automated gages maintained by USGS. As described above, the long-term average in both streams reaches a peak in April and then slides down to a low in late summer. Since the beginning of July 2021, the measured discharge in Nashoba Brook and the Assabet River have both been running substantially higher than the long-term average for those months.

So what kept the stream flow at Nagog, Nashoba, and Assabet so high this past summer? Anyone who lived in the area during the summer of 2021 can probably guess the answer: rainfall. July, August, and September were unusually rainy — surely an important cause of the increases in the streamflow data. Below is a comparison of the rainfall data for the July-through-September period for 2017–2021.

In conclusion, 2021’s high summer streamflow at Nagog Brook is certainly attributable to the summer’s increased rainfall, with the lack of withdrawal from the Nagog Pond reservoir as a likely contributing factor.

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Data notes:

• In the graph of Nagog Brook stage height, open circles mark dates when the brook was frozen. The x marks an estimated value at a time when silt obscured the base of the gage.

• In the precipitation graph, most data come from Hanscom Field in Bedford. However, the August 2019 data were missing from that data set, so for that month alone, the data come from the Massachusetts Department of Conservation & Recreation (DCR) monitoring station in Concord.