by Kim Kastens, Brewster Conant, Jr., and Katarina Spasojevic
Overview: Why Salt Pollution Matters
Organisms and ecosystems that evolved to live in fresh water suffer if they are submerged in salty water. Salty water can harm both plants and animals, and salt-stressed aquatic ecosystems experience a loss of biodiversity. Too much road salt getting into the groundwater can present a threat to public health in towns, like Acton, that draw their drinking water from wells. Although there are multiple ways that salt can get into streams, ponds, and groundwater, in inland New England the primary source is road salt, also called “deicer.” The most commonly applied deicers are sodium chloride (NaCl) and magnesium chloride (MgCl2).
The saltiness of Nashoba Brook varies on hourly, daily, seasonal, and decadal timescales. Harmful effects of pollution can come from brief, “acute” exposures to high levels of a pollutant and/or from longer term, “chronic” exposures to lower levels of the pollutant. Prior studies of salt in Acton surface waters were based on single-data-point field measurements made approximately seven times per year. Such measurements give only a sparse and imprecise picture of the variations over time experienced by aquatic organisms.
This post presents new data based on measurements made in Nashoba Brook every 15 minutes, for a total of six months spanning all seasons, during the period from February 8 to November 11, 2024. Such detailed data enable us to better unravel the natural and human-influenced processes that control the saltiness of brook water. The data indicate that the greatest stress to Nashoba Brook aquatic organisms is from chronic exposure in late summer and fall, when the water level is low and the salt level is high.
At the end of the post we introduce a conceptual model describing how deicer application, precipitation, evapotranspiration (loss of water from soil by evaporation plus transpiration from plants), and groundwater flow could interact to cause the observed variations in brook salinity on scales of hours, days, seasons, years, and decades. We conclude that most salt is entering the brook year-round as baseflow from groundwater, rather than in short bursts following road salt application. Remediating groundwater contamination is a slow process that can take years to decades.
Previous Studies of Salt in Acton’s Streams
The Green Acton Water Committee and OARS (Organization for the Assabet, Sudbury & Concord Rivers) have been collaborating since 2021 to understand salt pollution in the waters of Acton, and implications for the wider SuAsCo (Sudbury/Assabet/Concord) watershed. We assess the extent of salt pollution in surface waters by measuring the specific conductance (SC) of the water. This approach works because salt, dissolved in water, increases the water’s ability to conduct electricity. SC is easy, quick, and inexpensive to measure by inserting a probe into the flowing water. OARS and other researchers have documented that SC is strongly correlated with chloride ion concentration in freshwater streams of New England. Because all the deicers commonly used in the region contain chloride, measuring how SC varies across time and space gives us a handle on when, where, and in what quantities deicer salt has been entering brooks.
Prior to the present study, the Water Committee has reported on (1) how SC varies spatially across the southerly parts of Acton drained by Fort Pond Brook and its tributaries, (2) how SC varies spatially across the more northerly parts of Acton drained by Nashoba Brook and its tributaries, and (3) how much SC has increased during the last 50 years at one well-documented site in Nashoba Brook. Regional context for the present study is provided by OARS’ 30+ years of measurements of water quality across the SuAsCo basin. Previous studies of Acton’s streams indicated that the concentration of salt in surface water is highly variable over time. The current study was designed to provide more clarity on the salt concentrations to which aquatic life is being exposed, both seasonally and during storm events, and on what processes control changes in concentration.
Making New Measurements
For the first three decades of the OARS stream monitoring program, SC measurements were made six to eight times a year by sending volunteer teams out into the field to manually lower a probe into the stream. In the early 2020s, OARS acquired new instrumentation: a continuous conductivity data logger. This instrument can be mounted at one position in a stream for several months at time, and set to record temperature and SC every 15 minutes (Figure 1).
The first two deployments of this new OARS instrumentation were at sites with known pollution problems: downstream of a wastewater treatment system and in an urban tributary in Lowell. The Green Acton Water Committee proposed OARS site NSH-047, on Nashoba Brook near Wheeler Lane in Acton, as the next deployment for continuous SC measurements. At this site, Nashoba Brook flows southward, draining the northernmost part of Acton and the southeastern third of Westford. As a site surrounded by conservation lands, it presents an informative contrast to those two earlier deployments in more-urban sites. At this same site, the U.S. Geological Survey (USGS) operates an automated stream gage, which measures the volume of water per second flowing in the brook (“streamflow”) every 15 minutes. Examining streamflow and SC together provides more insight than either data set alone.
We knew from the USGS stream gage data that Nashoba Brook, like many brooks in Massachusetts, has a strong seasonal pattern of high streamflow in the winter/spring and low streamflow in the summer/fall. We also knew from OARS observations throughout the SuAsCo watershed that SC and streamflow seem to be linked: SC tends to be higher when streamflow is lower, as during unusually dry years. Thus, we decided to deploy the SC data logger twice: once in the winter/spring (February 8 to April 10, 2024) during the expected highest streamflow, and a second time during the summer/fall (July 29 to November 11, 2024) when the lowest streamflow was expected (Figure 2).
Short-Term Pattern: Storms Swell the Brook and Dilute Salt Concentration
Data from the winter/spring deployment are shown in Figure 3. For the first three weeks of the deployment, stream discharge (blue line) was relatively steady at around 30 ft3/s. After that, the weather changed, and the brook experienced a series of sharp peaks of elevated streamflow, each lasting a few days. By comparing the streamflow data with daily precipitation data recorded at nearby Hanscom Field, we confirmed that each of these streamflow spikes was associated with a storm event that delivered a significant amount (> 0.5 inches) of precipitation.
There is a strong relationship between streamflow (blue line in graph) and SC (red line in graph). When streamflow goes up sharply, SC tends to go down sharply. Conversely, when streamflow starts to go back down to pre-storm event levels, SC goes back up to pre-storm event levels. When streamflow is steady (as in the first 3 weeks of Figure 3), SC is also steady.
Our interpretation of this pattern is that precipitation events temporarily introduce a large volume of fresh water into the brook, diluting the concentration of salt in the surface water, and resulting in a reduction in SC. So in this dataset, precipitation events cause sharp peaks in streamflow that coincide with sharp troughs/reductions in chloride concentration.
This observation and interpretation align with our earlier findings that when OARS and USGS took a field measurement of SC on a day that followed or coincided with a peak in Nashoba Brook streamflow, the SC reading tended to be lower than readings taken in preceding or subsequent months. This pattern is much clearer in the new, continuously monitored data than it had been in the older, sparsely sampled field data.
Small SC Spikes Associated with Deicer Application
When OARS staff measured continuous SC in an urban stream next to a major road in Lowell, they observed that during winter storms the SC spiked drastically, by as much as 1,000 µS/cm (unpublished data, Ben Wetherill, OARS, personal communication). This phenomenon, in which the first water reaching a stream at the beginning of a storm carries high levels of pollution, is called “first flush.” The first rainfall of a storm washes salt and other contaminants off of roads and parking lots and into storm drains and streams. Such events can subject aquatic ecosystems to acute stress — short-duration exposures to very high pollution levels.
We looked to see if there were first flush events in the Nashoba Brook data. We were not able to obtain records of how much deicer was applied in the paved areas of the Nashoba watershed upstream from our sampling sites during specific storms. However, the micro-weather firm Weatherworks reports that Acton experienced “wintry mix” on March 22 and 23, 2024. A “wintry mix” is a combination of snow, sleet, freezing rain, and/or freezing drizzle. With limited visibility and likelihood of black ice, wintry mix presents dangerous driving conditions, so we think that the Acton and Westford public works departments and the Massachusetts Department of Transportation almost certainly applied deicer on the roads of the Nashoba Brook watershed during this event.
Focusing in on the SC data during that March 22 and 23 wintry mix storm event (Figure 4), we see a peak in stream discharge coinciding with a trough in SC, as discussed in the previous section. But soon after the stream discharge began rising and SC began dropping, there was a brief, sharp spike in SC. The spike began about 7 hours after the stream discharge first began to rise, and lasted about four hours. SC rose about 50 µS/cm above the trendline it had been following, and then resumed its downward trend.
We interpret this spike as the result of a first flush event that occurred upstream, mostly likely in the headwaters of the watershed, where our spatial surveys had mapped high SC values in the developed areas around route Rt. 495 and Rt. 110 in Westford. Salt coming from this area would have taken several hours to flow downstream through the forested conservation land and Bellows Farm Mill Pond, and would have been subject to dilution along that journey. This SC spike on March 23 is much smaller in amplitude (~50 µS/cm) than first flush events OARS has previously observed in urban settings (~1,000 µS/cm). Weatherworks reported other episodes of snow in Acton on February 15 and 17, March 11, and April 3–5, 2024, but none of these other events showed a detectable first flush event in our data. We conclude that aquatic ecosystems in the region of NSH-047 are unlikely to be stressed by frequent acute exposures to very high salt levels in the immediate aftermath of deicer applications.
Spring to Summer to Fall Trend in Stream Flow and Water Quality
The full data set is shown in figure 5. Note that as 2024 progressed from spring into summer, streamflow (blue curve) continued to diminish. This is the streamflow pattern observed most years in Nashoba Brook and other brooks in the watershed.
Specific conductance (red curve) trended higher during spring into summer and then stayed high in late summer and fall. As in the spring/winter data, brief increases in streamflow were accompanied by brief decreases in SC, which we again attribute to stream dilution following rainstorms. However, the magnitude of these brief dilution-driven events (<100 µS/cm) was small relative to the contrast between spring and fall (~300 µS/cm).
By August/September, stream discharge was down to only a few cubic feet per second, and SC was up to around 700 µS/cm. OARS considers 500 µS/cm to be the threshold below which the salt concentration is healthy for aquatic ecosystems. During our winter/spring deployment, the SC never rose above 500 µS/cm, whereas during the summer/fall deployment, 99% of the data points were above this threshold. These late summer/early fall conditions are the most challenging for aquatic life, as they struggle with minimal water and maximum salt. Dissolved oxygen concentration drops at this same time of year, further stressing aquatic animals.
So why is surface water saltiest in summer and fall — when it has been months since the last deicer application?
The Role of Groundwater
To understand the seasonal trend in salt concentrations in surface water from spring to summer to fall, we have to understand how water gets into Nashoba Brook, the role of groundwater, and which water is contaminated with salt and which is not.
The primary source of water to the watershed is precipitation. Water from rain or melted snow and ice can reach Nashoba Brook via five main processes:
- by precipitation falling into the brook or an adjoining wetland or pond
- by direct overland flow: precipitation and snowmelt flow downhill over the land surface and enter a brook, pond, or wetland, most commonly when the soil is saturated or frozen
- by human-mediated overland flow: precipitation and snowmelt flow through storm drains and drainage swales before emptying into a brook, pond, or wetland — a common pathway for drainage from parking lots and roads in our region
- as interflow: precipitation enters shallow surface soils and moves down slope within the shallow soil (above the water table) toward a brook, pond, or wetland
- via infiltration, then recharge, then groundwater flow: water sinks deeper into the ground until it reaches the water table and recharges the groundwater; groundwater then flows slowly underground until it discharges into a brook, pond, or wetland as “baseflow.”
Of these sources of water entering the brook, Type 5, baseflow from groundwater, is volumetrically the most important for most streams in New England, including Nashoba Brook. This may seem surprising, because groundwater and baseflow are invisible to people strolling along or fishing in the brook. However, a study by the U.S. Geological Survey estimated that groundwater, as baseflow, accounted for 77% of the average annual streamflow in Nashoba Brook. Baseflow sustains streamflow throughout the year, especially during dry periods. The remaining 23% of the water entering the brook annually comes during and following storms in the form of direct precipitation, overland flow, interflow, and flow through drainage swales and storm drains. These stormwater contributions to streamflow are typically short lived, lasting on the order of hours to weeks.
Looking at the new data above and 50 years of older data from OARS and USGS, we see many stretches of time when the streamflow was low and steady, with no recent rain or evidence of stormflow runoff. During these intervals, we infer that the brook’s water came almost entirely from baseflow. Yet during these intervals, the SC was high, so the chloride must have come from groundwater. So how is chloride getting into the groundwater — and thus, into the baseflow?
Our conceptual model (Figure 6) begins with deicer being applied during the winter to roads, sidewalks, and parking lots in the developed parts of the watershed. When snow melts or rain falls and washes deicer off the roads and parking lots, the water dissolves the salt. If a deicer application area is close to the stream, the salty water can travel into the brook as overland flow or interflow.
Figure 6. A simplified, conceptual diagram of groundwater flow showing how road salt applied on a road or parking lot can move towards and into a stream. Yellow indicates saltier water. Groundwater flow paths are labeled with travel times (days to decades) indicating how long the groundwater takes to travel before it discharges into the stream. (Diagram by Brewster Conant, Jr.)
If the deicer area is not near a brook, the salty water may infiltrate to become groundwater wherever it stops traveling laterally. Once in the groundwater, the dissolved salt travels with the groundwater, flowing down gradient toward areas where it will discharge into streams, rivers, ponds, or wetlands.
As shown in Figure 6, recharged water can follow flow paths that take days, weeks, months, years, or decades to reach its discharge location in the brook. Many factors influence how fast groundwater can move in the subsurface. In this watershed, it is likely feet per day in sand and gravel deposits or as little as feet per decade in clay-rich deposits. In our model (Figure 6), it can take years for salty water to travel the full distance from a site of deicer application to the stream. After each deicer application, it may take several precipitation events and perhaps months to dissolve away all the applied salt on the pavement, adjacent ground, and roadsides. Each time it rains, salty water can recharge the underlying groundwater and then start moving toward the brook as a groundwater plume. Year after year, deicing applications occur and so more and more salty recharged water is added to the groundwater.
To envision the role of groundwater flow, we will use an analogy. Think of flowing groundwater as a slowly moving conveyor belt going from underneath the deicer application location to the stream, frequently getting loaded with more salt at the deicer end. It will take a long time for that salt to travel the length of the conveyor belt’s path; when it does reach the stream end, the salt is dumped into the stream. Note that when the first salt from a newly established deicer location (e.g., a new parking lot) arrives at the stream, the whole conveyor belt “behind” it is now loaded with salt that is also on its way to the stream. Thus, there is a near-continuous source of salty water entering the stream throughout the year from this source — and others, as depicted by the plumes of Figure 6. Even if deicer stops being applied at the parking lot end of the belt, the entire conveyor belt will have to empty out into the stream before clean (low-salt) groundwater reaches the stream — which could take many years.
In reality, the Nashoba Brook watershed has numerous salt application areas along roads and parking lots, at various distances from the stream, with plumes that arrive at different locations along the stream. We think it is likely that some groundwater plumes in the watershed have not yet had enough time to reach Nashoba Brook, as depicted by the newly built parking lot shown on the right side of Figure 6. This means that the amount of salt entering the stream is likely to continue to increase in coming years.
But what causes the seasonal change in SC in the brook? In our conceptual model, the key factor is a change in the ratio between the amount of water reaching the brook as clean, newly recharged groundwater and the amount of water arriving via the older, saltier groundwater plumes. In our interpretation, winter base flow is a mixture of lower-salt (clean) water arriving along the “days” and “weeks” paths (see Figure 6) from the forested land near the sampling site, plus saltier water arriving along the “years” conveyor belt from more-distant sites of deicer application.
In the summer, the “days” and “weeks” pathways — carrying fresher water — are diminished for two reasons. First, there is slightly less precipitation in the summer than in the spring. More important, in the hotter, sunnier months, less of the water falling from the sky manages to recharge into the groundwater. During the hotter months, evaporation returns more water back into the atmosphere. During the sunnier months when deciduous vegetation is leafed out, more water is sucked up through the roots of plants and returned to the atmosphere via transpiration. A USGS study found that during July, August, and September, less than 15% of the precipitation falling in the Assabet watershed is recharged into groundwater. By contrast, in January through April, more than half the precipitation is recharged.
In summer, weeks and sometimes months can pass without significant amounts of groundwater recharge occurring. This is especially true in areas of dense vegetation, such as the strips of forest surrounding Nashoba Brook on both sides. Because the “days” and “weeks” pathways of Figure 6 have hardly been replenished, the stream receives much less groundwater input. We see this in the data (see Figures 2 and 5) as the trend from higher streamflow in spring to lower streamflow in summer and fall.
Meanwhile, the conveyor belt keeps delivering salty water along the “years” paths of Figure 6. With little clean water arriving via the “days” and “weeks” paths, the water in the stream is dominated by the salty water arriving via the “years” path. We see this in the data (Figure 5) as the trend from lower SC in the winter/spring to higher SC in the summer/fall.
In summary, the conceptual model of Figure 6 accounts for both of the seasonal trends that occur simultaneously in the Nashoba data:
- the transition from abundant streamflow in the winter/spring to trickling streamflow in the summer/fall, as evapotranspiration in summer intercepts precipitation before it can recharge the groundwater
- the trend from lower SC in winter/spring towards higher SC in summer/fall, as the fraction of water arriving in the stream via different pathways shifts to favor older, saltier water following longer flow paths from regions of deicer application
Readers might wonder if evaporation of water directly from the brook during summer heat could be causing the observed seasonal trend in SC. This is an insightful suggestion, building on the knowledge that evaporation pulls pure H2O into the atmosphere while leaving dissolved ions behind. Although this process is surely happening, we think it is not a very significant mechanism for making the summer surface water saltier in Nashoba Brook. The brook’s surface area is small relative to the total volume of water flowing in the brook. Also, if evaporation directly from the brook were a major influence, we should see the water getting more and more salty as it goes downstream, as is seen in some rivers flowing through arid terrain. We saw exactly the opposite in our spatial survey of Nashoba Brook: SC decreased downstream, even during a very dry year.
Implications for Public Water Supplies
If most of the salt in the brooks and rivers of the SuAsCo watershed has, indeed, gotten there by way of groundwater, then the measurements that OARS and Green Acton have been making in the tributary networks of the watershed reflect the quality of the regional groundwater. Sampling of streams in the watershed is actually an indirect way of assessing the amount of deicer in groundwater; this is directly relevant to public water suppliers that draw from groundwater wells. In effect, each sample represents an integrated composite of all the groundwater discharging into brooks and rivers upstream of that sample location (provided the samples were not collected during stormflow runoff events).
So how bad might the chloride levels in groundwater be? In our regional SC survey of the entire Nashoba Brook watershed during a drought year, we recorded SC as high as 2,148 µS/cm. This suggests that chloride level in this water was far above the Massachusetts Secondary Maximum Contaminant Level (SMCL) for drinking water, which is 250 mg/L (approximately 977 µS/cm). In OARS measurements, SC levels above 1,000 µS/cm were found occasionally at multiple sites in the SuAsCo watershed, suggesting that salt in groundwater may be developing into a regional problem.
Public water suppliers are not required to monitor chloride content or conductivity in drinking water. However, the Acton Water District (AWD) does have some chloride measurements of Acton well water, going back to 2004, as presented to the Acton Board of Health in November 2024. Although the data are sparse, there is a distinct upward trend in at least two of the wells, and a few data points approach the SMCL. The AWD has begun regular monitoring of chloride in its wells in 2026 (Shawn Case, AWD Environmental Analyst, email communication, Sept. 2026).
Summary
To summarize, the data shown above and in previous posts document that salinity in Nashoba Brook changes substantially on many different timescales. Organisms and ecosystems have to contend with these changes in real time. Deicer application, precipitation, evapotranspiration, and groundwater flow influence the saltiness of Nashoba Brook at time scales from decades to hours. Considering these timescales from longest to shortest:
On the multi-decade timescale, the average salt concentration in Nashoba Brook (as inferred from SC measurements) has been going inexorably upward. We and others attribute this long-term trend in New England streams to increased use of road salt over the last 50 years, plus build-up and migration of salt within the groundwater system. Our conceptual model has groundwater salinity increasing most rapidly and drastically in areas with lots of roads and parking lots. From the deicer application areas, the salty water moves slowly through long, deep groundwater flow paths to eventually reach Nashoba Brook. If this model is correct, then it would be a long, slow process to undo the damage. Even if all deicer application ceased tomorrow, it would take years to decades to flush the salt out of the groundwater conveyor belt.
On the seasonal timescale (from spring, into summer and early fall), streamflow decreases and SC increases. We attribute this to diminished groundwater recharge in the summer, due to less precipitation and more evapotranspiration. As a result, summer baseflow entering the brook is dominated by older, saltier groundwater following longer, deeper flow paths that originated in areas and seasons when deicer was being applied.
On the scale of days to a week or so, saltiness of the brook can be strongly influenced by precipitation. During and immediately after a moderate-to-heavy rainfall or snowmelt, stormflow runoff results in a burst of clean water entering the stream and diluting the saltiness of the brook. Such events appear in our dataset as upward peaks in stream discharge coinciding with downward troughs in SC. In between the precipitation-induced peaks in stream discharge, during periods when streamflow is low and nearly constant, we infer that most of the water in the stream is baseflow, carrying the saltiness signature of its groundwater source.
In the hours following a deicing application, there can be a brief but significant bump up in saltiness, if rain or snowmelt washes deicer salt overland and through storm drains into the brook and its tributaries. In this data set — taken within conservation land — the increase in saltiness delivered by the first flush effect was minimal and was overwhelmed by the decrease in saltiness caused by freshwater dilution from stormwater runoff.
Nashoba Brook’s aquatic ecosystem and organisms have been exposed to a range of salt concentrations that have varied over time and are high enough to be of concern. Our data show that the stressors are most likely to be chronic exposure to moderately high salt levels rather than episodic exposure to extremely high salt levels. The most stressful season is likely to be summer and early fall, when the stream flow is at a seasonal low and saltiness is at a seasonal high.
Both the data and our model suggest that salinity in the brook may continue to get worse in the future. Comparing observed concentrations to existing freshwater aquatic life standards suggest that adverse effects may already be occurring, but to our knowledge no direct assessments of salt impacts on this ecosystem have been performed.
* µS/cm = microsiemens per centimeter, a standard unit of electrical conductance.
Acknowledgments
We thank OARS for the loan of the conductivity sensor, and OARS water quality scientist Ben Wetherill for help in planning the deployment and processing the data. We thank Ben Wetherill and Abbie McCarthy of OARS for access to, and permission to cite, unpublished data from OARS deployments of the continuous conductivity sensor elsewhere in the SuAsCo watershed. Matt Mostoller and Shawn Case of the Acton Water District informed our work by providing unpublished sodium and chloride data from their wells. Acton’s Conservation Commission encouraged this work and gave Green Acton and OARS permission to deploy the equipment in the Nashoba Brook Conservation Land. Acton Land Steward Dale Chayes designed the mounting system. Chayes, Wetherill, and Acton Community Conservation Specialist Ian Bergemann installed the system in the river. These findings were first presented as the keynote talk at the 2025 OARS Annual Meeting.
Appendix: Notes on Terminology
“Streamflow” versus “discharge”
When referring to the volume of water that passes through the Nashoba Brook measuring station per unit of time, this post uses the term “streamflow” — following the usage of both the U.S. Geological Survey and the U.S. Environmental Protection Agency in their communications with the public. However, the data webpage for the U.S. Geological Survey stream monitoring station on Nashoba Brook uses the more-technical term “discharge.” At that website, the default data type graphed is gage height. To see, instead, the latest streamflow data, scroll down below the graph to the Data Type selection field, find “Discharge, cubic feet per second,” and click “Graph it.”
“Discharge” (noun) versus “discharge” (verb)
In water science, the term “discharge” has two distinct meanings. As a noun, “discharge” is a synonym for streamflow as we defined it above: the volume of water passing through a specific point in a river, stream, or conduit per unit of time. As an action verb, “discharge” refers to the act of water leaving or being released from a source. In this post, to avoid confusion, we use “discharge” exclusively in the second sense. (See “baseflow” and “interflow,” below.)
“Water table” and “groundwater”
The “water table” is the boundary that separates unsaturated soil above from water-filled, saturated soil and rock below. In “saturated” soil or rock, all the pore spaces between the grains of sand, silt, or clay are filled with water; in “unsaturated” soil, the pores contain water plus air. The water table rises and falls during wet and dry periods. “Groundwater,” strictly speaking, refers to water below the water table, and that is how we have used the term in this post. But the term is sometimes used loosely for any underground water, including water moving through the unsaturated zone.
“Infiltration” versus “recharge”
Both terms relate to water moving downward. “Infiltration” refers to water passing downward out of the air and into the soil. “Recharge” refers to water leaving the unsaturated zone and becoming part of the groundwater. The amount of infiltration is what matters for the health of plants. The amount of groundwater recharge is what matters for the welfare of the public water supply. Only a fraction of the water that precipitates in a given area infiltrates, and only a fraction of the water that infiltrates in that given area recharges into groundwater.
“Baseflow” versus “interflow”
“Base flow” and “interflow” both describe water that has infiltrated down into the soil and then moved laterally toward a stream — but they differ in speed, depth, and timing. Interflow is rapid flow through shallow, unsaturated soil during or shortly after a storm. “Baseflow” comes from more slowly moving groundwater that discharges into streams nearly continuously, sustaining streams and rivers between precipitation events. Interflow travels in the unsaturated zone (above the water table), whereas baseflow comes from the saturated zone beneath the water table.
“Stream” versus “brook”
In water science, “stream” can be used to describe any flowing body of water, from a mere trickle to a vast river. In everyday usage, “river” is used for larger bodies of flowing water, while “brook,” “creek,” and “stream,” refer to lesser flows. “Brook” is the favored term in New England, echoing the usage in eastern England, the source of early European settlers in New England. In this post, we use “brook” when referring specifically to Nashoba Brook, and “stream” when making broader generalizations that we think pertain to all or most flowing “bodies” of water in the watershed.