Rivers Alliance of CT
Rivers Alliance of CT logo

Rivers Alliance
Connecticut's United Voice for River Conservation

donate and join button










Follow Us on Facebook!

River Alliance of CT > Priority Topics > Streamflow > Streamflow and Groundwater Overview in CT

Know Your Flow!

Updated Nov 9, 2017
Sections of this page updated as needed; refer to dates in each section

Link to webpage
Rivers Alliance thanks Loureiro Engineering Associates, Inc. for their generous sponsorship of the Know Your Flow page.

CT Conditions At A Glance

(Click on any graphic below for more information)
Streamflow in CT Now
(click map to go to the data page)

 [color code for]  color code for  color code for color code for  color code for  color code for  color code for               [color code for]       
   High Flow     Low Flow        Not Ranked
CT Current Streamflow. Click to go to source page.
Recent Flow At a CT Stream
(click graph for details)

Link to Burlington Brook flow data
Groundwater Level
(click graph to see more well data)
USGS monitoring well. Click to see more.
Check your forecast here
(click map to go to state weather page)
Link to NWS graphic page
Graphic courtesy NOAA showing CT area watches and warnings if any.

Streamflow Discussion 

On Nov 9, most of Connecticut's rivers and streams had returned to near-normal flow rates, down from high and very high levels that had been due to unusually heavy rainfall. There are a few exception: 3 of  61 CT stream gages reported flows have dropped to levels low for that stream for the date, while 6 gages are showing flows that are steady at levels well above those gages' recorded average flows. Tables with links to these stream gages are below

In general, between-storm streamflow across the state had been dropping to low levels since the middle of August, not only because of less rain overall but also because rain was fairly localized along persistent storm paths, causing some rivers and streams to peak while not affecting others. Until this pattern shifted at the end of October, most tropical systems ended up saying off the coast. 

See below for a discussion of what common streamflow patterns look like.

Click here for the USGS Connecticut stream gage web page with graphs of flow from every gage for the past 30 days compared to their historic daily flows.

Why do we countThe flow between storms is an indicator of stream conditions. When water levels are low, portions of the streambed become too dry to support the aquatic life that usually colonizes those areas. Groundwater seeping from the stream banks can help organisms survive for short times; therefore, well measurements can also be used to identify areas where aquatic life might be in jeopardy.

A count of the number of USGS stream gages reporting low flow at their lowest point between storms is valuable as an indicator of conditions statewide. If the number of streams that reach low levels between storms is increasing over time, it raises concern for the recovery of stream ecosystems from dry conditions. If the count decreases, we can look for recovery. If most of the low-flow gages are in the same area of the state, it could indicate a regional problem. We also note any record low flows for the date (see Sep 14 discussion).

Brief high water, even flooding, caused by sporadic heavy rain does little to alleviate long-term effects of dry conditions, because it takes time for ecosystems to recolonize streambeds after a drought has killed off aquatic organisms. Direct observation of any particular segment of stream is necessary to determine actual streambed conditions, because there are only 61 stream gages in Connecticut and the conditions they report may not represent the conditions in the hundreds of streams with no gages.

 On Nov 9, 3 out of 61 Connecticut USGS stream gages (see table below) were reporting low flow*.  6 stream gages were showing steady flows well above normal (second table below).

Here are the numbers from previous weeks:

Nov 6: only the Indian River gage near Clinton, CT was reporting low flow.
Oct 23: 48 of 61 stream gages were reporting low flow, 14 of them setting low-flow records
Oct 16: 50 of 61 stream gages were reporting low flows.
On Oct 12, 33 of the 66 gages  at this link were below their 25th percentile (the USGS datafile we normally use to count low-flow gages was not available).
Oct   5: 53 of 61 USGS stream gages reported low flows.

Stream Gages Reporting Low Flow* Thursday November 9, 2017

25th percentile,
25% of mean,
or both
Station Name Station Website
m ASPETUCK RIVER AT ASPETUCK, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=01209105
pm FIVEMILE RIVER NEAR NEW CANAAN, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=01209761
pm MUDDY RIVER NEAR EAST WALLINGFORD, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=01196561

Stream Gages Reporting High Baseflow* Thursday November 9, 2017

75th percentile,
175% of mean,
or both
Station Name Station Website
p COGINCHAUG RIVER AT MIDDLEFIELD, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=01192883
p CONNECTICUT RIVER AT THOMPSONVILLE, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=01184000
p QUINEBAUG RIVER AT QUINEBAUG, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=01124000
pm QUINEBAUG RIVER AT WEST THOMPSON, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=01124151
p RIDGEFIELD BROOK AT SHIELDS LANE NR RIDGEFIELD, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=012095493
h WILLIMANTIC RIVER AT MERROW RD. NEAR MERROW, CT https://waterdata.usgs.gov/ct/nwis/uv?site_no=01119382

 *We define low flow as below the 25th percentile for that stream, or below 25% of the mean flow for that stream for the date from the USGS data file at https://waterwatch.usgs.gov/webservices/realtime?region=ct&format=xml at a time when rivers are near their lowest points between storms. High baseflow we define as above the 75th percentile for that stream or above 175% of the mean flow. 

Click here for a list of all CT stream gages comparing their flow right now to their mean and median flows for today's date.

The Pequabuck River gage in Forestville, which previously had not been sending in live data, was added to the live-data file on Sep 14, 2017. Meanwhile, two other gages that had lost funding from the town that was helping to pay for them have not had their data collected since June 30, 2017. Before that date, there were 62 gages total in CT. Although historic data will remain accessible for those two gages, no new data will be collected unless one or more new funding partners are found. Users who can contribute funding for the non-federal share of costs to continue operation of the stream gages for Rooster River at Fairfield (39 years of records) and Mill River near Fairfield (44 years of records) should contact Jon Morrison at the USGS New England Water Science Center - Connecticut Office (860-291-6761) or email at jmorriso@usgs.gov. 

Public Water Supply Information

Your water utility and town web pages are the best sources of information on any water use restrictions that may apply specifically to you. 

As reported by the September DPH Monthly Reservoir Status Summary (issued Oct 19), the reservoirs of 34 large water companies averaged near their normal 80% full at the end of September, so DPH reports they are at 99% of normal for the time of year. Individual systems' report varied widely however.  Norwalk reported a Stage 2 Drought Watch with 55% Usable Storage. At the other end of the spectrum is Windham Water Works, reporting 100% of capacity and that they have always been full in September over 33 years of record.

These reservoirs were at 89% full at the end of August, or 105% of normal for that time of year.

UConn declared a Stage IA Water Conservation Alert because seasonally dry conditions have reduced area stream flows.

On Oct 18, the Town of Greenwich and Aquarion Water asked for immediate, voluntary water use reduction of 10% in Greenwich, Stamford, Darien and New Canaan.  In Darien, representatives of Aquarion Water Company presented "Water Conservation: How We Got Here and Where Are We Going and their Drought Response Action Plan to the Board of Selectmen on Monday, October 23. 

Aquarion maintains on its website a weekly updated water usage report that includes water demand graphs, system transfers, system capacities, days usable storage remaining, and precipitation. On Oct 19 they noted they had passed their Drought Advisory trigger of a 10% chance that their system's storage capacity could drop to 90 days water supply. 

Groundwater Discussion

In Connecticut, the USGS has 10 real-time groundwater monitoring wells. 8 of these have multiyear records. 6 of these 8 were near normal on Nov 9, the Greenwich well was below normal, and the Scotland well was above normal.

All 10 live-data wells showed a significant jump due to the storms at the end of October and the first week of November. The rain fell for longer periods of time than usually occurs with heavy downpours, so even though most of that precipitation ran off into streams and rivers, the long duration of maximum infiltration significantly recharged groundwater. Looking back over the graph of levels for individual wells shows a few other times in the past couple years when any individual well showed similar jumps in its water table, but rarely do all the wells in the state show such a jump at the same time. 

Before those storms, on Oct 24, 4 of the wells were below normal for the time of year, up from 2 on Oct 12. The other 4 of these 8 wells were near normal, down from 6 on the 12th. No wells had been above normal since the beginning of September. Well measurements across the state were trending lower before the rain events at end of October. Groundwater levels normally fall steadily during the summer through September, then begin to rise during October. This year, the water continued to drop steadily through most of October, then rebounded suddenly at the end of the month. 

Monitoring the water table can provide clues to future conditions. There was an apparent pattern in 2013-2016 in which normal groundwater levels in winter and spring alternated with increasingly well-below-average levels in summer and fall. 2017 was on track to continue this until this until the unusual storms reversed the trend. Click here to see all the USGS graphs for their real-time groundwater measurements that compare levels from the last four years to the median level for each day of the year (lots of data, so it takes a long time to load). 

Groundwater levels influence streamflow. The water level in streams and rivers between storms depends on the flow of water out of the ground into the stream channel. The higher the water table, the more water is available to keep streams flowing. During a drought, smaller upland streams dry up faster than the larger rivers (details below). An occasional brief, powerful rainstorm does not do much to help groundwater levels rise. The rain falls faster than the ground can absorb, and thus much of it becomes stormwater runoff. In contrast, public water supply systems that depend on reservoirs do benefit from heavy rains because the reservoirs collect the runoff. The runoff, however, can wash pollutants into the reservoirs, lowering water quality. Streamflow after the storms showed the expected drop that occurs after storms, but flow rates became fairly steady near what they normally are due to groundwater levels returning to near normal. 

The USGS Groundwater Watch website displays data from other wells that are measured once a month, comparing the most recent measurement to each well's records. The first of the heavy rain events at the end of October was along a fairly narrow corridor through the state that only raised the water table for some of those wells. The rain that soaked the rest of state came after most of the wells were measured. 

Consequently, at the end of October, 6 of the 59 wells were higher than any of those wells' previous October records. 3 of those were even higher than any of their November records. 4 wells were rated much above normal; 11 were above normal. 25 out of 59 wells were near normal compared to previous October records. The total of 46 wells near or above normal, compared to 50 at the end of September and 56 in August, was due to the way storm paths had been consistently missing the state.

13 wells were still below normal near the end of October, compared to 10 for September and 4 in August.

U.S. Drought Monitor

All Abnormally Dry and Moderate Drought areas of the state were removed from the U.S. Drought Monitor ratings on November 2. The Moderate Drought category had been expanded to 83% of the state one week earlier.

Precipitation Patterns 

The 4 to 8 inches of rain that got dumped on Connecticut at the end of October and beginning of November abruptly ended a persistent pattern of storms tracking out to sea or to the northwest of us. 

Total rainfall in the 30 days through Nov 9 were from 50% to 200% above normal over most of Connecticut. About a quarter of the state, in the northwest and southwest corners, saw 30-day totals near normal for this time of year. The 60-day totals still show below-normal rainfall for western portions and near-normal for the remainder of the state. 

Go to the National Weather Service (NWS) Advanced Hydrologic Prediction Service at this link for updated, detailed maps and data. 

The National Weather Service 7-day forecasts show a relatively dry week; the long-range (16-day) computer models were predicting normal rainfall. Connecticut normally receives 3 to 5 inches of rain in November.

Each month, the NWS Climate Prediction Center issues analyses of global atmospheric conditions that could influence general temperature and precipitation patterns. Their October 19 prognostications for the the next year show equal chances for above-normal, normal, and below-normal precipitation until next summer, when they give elevated chances for above-normal rainfall. Above-normal temperatures are more likely than normal temperatures for most of the next year. 

The Oct 19 release of the monthly U.S. Seasonal Drought Outlook from the National Weather Service Climate Prediction Center indicated drought removal likely for northeast Connecticut. Here are important points from their discussion for the northeast: "Moderate drought (D1) expanded recently to include... parts of Connecticut, ... based on 30 to 90-day precipitation deficits and low streamflows. ... Despite the increasing precipitation deficits, the late fall and winter is a favorable time for drought recovery with low evapotranspiration rates, dormant vegetation, and soil moisture recharge .... There is the potential for heavier precipitation amounts across the Northeast, ... Given the likelihood for moderate to heavy precipitation during the remainder of October coupled with climatology, drought removal is forecast for the Northeast."

Streamflow Graphs and Storms

Click to enlarge. Streamflow graphics courtesy USGS The bottom graph to the right illustrates what can be considered an average pattern of streamflow over time, for example, a month -- streamflow peaks above its average for the time of year but then drops to below average. Streams in nature maintain healthy flows between storms if the water table level in the watershed of the stream is near or above average for the time of year.

How quickly the flow drops after each rainstorm and how low it drops are unique for each stream, because those factors depend on how much water ran off the surface compared to how much soaked in to increase groundwater levels. If the watershed of the stream has not received average long-term rainfall, the low water table will allow the flow between storms to drop to unusually low levels. In watersheds with lots of impervious roads, roofs, parking lots, patios, and compacted soil in lawns, a lot of the rain typically flows over the surface or through storm drains directly into the streams, leading to flooding problems far more often than in watersheds with mostly natural surfaces. The streamflow then drops quickly after the storm ends, and the flow levels off far lower than in streams whose watersheds have more natural surfaces.

How did the 2016 drought compare to historic droughts in Connecticut? The 1960s' drought was clearly the worst of the past century, but our recent drought may have been the second worst. The graph is from NOAA's National Environmental Satellite, Data, and Information Service (NESDIS), displaying the Palmer Hydrologic Drought Index (PHDI) for our state for the past century (click on it for a larger view in a new tab). Since groundwater and streamflow did not fully recover from the annual return of drought conditions, we can consider the drought as starting in 2010 or 2012. The overall length of time we had these intermittent drought conditions is comparable to the continuous drought of the 1960s. At times in 2016, the severity of the drought equaled the maximum severity in the 1960s. It is therefore fair to say the recent drought was one of the worst in 100 years. In November 2016, area weather forecasters were reporting that the drought was the worst since the 1960s.  

Are small and large streams affected by dry conditions equally?

This graph plots the rate of flow on September 4, 2015, as a percentage of the median for each stream gage. For example, a dot at the 40% line means that stream that day was flowing at 40% of what could be considered normal for that date. The few rivers and streams flowing at or above 100% of their median rate are mostly those with flows managed by dam releases.Comparison of stream sizes


Climate in the Draft State Water Plan

(Click here to return to top of page)

Rivers Alliance of Connecticut
PO Box 1797, 7 West Street 3rd Floor, Litchfield, CT 06759-1797
rivers@riversalliance.org, www.riversalliance.org