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CT Streamflow and Drought Analysis

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

  Know Your Flow!
We update this webpage with the latest information weekly.

Most recent update May 19.
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.
US Drought Monitor Map of CT
(click map for details)
Current US Drought Conditions. Click to open                  source page.
The U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. Map courtesy of NDMC-UNL.
Map of Below Normal 28-Day Average Streamflow

Recent Flow At a Typical 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.

Summary (details follow)

As of May 18, 85% of Connecticut is rated as no drought on the map of the state published by the US Drought Monitor (click here for an analysis). 15% of the state remains Abnormally Dry. See the discussion below for a comparison to previous years. 

The Interagency Drought Workgroup (IDW) met on May 3 and recommended improving "Drought Watch" to "Drought Advisory" for all counties; continue monitoring conditions.

Press release:

Connecticut Interagency Drought Workgroup Ends First-Ever Drought Watch, Maintains Statewide Drought Advisory
Urges Residents And Businesses Served By Public Water Suppliers To Continue Following Requests From Their Suppliers

Some water companies in May were still asking their customers to conserve water; see below.

On Friday, May 19, the Northeast River Forecast Center reported none of the 20 river gages they monitor were near flood, and they were all expected to drop in the next three days.

Groundwater level is generally recovered from its drought conditions, with most wells being near normal for this time of year and a few being above normal.

Streamflow Discussion 

On May 19 only the Aspetuck River stream gage in the Aspetuck section of Easton, CT was reporting low flow. By Monday May 22 however, 4 more streams were added to our low-baseflow list. See below for a discussion of how we count low flows.

The USGS's 30-day graphs of streamflow in Connecticut (link below) show similar patterns of the normal rapid jumps in flow during rain events. The rain events have been often enough recently to prevent the flows from receding below the average flows for the date, however. Earlier in the 30-day period, many streams stayed near average without high peaks due to a long interval between storms. See below for further discussion of streamflow patterns.

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 daily average flows.

The USGS's 28-Day Average Streamflow map now categorizes all of Connecticut as having normal or higher-than-normal streamflow.

NOTICE (03/30/2017) -- Data collection at the following gages may be discontinued on June 30, 2017, due to funding reductions from partner agencies. Although historic data will remain accessible, 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 these stream gages should contact Jon Morrison at the USGS New England Water Science Center - Connecticut Office (860-291-6761) or email at jmorriso@usgs.gov.

North Branch Park River at Hartford-----_-52 years of records
Rooster River at Fairfield------------------39 years of records
Mill River near Fairfield--------------------44 years of records

Public Water Supply Information

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

The CT Department of Public Health (DPH) Monthly Reservoir Status Summary dated May 3 reports capacity was at 100% of normal for the end of April, meaning the reservoirs of 34 large water systems averaged 97% of full capacity. More details:

  • 25 of the 34 systems reported no restrictions on water use at the end of April.
  • "Voluntary Water Restrictions" was noted for four systems, but no further information was available on their websites as of May 11.
  • An "Alert" is noted for Bristol; their website has an April 20 press release noting that mandatory water restrictions were lifted and that they were asking for voluntary odd-even watering outdoors.
  • New Britain also listed "Water Supply Alert" on their last report to the DPH in April. They reported at that time that their reservoirs were down to 80% of capacity. As of May 11, no recent information on what their Alert means could be found on their website.
  • "Water Supply Emergency" is listed for Aquarion Water's Darien, Greenwich, New Canaan and Stamford systems. Their websites have a March 28, 2017, statement: Ban On Outdoor Irrigation Continues.

DPH's Monthly Reservoir Status Summary is one of several links on DPH's Reservoir Data monthly web page.

Aquarion maintains on its website a weekly updated water usage report that includes customer usage trends, system transfers, system capacities, days usable storage remaining, and precipitation. They note that as of May 15 their Greenwich Reservoir was 99% full, slightly higher than last year and slightly higher than usual at this time of year based on 30 years of records. Their Stamford system is still a little lower than where it was last year or on average for May 15. Their Bridgeport system reservoirs are at 99.9% full, same as last year and higher than average.

The graphs show that last year their reservoir levels did not begin falling at rates faster than usual until June. That part of the state last year was rated Abnormally Dry by the U.S. Drought Monitor in May and June and was the first area to again be rated Moderate Drought in June 2016. The USGS live-data monitoring well in Greenwich also did not began to fall faster than usual until June 2016.

Groundwater Discussion

In Connecticut, the USGS has seven real-time groundwater monitoring wells with multiyear records. Comparing the May 19 water levels with all measurements for that date, 4 were higher than average and 2 were near normal.

The water table at those wells has recovered very well from previous months' levels. March 30 was the first time in months that none of the seven monitoring wells were breaking records for lowest measurement for the date. The water table's lowest levels occurred last September, with more stations breaking low-water-level records than those that were merely very low until November. The water table usually begins to drop each year in June. In previous years drought conditions began to show up in groundwater measurements when the water levels fell at a faster rate than normal. So far this year levels seem to be pretty steady.

The USGS Groundwater Watch website displays data from 60 wells with monthly measurements along with 10 continuously monitored wells. As of May 12, the most recent monthly measurements were from late April, when 4 of the 60 wells were rated above normal, 45 were normal, and 14 were below normal compared to their records for April measurements.

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 that situation, public water supply systems that depend on reservoirs are affected less than those with wells because the runoff is collected by the reservoirs.

graphThere has been an apparent pattern for the last five years in which normal groundwater levels in winter and spring have alternated with well-below-average levels in summer and fall. Click the graph to the right for a larger version in a new tab of a modified USGS graph. There are two trends of great concern. One concern is that the below-normal levels in summer and fall got worse in each of those years. The other concern is that the water table has not recovered yet this year as it did in previous years. Five years is not long enough to make any statistically valid conclusions about climate, however, and those station records only go back seven to 14 years. 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). USGS manually measures other wells once a month or so, and a quick look at those records verified the possibility that this pattern may apply to all our groundwater.

The USGS Groundwater Watch website is an excellent source of data for examining historic and current trends. It has a map of Connecticut that is colored to show how the most recent measurements compare to historical records for each station.

Precipitation Patterns

The National Weather Service (NWS) Advanced Hydrologic Prediction Service has detailed maps and data at this link to compare actual precipitation to what is normal for the time of year. Recent rainfall has brought Connecticut's 30-day rainfall totals to near or above average across the state. Just last week the 30-day totals were 25% below normal except for about a quarter of the state along the Rhode Island border and along Long Island Sound.

Storm Tracks: For most of 2015 and 2016, storm centers repeatedly went out to sea to our southeast or passed us by to our northwest, instead of a more usual pattern of random storm paths moving generally west to east. In addition to this unusual pattern, rain that did mange to track over us tended to be fairly heavy. Generally, as our climate changes, heavy precipitation events are becoming more frequent, but heavy rain does not recharge groundwater aquifers as much as the same amount of rain over longer periods would. This means that even when long-term total rainfall returns to normal, streams and rivers could still be afflicted with unusually dry conditions between storms, because flow will drop to unusually low levels due to a lower than normal water table.

Why the unusual storm tracks? Some studies connect a shift in wind systems such as the jet stream to the unprecedented summertime warming in the north related to the loss of Arctic sea ice. For more information, see these articles about the studies: Scientists Link California Droughts, Floods To Distinctive Atmospheric Waves (2017-04-06), Extreme Weather Events Linked to Climate Change Impact on the Jet Stream (2017-03-27), .

Rain? 3 to 5 inches of rain is normal for the month of May in Connecticut, according to the National Weather Service (NWS), based on data from 1981 to 2010. On May 19, the NWS's seven-day forecasts were predicting normal rainfall, and so were the long-range (16-day) computer models

Each month, the NWS Climate Prediction Center issues analyses of global atmospheric conditions that could influence general temperature and precipitation patterns. Their May 18 analyses for trends through the end of the calendar year show equal chances for above-normal, for normal, and for below-normal precipitation. Above-normal temperatures are a little more likely than normal temperatures. This is pretty much the same three-month forecast we have seen for over a year, but actual precipitation was much lower than normal. Before the April analysis, for three months in a row, their prognostications for the summer predicted above-normal rainfall, but that prediction has now disappeared.

Lowest Baseflow (Between Storm) Gaged Streams and Rivers

The flow between storms is an indicator of drought conditions. We define low flow as below the 25th percentile for that stream, or below 25% of the mean or below 25% of the median 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. The 25th percentile is a flow rate lower than 75% of all average daily flows recorded on that date for that stream.

Why do we count? A count of the number of USGS stream gages reporting low flow at their lowest point between storms is valuable as an indicator of drought conditions. There are only 62 stream gages in Connecticut, so they may not represent the hundreds of streams with no gages. 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 the drought. If the count decreases, we can look for continued recovery from drought conditions.

Variable rainfall this spring has greatly affected the count of low-flow gaged rivers. Instead of the fairly steady increase in the number of storms and rainfall intensity one expects to see in spring, there has been wide variation week to week. The double-digit count of low-flow streams out of the 62 measured by USGS first dropped to single digits (about 15% of the 62) in mid-February, lasting until the beginning of March. The count of low-flow gages jumped in March but went back to single digits again as storms increased in early April. Since mid-April, streamflow has generally been near normal for the time of year, but there have been stretches of time between storms during which some stream flows have dropped to low levels. 

Here are the recent numbers:

May 22: 7 out of 62 stream gages reporting low flows. List is below.
May 19: only the Aspetuck River stream gage in Aspetuck, CT was reporting low flow.
May 12: 4 out of 62 gages were in low-flow status.
May 1:  15 stream gages reporting low flow.

Low Baseflow USGS Stream Gages Monday , May 22, 2017
Station Name
Station Website
ASPETUCK RIVER AT ASPETUCK (Easton/Weston) https://waterdata.usgs.gov/ct/nwis/uv?site_no=01209105
BROAD BROOK AT BROAD BROOK (East Windsor) https://waterdata.usgs.gov/ct/nwis/uv?site_no=01184490

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.

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, in that the stream shows a flow that 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 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, is unique for each stream, because it depends 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.

U.S. Drought Monitor

 As of May 18, only 15% of Connecticut remains Abnormally Dry. The 85% of the state with a U.S. Drought Monitor rating of "None" is the largest portion with that rating since December 2014 through April 2015. See the graph below. Most of Connecticut (76%) had finally reached the category of "None" on May 11. More than half the state had been at least Abnormally Dry since April 2016.

Their May 11 narrative for the Northeast said: "Another soaking storm further reduced the coverage of lingering, long-term dryness (D0). Vestiges of dryness, reflected mainly in spotty groundwater shortages, remained in Maryland, Pennsylvania, and Connecticut."

Comparisons to past years' Drought Monitor ratings.

This graph compares the extent of drought conditions in Connecticut over the past four years. As the graph below shows, Connecticut has been labeled Abnormally Dry, Moderate Drought, or worse pretty often over the last few years.

Drought Monitor Connecticut graph

Last year the state was never completely free of abnormally dry conditions though the area rated None reached 70% during March, two months earlier than this year. The None area then began decreasing again in April 2016 until all of the state was at least Abnormally Dry by mid-June 2016. Two years ago, all of the state had been rated None from December 2014 through April 2015. The 2013 dry period lingered until April 2014 when all of the state was rated None. Abnormally Dry areas began again in July 2014. 

The persistent low streamflow levels during the drought had harmful effects on river ecosystems. Last November, for example, fishermen reported long stretches of rivers with no catch at all. How well river ecosystems recover from the drought will depend on flow from groundwater. Where groundwater is near normal, streamflow should remain healthy between storms unless rainfall patterns change in late spring as they did in the past four or five years. 

The past U.S. Drought Monitor data for Connecticut used for these comparisons can be found at this link. From the end of January to the beginning of April, Connecticut had the dubious distinction of being the most drought-afflicted state of all 50 based on percentage of area. Click here for details from an archived Know Your Flow! page.

The U.S. Drought Monitor publishes a weekly analysis of drought conditions across the entire nation based on a variety of types of data that include streamflow. The report comes out every Thursday based on data from that Tuesday. The U.S. Drought Monitor is produced by the National Drought Mitigation Center, which is also on Facebook.

How does the current drought compare to historic droughts in Connecticut? This 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 of the past few years, we can consider our current drought as starting in 2010 or 2012. The overall length of time we have had these intermittent drought conditions is comparable to the continuous drought of the 1960s. At times in 2016, the severity of the current drought approached the maximum severity in the 1960s. It is therefore fair to say the current drought is one of the worst in 100 years. Area weather forcasters have been reporting that the current drought is 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.

Click here for a search of news articles about the drought.


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