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

Know Your Flow!

Updated Aug 7, 2018
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 

The middle of July saw a distinct change in streamflow patterns. A trend of  lower and lower baseflows between widely space storms reversed itself to a pattern of increasing flow in most streams. Before this, storms had been repeatedly tracking west of Connecticut, but beginning July 17,  an eastward shift in weather patterns started to bring significantly more rain and severe weather than Connecticut had seen for months. We may have missed the worst of it though. Although there were many storm based statements from the National Weather Service from July 17 through Aug 7 for Connecticut, there were fewer reports of flooding in Connecticut than in neighboring states. Perhaps the low river levels before the storms arrived kept them from flooding. The Indian River near Clinton peaked near its two-year recurrence level, something usually seen in the spring. 

Although the storms were not enough to produce much flooding, they were enough to result in 71 reports of combined sewer overflows of somewhere between 6.7 million to 31.2 million gallons of  combined runoff and sewage in four of our cities. 4 additional reports of sewage bypass events were also blamed on the rain.  Click here for a zoomable map of any sewage spills reported to CT DEEP in the last week.  

Since mid July, most stream gages in the state show streamflow climbing above their average daily flow and not falling back below average before the next rain event. Notable exceptions to this are a few gages where reservoirs did not overflow during storms and operators were not required to release much, if any, water. The table below lists these gages. 

Here are the numbers from recent Know Your Flow reports:

Aug 7:     3 of 64 USGS stream gages in CT were reporting low flows*; 2 were setting low-flow records for the date.
July 26:   3
July 21: 18
July 17, 42 were reporting low flows;  3 gages were setting  records.
July   6: 10  
June 27: 33 were reporting low flows* ; 2 gages setting records.

Stream Gages Reporting Low flow*
Tuesday Aug 7, 2018

25th percentile,
25% of mean,
or both
Station Name
(click for the Station website)

Some notes regarding flow,
quotes from the USGS Station Yearly Summary are in italics


Flow on Aug 7 was lower than any daily average flow for the date over 19 years of record.
Flow regulated from New Canaan Reservoir by Second Taxing District Water Department


Flow on Aug 7 was lower than any daily average flow for the date over 12 years of record.
The natural flow of Muddy River has been altered by flow diversion for municipal supply of Wallingford, CT. Flow regulated by Mackenzie Reservoir directly upstream. 

p SAUGATUCK R BELOW SAUGATUCK RES NR LYONS PLAIN Flow dropped to low levels only briefly on Aug 7.  
Flow regulated by reservoir releases.

 *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 on a day when rivers are near their presumed lowest points between storms. High flow streams are those reporting over their 75% percentile for that date.

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 of previous years. 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. A discussion of normal streamflow patterns can be found here.   

Informative Links:
Click here for today's USGS Connecticut stream gage web page with graphs
of flow from every gage for the past 30 days compared to their historic daily flows

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

Click here for a discussion from an archived Know Your Flow!
web page of what streamflow patterns usually look like

Check for electronically reported sewage spills in the last three days

U.S. Drought Monitor

link to websiteBased on July 31 data, the U.S. Drought Monitor rates all of Connecticut "No Drought".  They had dropped the "Abnormally Dry" area of Connecticut down to 18% on July 24, along the eastern side of the state and its northwest corner. It had rated 40% of Connecticut as "Abnormally Dry" from June 9 to July 3, then 59% between July 10 and 17. Before June, the whole state had been "No Drought" since February. Click here for their complete discussion of current data.  Northern New England however still has dry conditions. 

Public Water Supply Information

Your water utility and town are the best sources of information on the status of your water supply. A few systems post their current capacities regularly on their websites.

Aquarion maintains on its website a weekly updated water usage report for their systems in southwest CT that includes water demand graphs, system transfers, system capacities, days usable storage remaining, and precipitation. As of July 24, all those systems were where each system usually is at this time year based on a 20-year average. Their Greater Bridgeport System was at 93%, Stamford system 87%, and the Greenwich System was at 83% of Usable Storage. Their graphs also showed that the Stamford and Greater Bridgeport systems were close to where they were last year at this time, but the Greenwich system was lower than last year.

As of July 1, MDC reports its reservoirs were are at 97.9% of capacity. The South Central Regional Water Authority reported  as of May 31, its reservoirs were at 96% of capacity. 

The June CT Department of Public Health's Monthly Reservoir Status Summary (dated July 13) reported that the reservoirs of 34 large water systems averaged 96% Usable Storage, which was 102% of where they usually are at the end of June.  All but 1 system reported levels above 90% Usable Storage. New Britain Water Department reported its reservoirs were 82% full, down from 88% at the end of May. 

In the May report the 34 systems averaged 99% Usable Storage, which was 102% of where they usually are at the end of May.

On the CT Department of Health Reservoir Data webpage are links to the Current Statewide Reservoir Capacity Chart, and Historical Reservoir Capacity Charts going back to 2002. These have bar graphs showing each month's percent usable storage compared to that month's historical average. Looking through the graphs over the past few years, you can see that the average percent usable storage has dropped below the historical average for more than a month each year since 2012. Here are the months in which the usable storage first dropped below average and the number of months it stayed below average: 

September 2017 — 2 months
May 2016 —at least 8 months (early 2017 data missing)
August 2015 — 6 months
August 2014 — 4 months
October 2013 — 4 months
August 2012 — 2 months

Before that, the years in which usable storage was below or above average for months at a time nearly alternated:

Stayed above average in 2011
Dropped below average in June 2010
Stayed above average in 2009
Stayed above average in 2008
Dropped below average in May 2007
Stayed above average in 2006
Dropped below average in June 2005
Dropped below average in August 2004
Stayed above average in 2003
Dropped below average in July 2002 

Groundwater Discussion

In Connecticut, the USGS has 10 real-time groundwater monitoring wells. 6 of these wells publish reports comparing the current depth to water table with the average for the date over multiple years of record.  On July 22, 4 of these 6 were near-normal for the date over 7 to 15 years of record, 2 were below normal over 9 and 15 years of record. 

The USGS Groundwater Watch website displays data from 60 other wells that are measured once a month, comparing the most recent measurements to each well's measurements taken in that month in previous years. At the end of June, 39 of the 60 wells had near-normal groundwater levels, down from 47 at the end of May. 9 of them were above or well above normal (7 in May). 12 were below normal at the end of June (6 in May).  At the end of April, none of the wells were below normal.  Click here to see their current data. 

Monitoring the water table can provide clues to future conditions. There was an apparent pattern from 2013 to 2017 in which normal groundwater levels in winter and spring alternated with increasingly well-below-average levels in summer and fall. Although 2017 groundwater levels did get as far below average as they did in 2014 or 2015, they did not drop as fast as in 2016. 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 (click here for details from a previous Know Your Flow! webpage). 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.

Precipitation Patterns 

NOAA's June 2018 Synoptic Discussion:  "... a large broadscale/long-wave ridge dominated the overarching circulation ... The ridge was responsible for another month of widespread much warmer-than-normal temperatures ... and, for the first half of the month, mostly drier-than-normal weather... the ridge inhibited precipitation and enhanced evaporation, with below-normal precipitation occurring across ... parts of the ... Northeast... As a result, drought and abnormal dryness developed, expanded, or intensified in parts of the ... Northeast." 

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


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