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

Know Your Flow!

Updated June 12, 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 

Streamflow graphs since April 1st (caution: long download) are dominated by the very high flows from the extreme rainfall events April 15-16. Since then, other heavy rain events have also put high flows in the news and spiked the streamflow graphs, especially in mid-May. These high flows tend to mask awareness of how low our streams and rivers get between rain storms. See the table below for a list of gages reporting low flows. 

Before mid-January, then again from mid-March to mid-April, storms were diverted to our northwest or southeast by persistent blocking patterns. From mid-January to mid-March, above-freezing temperatures melted the snowpack, and storms tracked more consistently over the state. A look at all USGS Connecticut streamflow graphs for the past five months (bigger download, takes  even longer) shows that flows were consistently near or below normal until mid-January, then suddenly went to higher levels for about two months. From mid-March to mid-April, although streamflow patterns showed normal variations from above to below normal that are expected for that time of year, there was an overall trend to lower flows.  A discussion of normal streamflow patterns can be found here.

Since mid April, flows have been jumping to high levels less often than in February and March, causing flows to drop to lower levels between storms. In addition to the 23 gages reporting lows flows below, an additional 30 gages were reporting flows below their median levels on June 8. Only 12 gages were still above their median flows for the date.  

On Fri June 8, 22 out of 64 USGS streamgages in CT were reporting low flows*. See the table below. By Tuesday June 12, that number had risen to 37 out of 64, or 50% of the gages. 

Here are some numbers from previous Know Your Flow reports:

May 31: 28 out of 64 USGS streamgages in CT were reporting low flows.
May 10: 11 out of 62 streamgages. The Fivemile River near New Canaan was setting a low-flow record for the date
May 1: no stream gages were reporting low flows. 
April 12, 27 of 61 By April 15, (just before the heavy precipitation of April 16) that number had risen to 36. 
Mar 29: 30 of 61.
Mar 19: 3 gages,  on the Natchaug River at Marcy Rd. near Chaplin, the Naugatuck River at Thomaston, and Bunnell Brook near Burlington were reporting low 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.


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


Stream Gages Reporting Low Flow* 
Friday June 8, 2018

25th percentile,
25% of mean,
or both
Station Name Notes from the Station's Yearly Summary Station website
p BROAD BROOK AT BROAD BROOK 50 years of record. Flow regulated by reservoir and mill upstream. https://waterdata.usgs.gov/ct/nwis/uv?site_no=01184490
m BYRAM RIVER AT PEMBERWICK No flow regulation mentioned https://waterdata.usgs.gov/ct/nwis/uv?site_no=01212500
m COGINCHAUG RIVER AT MIDDLEFIELD Infrequent regulation from Beseck Lake https://waterdata.usgs.gov/ct/nwis/uv?site_no=01192883
p CONNECTICUT RIVER AT THOMPSONVILLE Flow regulated by power plants, by diversion from Chicopee River Basin and by First Connecticut and Second Connecticut Lakes, Lake Francis, Moore and Comerford Reservoirs, Quabbin Reservoir, and other reservoirs, combined usable capacity, about 107 billion ft³ https://waterdata.usgs.gov/ct/nwis/uv?site_no=01184000
pm FENTON RIVER AT MANSFIELD Flow lower than any average daily flow for the date over 11 years of record. No flow regulation mentioned. https://waterdata.usgs.gov/ct/nwis/uv?site_no=01121330
pm FIVEMILE RIVER NEAR NEW CANAAN Flow lower than any average daily flow for the date over 18 years of record. Flow regulated from New Canaan Reservoir by Second Taxing District Water Department https://waterdata.usgs.gov/ct/nwis/uv?site_no=01209761
p FRENCH RIVER AT NORTH GROSVENORDALE Flow regulated by Hodges Village and Buffumville Reservoirs, by Lake Chaubunagungamaug and other smaller reservoirs upstream https://waterdata.usgs.gov/ct/nwis/uv?site_no=01125100
p HOCKANUM RIVER NEAR EAST HARTFORD Streamflow regulated by Shenipsit Lake, smaller reservoirs, industrial plants, and augmented by releases from upstream waste-water treatment facilities https://waterdata.usgs.gov/ct/nwis/uv?site_no=01192500
pm HOUSATONIC RIVER AT STEVENSON Flows due to power generation varying from above 75th percentile to below 25th percentile daily https://waterdata.usgs.gov/ct/nwis/uv?site_no=01205500
p MOUNT HOPE RIVER NEAR WARRENVILLE Occasional regulation from ponds upstream. https://waterdata.usgs.gov/ct/nwis/uv?site_no=01121000
m NATCHAUG RIVER AT MARCY RD. NEAR CHAPLIN No flow regulation mentioned https://waterdata.usgs.gov/ct/nwis/uv?site_no=01120790
p NEPAUG R NR NEPAUG Live data began May 2017 but daily average flow is based on 52 years of record.  https://waterdata.usgs.gov/ct/nwis/uv?site_no=01187800
pm NONEWAUG RIVER AT MINORTOWN Minor alteration to the natural flow from Lockwood Reservoir and by out-of-basin flow diversion for municipal supply for the town of Watertown https://waterdata.usgs.gov/ct/nwis/uv?site_no=01203600
p PEQUABUCK R AT FORESTVILLE Flow regulated by Whigville Reservoir and mills upstream. Diversion for municipal supply of City of New Britain from Whigville Reservoir, Whites Bridge wells, and Copper Mine Brook https://waterdata.usgs.gov/ct/nwis/uv?site_no=01189000
p POOTATUCK RIVER AT SANDY HOOK No flow regulation mentioned https://waterdata.usgs.gov/ct/nwis/uv?site_no=01203510
p QUINEBAUG RIVER AT QUINEBAUG Peak flows are affected by flood-control regulation at East Brimfield Lake and Westville Lake since 1960. The natural flow regime can be altered by regulation at East Brimfield Lake, Westville Lake, and other small reservoirs in the basin. https://waterdata.usgs.gov/ct/nwis/uv?site_no=01124000
pm QUINEBAUG RIVER AT WEST THOMPSON Peak flows are affected by flood-control regulation at East Brimfield Lake, Westville Lake, and West Thompson Lake since 1960. The natural flow regime is altered by regulation at East Brimfield Lake, Westville Lake, West Thompson Lake, East Brimfield Lake, and other small reservoirs in the basin. https://waterdata.usgs.gov/ct/nwis/uv?site_no=01124151
pm RIPPOWAM RIVER AT STAMFORD The natural flow is altered by Mallard Lake, Trinity Lake Reservoir, Laurel Reservoir, Suscowit Reservoir, and by storage and diversion at North Stamford Reservoir.  https://waterdata.usgs.gov/ct/nwis/uv?site_no=01209901
pm SAUGATUCK R BELOW SAUGATUCK RES NR LYONS PLAIN Flow lower than any average daily flow for the date over 8 years of record. Flow regulated by reservoir releases. https://waterdata.usgs.gov/ct/nwis/uv?site_no=01209005
m STONY BROOK NEAR WEST SUFFIELD No flow regulation mentioned https://waterdata.usgs.gov/ct/nwis/uv?site_no=01184100
pm WILLIMANTIC RIVER AT MERROW RD. NEAR MERROW Flow lower than any average daily flow for the date over 7 years of record.
p WILLIMANTIC RIVER NEAR COVENTRY Effects of flood-control dams in the Middle River Basin (Ellithorpe Reservoir Dam No. 5, Ellis Reservoir Dam No. 2, Whitney Reservoir Dam No. 1, Shenipsit Reservoir Dam No. 6, Pomeroy Reservoir Dam No. 3, and Bradway Reservoir Dam No. 4) on peak flows are minor. Usable storage at the gaging station from flood control is about 31 acre-feet per square mile. The natural flow of stream can be altered by regulation from Staffordville Reservoir and the flood-control dams. https://waterdata.usgs.gov/ct/nwis/uv?site_no=01119500

 *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.

U.S. Drought Monitor

The U.S. Drought Monitor has been rating all of Connecticut "No Drought" since February. Their June 5 Summary reports an expanding area of "Abnormal Dryness" from eastern Massachusetts through New Hampshire and southern Maine. The Monitor lists streamflow as one of the reasons for that ranking: "Stream flows in this area are within the lowest quartile of the historical distribution for this day of the year..." Almost a third of Connecticut's streams can be described the same way. Click here for their discussion of current data. 

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. For example: 

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. As of June 4, their reservoirs ranged from 97% to 100% of Reservoir Total Capacity, at or above where they usually are at this time of year.  

As of June 1, MDC reports its reservoirs were at "99% of capacity with 39.5 billion gallons in storage representing 659 days of supply assuming typical water use and no precipitation". The South Central Regional Water Authority reported  as of May 31, its reservoirs were at 96% of capacity. 

The most recent Monthly Reservoir Status Summary from the CT Department of Public Health available (as of June 12) was the March report issued April 23. In it, CT DPH reported that the reservoirs of 34 large water systems averaged 99% Usable Storage, which was 103% of where they usually are in March. 

Note: the CT Department of Health recently moved its Reservoir Data webpage and old links to it may not work. The new page can be found here.

Groundwater Discussion

In Connecticut, the USGS has 10 real-time groundwater monitoring wells. 6 of these compare their current measurements to multiyear records. On June 12, 3 of these 6 monitoring wells were reporting low water tables compared to the historical records for the date based on 8 to 13 years of record. The other 3 were reporting near-normal levels. All 6 of these wells were reporting near or above-normal levels in May.   Click here to see their current status. 

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 May, 6 of the 60 wells  were below normal, 47 were at normal levels, 5 were above-normal, 2 were well above-normal. 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 in 2013-2016 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.


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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