Connecticut Streamflow and Groundwater Overview
Know Your Flow!
Thursday, August 25, 2016 update: Although the return to normal summer rainfall in recent weeks across CT stopped the deepening of the drought, clear weather is predicted, which could return most rivers and streams to low-flow conditions. The U.S. Drought Monitor lists the bulk of Connecticut in the Moderate Drought category, with 5% of the state in the Severe Drought category. As reported by USGS stream gages, one-half of CT rivers and streams have seriously low flows, but that fraction is expected to increase. Unlike in recent reports, there are no record-low flows reported by the USGS stream gages. Groundwater levels show little or no improvement, which is why most streamflow graphs still show unusually low baseflow between storms. A few streamflow graphs started to show the normal pattern of equal graph areas above average and below average. Details below.
News media are reporting our 30-day rain deficit has been erased for much of the state, but some areas still have 60-day rain deficits of 1 to 2 inches with small areas up to 3 inches. Even where the heaviest rain occurred, groundwater remains low because the rain fell faster than the ground could not absorb. As of Aug. 22, less than half the state still had a 60-day rain deficit. Other drought stories in the news include banning of fishing in some areas of the Farmington River, closed beaches on Bantam Lake, and water conservation.
Streamflow continues to fall to seriously low levels between rain storms because long-term rainfall has been a lot less than what we normally get, which causes groundwater levels to drop. These topics are discussed in more detail below. In June, the CT Department of Public Health issued the first Drought Advisory since 2010. They cited the low rainfall but emphasized that public water supplies were doing fine at that time. The July 2016 monthly report from the CT Department of Public Health reported 11 of 34 water companies have asked for voluntary water restrictions, and 1 company has issued manadatory restrictions. State reservoirs were at an average 81% of their capacity (down from 91% in June), ranging from 62% to 100%.
Streamflow: Click here for a display of USGS Connecticut stream gages with graphs of their flow for the past 30 days compared to their daily average flows. An alphabetical list of low flow rivers is below. Recent rains have brought relief to streams where storm paths brought heavy rain, but streams in areas that did not get much rain were not affected.
Stream gages normally show pulses of above-average flow when it rains, then below-average between storms. Only a few of the records show Connecticut's streams rising above average at all during storms; most are barely reaching average, and a few have not even come close to average. Dry conditions that cause flow to be below average more often than it is above will affect stream life if these conditions continue for too long. For a review of the localized relationship between streamflow and precipitation, see below.
Groundwater: 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 (see the graph below). Groundwater levels recovered during the first three months of the year but have been dropping ever since. 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.
Recent rains produced little or no improvement at the seven real-time monitoring wells run by the US Geological Survey (USGS). All of them report levels lower than 75% of their records for the date; three of them are lower than ANY of that station's records for the date. This continues a possible pattern of the last four years in which normal water levels in winter and spring have alternated with below-average in summer and fall. What is worrisome, however, is that the below-normal levels got worse each of those four years. Four 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. It was hoped that the El Nino would break the pattern (if there is a pattern), but the El Nino ended in May. USGS manually measures other wells once a month or so, and a quick look at those records verified the possibility that the abnormally low yearly pattern seen in the real-time data may be getting worse.
Precipitation. Although 30-day rain deficits over most of the state have been erased by recent heavy rain, streamflow is expected to drop quickly because not all the rain soaked into the ground to supply the rivers between storms. The National Weather Service Advanced Hydrologic Prediction Service (at this link; choose CT from the state drop-down list to see current data) shows that over the last 60 days most of CT received 75% to 90% of normal rainfall, which is better than we have seen for months. Some areas now have a 60-day rain surplus.
The dire situation last year (see below) was caused not only by less total rainfall than normal over a long period of time but also by the intensity of the rain we did get.
Rain? For most of the last six months, storm tracks brought most precipitation out to sea. Recently though, weather systems have tracked over Connecticut, creating a return to more normal summer thunderstorms and storm systems. This normal precipitation did not end the drought due to the serious long-term rain deficit, but it kept it from getting worse for a while. Some areas of the state received well above normal precipitation, erasing even the 60-day rain deficit in those places. However, because recent one-week forcasts show almost no chance for rain, streamflow will probably drop back to low levels across the state. Long-range computer models also show only a small chance for a little rain at the end of next week. If Hurricane Gaston comes our way instead of heading northeast as predicted, our rivers will come up in a hurry.
The NWS Climate Prediction Center's one- and three-month prognostications give equal chances for normal or above-average or below-average precipitation. These forecasts said nearly the same thing for most of the dry period last year, yet we consistently got less than normal precipitation.
The amount of rainfall that has come during extreme precipitation events has risen faster in the northeastern United States than in any other region of the nation. Whenever there is a powerful storm, watch for flooding! Flash flooding can easily take people by surprise. Remember, if water is across the road, Turn Around, Don't Drown! See below for general flooding information.
On Aug. 25, 2016, 33 out of 62 USGS stream gages in Connecticut (list below) reported low flows.* Stream gages reporting flows lower than their records for the date are in bold red. To see their current status, click on the link to the right of the name.
Click here for a list of all CT stream gages comparing
current flow to their mean and median flows for today's date.
The U.S. Drought Monitor: As this graph shows, we have been labled Abnormally Dry or worse pretty often over the last few years.
For a good analysis of last year's stream problems across the state, see this press release from Pomperaug River Watershed Coalition (PRWC). Here is a key quote:
The September and October low river flows demonstrate that rivers were not supporting the critical flows needed for a healthy river habitat. In fact, sections of the Weekeepeemee River had dry river beds and the Pomperaug River was flowing only at a rate of 15% of the critical flow requirement. DEEP officials confirmed that they would expect fish mortality to increase significantly during such conditions...
Meanwhile, parts of Coppermine Brook in Bristol were drawn down to rocks and dirt, as shown in the photo below taken Dec. 11 in Bristol. The sandy ditch in the foreground is the stream channel. Bristol Water Company and New Britain Water Company have eight registered diversions near Coppermine or its tributaries with a combined registered water withdrawal of 36.6 million gallons per day (source: CT DEEP). Company representatives report they do not withraw anywhere near that much water. Bristol Water says they stopped pumping from their well near this photo for a day but with no apparent effect on the stream. This is a perennial stream with a state Trout Management Area below where this picture was taken. Click on the photo for a full-screen version in a new tab.
When floodwaters fill the part of the stream channel that is called a floodplain, the water may find that someone has built a building or two in the channel. The water then saturates and fills any leach fields, often flushing untreated sewage out into the flow.
The floodwaters also find these really nice holes in the ground called wells to flow down into. All kinds of interesting things can be delivered to the bottom of the well, such as the aforementioned sewage, and soil, bugs, leaves, pesticides such as weed killer and insect poison, even the dog droppings from the backyard.
Very important: Any well that was flooded should be pumped and flushed out thoroughly and the system sanitized or "shocked."
The Connecticut Department of Health website has a good guide called:
There are places in Connecticut where the storm drains and the sewage pipes are combined into one system. With high rainfall, many of these combined pipes are designed to overflow into rivers and streams so the wastewater treatment plants are not overwhelmed. You really do not want to be downstream when raw untreated human sewage is entering the water. The DEEP has a map of Combined Sewer Overflows that shows the six urban areas where these can occur. Zoom in to any of them to see exactly where the combined flow may enter streams and rivers. Not every rain event is enough to cause these overflows, but it's a good idea to avoid contact with the water downstream from them after significant rain.
Unless you are an expert paddler, do not attempt to canoe or kayak on floodwaters; there are usually one or two fatalities per year. Our Connecticut Water Trails website and the webpages of the many paddler groups in the state all have good safety procedures. Here is a good article.
As the graphic to the right illustrates, where a storm moves across the state, the streams show flows that peak above their averages for this time of year but then quickly drop. The streams that maintain healthy flows between storms are those draining the parts of the state where there has been decent rainfall or those controlled by managed dams.
What IS Normal?
The National Oceanic and Admospheric Administration (NOAA) uses a rolling 30-year average of modern rainfall data to calculate its "normals." A discussion of the 1981 to 2010 precipitation calculations can be found at http://prism.oregonstate.edu/normals. To see Connecticut's average annual precipitation, go to http://prism.oregonstate.edu/gallery/view.php?state=CT_RI.
Recently there has been much discussion of what constitutes a "100-year-storm" or a "50-year storm." These traditional terms can be misleading unless you keep in mind they are an old-fashioned way of describing probablities. A "100-year" rainfall event means that in any year there is a 1% chance of that rate of rainfall.
Here is a map showing that in Connecticut we have a 1% chance of getting 7 to a little over 8 inches of rain in a 24-hour period, depending on where you are in the state. (Image source: http://precip.eas.cornell.edu/)
Comparing this to Table 7.2 (copy below) of the CT Stormwater Manual, we note that the design criteria for managing runoff possibly should be updated if we are building our landscape to control the flow from a 100-year-storm.
Photos by Joan Smith GOSA taken 9/20/2015 at The Merritt Family Forest in Groton. The stone slab bridge lies over Eccleston Brook, which had only a few puddles despite last week's downpour. The muddy bottom is also part of EB, further downstream. The photo with the two upright stone slabs is Cowslip Brook, a tributary to EB, and the wooden bridge traverses another small tributary to EB, flowing from a vernal pool. Used with permission.
State of CT - Drought (Search)
Water Conservation tips
Water Conservation is not just for droughts; it is important because:
What Can I Do?
Model Water Use Restriction Ordinance (PDF, 28KB)
Connecticut's Draft Drought Management Plan is being updated by the CT Water Planning Council Advisory Group Drought Plan Work Group.
News article: Researchers reveal cost-effective path to drought resiliency (July 21, 2016) "Published in San Francisco Estuary & Watershed, the study reveals the costs and benefits of using groundwater recharge and storage across the state. This process, known as "managed aquifer recharge," or MAR, can incorporate co-benefits such as flood control, improved water quality and wetland habitat protection. The study found the median cost of MAR projects is $410 per acre-foot (the amount of water required to cover an acre of level land at a depth of 1 foot) per year. By comparison, the median cost of surface water projects is five times more expensive -- $2,100 per acre-foot."
Some archived Know Your Flow pages
Rivers Alliance of Connecticut