Connecticut Streamflow Overview
Know Your Flow!
Tue May 19, 2015 Summary: Some rivers and streams have seen a slight improvement in streamflow since last week's analysis, but most continue to show us more rocks than water. Streamflow has been dropping since the end of April.
The USGS streamflow map shows 9 of the 51 ranked streams and rivers are temporarily near or above average flow for this time of year in the parts of the state that got some rain recently (why? see below). This is an improvement from only 2 last week. The rest of them are at flows lower than 75% of the records for today's date. 26 of those have less water flowing than 90% of the records for that station for today's date. Six of them, the Coginchaug River at Middlefield, the Westfield River near Westfield MA, the French River at Webster MA, the Eightmile River near North Lyme, the Pawcatuck River at Wood River Junction RI, and the Hubbard River near West Hartland, are near or below the lowest flows ever recorded for those rivers. Last week two were in that condition.
According to the Advanced Hydrological Prediction Service of the National Weather Service, Connecticut rainfall during the last 30 days has been about half what we normally have at this time of year, with an area of southeast CT getting less than 25% of average. Connecticut rainfall during the last 60 days, 90 days, even 180 days also show below average precipitation. It is the long-term rainfall totals that control groundwater levels, which in turn keep streams flowing between storms. Last week the U.S. Drought Monitor labeled all of the state "Abnormally Dry." There is not a lot of hope in the weather forecasts for better flow conditions. The long-term forecast does not show much rainfall until about two weeks from now.
The groundwater graphs for CT show two of the seven instumented wells near average, though falling, levels. The other five wells have to levels lower than recorded for 75% of the records for today's date. The slow melt of the large snowpack this past Winter kept streamflow from falling even faster than it has, but if we do not get significant rain soon, we could see streams actually going dry when the water table drops below the bottom of the stream.
In April, our public water supply reservoirs were 98% full on average, same as in March. Empoundments of 29 water companies were about where they usually are in April and 4 were more than 5% higher than average, almost the same numbers as in March. Only the New London Dept. of Public Utilities (Veolia NA) and the Bristol Water Company reservoirs were more than 5% under average. However, groundwater provides for about half the state's drinking water and monthly data for public water supply wells is hard to come by.
If We Do Get Thunderstorms, Watch for Glooding! Flash flooding can easily take people by surprise during dry periods. Remember, if water is across the road, Turn Around, Don't Drown!
Some General Flood Information: 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."
Also, 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.
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.
How quickly the flow drops after each rainstorm is unique for each stream, because it depends on how much water soaked in to increase the groundwater baseflow. 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. Their streamflow then drops quickly after the storm ends, and the flow levels off far lower than in streams whose watersheds have more natural surfaces.
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.
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.
Rivers Alliance of Connecticut