Connecticut Streamflow Overview
Know Your Flow!
Fri Mar 27, 2015 Summary: Streamflow has jumped over the past two days due to rainfall instead of snow, and temperatures staying above freezing. The USGS streamflow state map shows almost all streams and rivers are at or above average flow. Before this week, most streams and rivers had flow rates well below average for this time of year due to slow melting of the snow pack as temperatures dropped back to below freezing every night. Most rivers and streams peaked well above average today, but we can expect rates to fall steadily to below average levels between rainfall events. Stream levels rising and falling with the rain is normal, of course, but how low the flows drop after the snow is gone depends on how much water was able to soak in to re-supply groundwater (more about this below).
Remember that groundwater provides for about half the state's drinking water as well as streamflow between storms. Groundwater levels have recovered from their low levels at the beginning of Winter because the slow melting of the snowpack allowed water to soak in. All the USGS measuring stations in CT but two are recording levels at or above average for this time of year. Click here to see the groundwater graphs.
Watch for Flooding due to ice jams! National Weather Service offices in the area have been issuing advisories about ice jams. Flash flooding can easily take people by surprise at the end of Winter the when ice on rivers begins to break up and move downstream. The ice can build up at obstructions, blocking the river, quickly flooding low-lying areas. Remember if water is across the road, Turn Around Don't Drown!
Some General Flood Information: When floodwaters return to use that 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 back yard.
It is very important that any well that was flooded 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 its a good idea to avoid contact with the water downstream from them after significant rain.
Only crazy people attempt to canoe or kayak on floodwaters, but apparently there are enough of them to cause 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 they have had decent rainfall, or on streams 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. Watersheds with lots of impervious roads, roofs, parking lot, patios, and compacted soil in lawns typically cause a lot of the rain to flow over the surface or through storm drains directly into the streams, leading to flooding problems far more often than natural surfaced watersheds. Their streamflow then drops quickly after the storm ends and the flow levels off far lower than streams with more natural surfaces in their waterheds.
What IS normal?
Please note that 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 are traditional terms that 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 each and every year there is a one-percent 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 maybe the design criteria for managing runoff should be updated if we are building our landscape to control the flow from a 100 year storm.