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Streamflow is dropping across the state, down from the memorable high flows a couple months ago. Most stream gages are now reporting below average conditions, though only 15 of the 63 gages are at low levels.
Looking back over three months, graphs of streamflow in Connecticut over the last three months show the very high flows in April gradually dropping until by the end of May, flows between storms began to reach average for the time of year. By June 10, 14 of 62 stream gages were reporting low flows. The period of near-normal flow patterns only lasted 7-10 days however, before storms once more began delivering above normal rainfall and arrived often enough to keep streamflow above average for the time of year.
Most storms have missed the state since the last week of June, allowing streamflow to drop to near normal by the end of the month. Since the beginning of July, most of the state has seen less than half of what is normal for the time of year, with many areas getting under 25 percent or even under 10% of normal for the first two weeks of July. Looking back over the last 30 days, most of Connecticut has seen under 75% of normal rainfall for the time of year, with much of northwest CT under 50%. Click here to see all of the USGS streamflow graphs for the last thirty days.
11 gages were reporting low flows on July 11, with four of them at very low levels. Although the state got some rain since then, on July 15, 15 gages were reporting low flow, with 7 of them very low. 2 gages, on Muddy River near East Wallingford, and the Still River at Route 7 at Brookfield Center, were at flow rates lower than any daily average for the date based on 15 years of record.
Future Flooding? Although all rivers and streams should be watched carefully for flooding whenever there is heavy rain, this is especially true for those with higher than usual levels before the additional runoff from storms. Flood control dams can prevent or lessen flooding downstream, but do nothing to lower flash flood risk up stream. Go to the Northeast River Forecast Center for flood probabilities at the gaging stations they monitor.
We define high flow as at or over the stream's 75th percentile for that date, very high at 90th or higher, from the data file at https://waterwatch.usgs.gov/webservices/realtime?region=ct&format=xml when rivers are near their lowest points between storms. Low streamflow is below the 25th percentile or 25% of the mean flow for that date.
Why do we count? The flow between storms is an indicator of long-term stream conditions. Although expansion of a river or stream channel onto its floodplain is a normal, natural occurrence, human interference can cause higher than normal and more frequent flooding. The higher the water level remaining in streams and rivers before a storm, the more likely there will be dangerous conditions, streambed scouring, and other detrimental effects of high water.
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.
In Connecticut, the USGS has 10 real-time groundwater monitoring wells, six of which have records long enough to compare to other years. On July 15 all of them were reporting near-normal water levels. We should note however that none of the live-data wells with long records are in northwest CT where the least rain has fallen. There is a fairly new live-data well in Salisbury reporting the water table is lower than in either of its previous two July 15 reports, though that well's July monthly measurements over 31 years of record show its July measurements so far this year are near-normal.
The USGS Groundwater Watch website displays data from 57 other wells that are usually measured once a month, comparing the most recent measurements to each well's measurements taken in that month in previous years. All the measurements near the end of June were still near or above normal for that month.
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. High water tables keep streamflow from dropping very far or very fast between storms, so when the next storm hits, the runoff pushes streams to higher levels with more flooding.
For technical information about past broad-scale weather patterns, see NOAA's National Centers for Environmental Information State of the Climate Synoptic Discussions.
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. The report includes water demand graphs, system transfers, system capacities, days usable storage remaining, and precipitation.
The most recent CT Department of Public Health's Monthly Reservoir Status Summary was for Sep 26. At a Water Planning Council meeting, a DPH representative reported that a new, more comprehensive and timely report system should be online in 2019.