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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.
Here are some numbers from previous Know Your Flow reports:
May 31: 28 out of 64 USGS streamgages in CT were reporting low flows.
Why do we count? The 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.
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.
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.
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.