2006 Severn Water Quality Monitoring Results -- Salinity

    Although the primary objective of the 2006 Severn Monitoring Project was to characterize summer hypoxia, monitoring personnel also recorded salinity and temperature from the YSI 85 meters along with the DO readings.  The Severn is located about 4/5 of the way up the Chesapeake estuary, and its salinity is roughly 1/5 of that of sea water, which is 35 parts per thousand salt.  Actually, the salinity in both the Severn and the Chesapeake varies seasonally, with lows in the spring and highs in the winter, as shown by DNR monitoring data.  Because rainfall was unusually low during the spring of 2006, the normal seasonal high fresh water inflow from the Susquehanna and other rivers did not occur.  By early June, the salinity in most of the Severn was about 11 parts per thousand (ppt) rather than the seasonally normal 5-7 ppt.  As seen below, our monitoring on June 19 showed a rather even salinity distribution throughout the Severn, with fresh water inflow from Severn Run apparent at the head of the Severn, and some saltier, denser water entering the Severn along the bottom near its mouth.  This pattern was representative of what we found in early June, as our monitoring project was gearing up.

    What was most unusual in 2006 was a major rain storm June 25-27, which dropped 5-12 inches of rain on the local area and most of the Susquehanna watershed.  This led to a marked decrease in salinity in the Severn and northern Chesapeake.  Data from the Susquehanna's Conowingo Dam shows a ~20 fold increase in water flow from June 24 to June 30 (below, left), which was followed by a dramatic decrease in surface salinity on June 29 in the Chesapeake at Sandy Point near the Bay Bridge (below, right). 

    Because our monitoring program was in place at this time, we were able to follow the subsequent salinity changes up and down the Severn, as shown by our data below.   On June 27, the Arlington Echo team found fresh water at the surface near the head of the Severn (Station SR7), although the nearby deeper station SR6 showed a salinity profile quite to that of June 19.   Several days later on June 30, the Riverkeeper team found the mid-Severn stations SR2 and SR3 to show similar salinity profiles to those of June 19, while station SR1 near the Severn's mouth showed fresher water entering at the surface.

    On July 3, a striking "reversed salinity" picture was found, with the salinity decreasing progressively from station SR1 near the Chesapeake through station SR6 near the head of the Severn.  The shallow top station SR 7 had increased its salinity compared to June 17, but was fresher than nearby station SR6.  On July 12, we found that even the saltier water in the deeper stations had been replaced with fresher water, and at depths shallower than 6 m, the Severn's salinity had almost halved compared to pre-storm levels.  These data clearly show that the major influx of fresh water throughout the tidal Severn came from the top layer of the adjacent Chesapeake, which in turn is derived from the Susquehanna. 
    July and August 2006 were very dry months in this region, and the salinity rose steadily throughout this period. As expected, our monitoring showed that this heavier saltier water came in from the Chesapeake along the bottom (e.g., station SR1 on August 14)  displacing the lighter, fresher water from the Severn.  During the first week of September, hurricane Ernesto and associated storms dropped over 5 inches of water locally, and a smaller scale influx of fresh water occurred, again from the Chesapeake as seen in late June.  On September 18, our lowest station SR1 was again fresher than other Severn stations except for SR7 at the mouth of Severn Run, with salinity increasing through mid-Severn station SR3.

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