Salinity monitoring shows most water in the tidal Severn comes from the Chesapeake

   Our 2006 monitoring project was fortunate to be able to document the salinity changes following the unusual rain event at the end of June, as similar large influxes of fresh water normally occur earlier in the spring.  Over the course of the next few weeks, this rainfall caused the Severn's salinity to drop by a factor of two.  While many would have envisioned the fresh water source for this as the Severn watershed, especially Severn Run, our results show clearly that the great majority of new fresh water entered the Severn from the Chesapeake, spreading up past Round Bay and into the upper Severn within a week.  At our SR7 (Indian Landing) station, at the head of the tidal Severn, fresh water from Severn Run is seen immediately after the rain ended, but within a week it is replaced by the saltier water that came in from the Bay, similar to that dominating the rest of the Severn.  This "salinity reversal" is a regular occurrence in the Severn and neighboring Chesapeake tributaries during the spring months, when large volumes of fresh water normally enter the head of the Chesapeake from the Susquehanna (USGS data).  This spring salinity reversal in the Severn has been nicely mapped by the MD DNR's surface monitoring program, although our data provided depth profiles showing additional features during the June rain event.
   After this rain in late June, minimal rain fell during the summer until hurricane Ernesto at the end of August.  As a result, the Severn's salinity increased slowly due to the denser saltier water intruding along the bottom from the Chesapeake, which was itself getting saltier due to the decreased fresh water influx from the Susquehanna.  The Severn's salinity changes are caused density-driven water exchanges from the top layer of the Chesapeake, whose salinity in turn reflects a balance between fresh water coming from the Susquehanna and the intruding denser saltier water coming from the Atlantic.  These shifts are nicely protrayed by the Chesapeake Bay Program's salinity profiles of the Bay.  
   The model at the left schematically depicts the Severn's rapid salinity decrease after the rain event in June, with dense salty water shown in dark blue and fresher water in light blue.  The diagram shows a simplified east-west vertical section of the Severn and adjacent Bay as seen from the south.  Panel a shows the pre-storm situation, when the Severn was well-mixed and had a moderate salinity level.  Panel b shows the effect of the storm-induced fresh water flow making the surface of the Bay fresher, while the Severn still contains denser, saltier water from before.  Panel c shows the density-driven flow of this saltier water out of the Severn into the adjoining bay.  This flow continues, resulting in the layering of incoming fresher water on top of outgoing saltier water, as shown in panel d.  When this dense older, saltier water has emptied out into the Bay, the Severn becomes filled with well-mixed fresher water with the same salinity as the adjacent Bay (panel e).  This model does not depict the detectable "sub-estuary" which exists at the tidal head of the Severn and undoubtedly also at the tidal heads of other Severn Creeks that have significant fresh water streams feeding them.  However, our salinity measurements showed that these are only minor influences, and the great majority of the Severn's fresh water is derived from the Susquehanna via density driven exchanges with the adjacent Chesapeake.  These exchanges were described for the Severn and neighboring tributaries in a 1982 paper in the journal Estuaries by Schubel and Pritchard (vol 9, p. 236). 
   Because the Severn is named a river and looks like a river on a map, most people assume that fresh water flowing in from its watershed makes it a sub-estuary of the Chesapeake.  For the great majority of the tidal Severn, our data do not show a salinity profile compatible with a sub-estuary, and it is more realistic to think of the Severn as an elongated bay off the Chesapeake.  Except for density-driven exchanges of the kind described above, water movement into and out of the Severn is limited.  The Severn has the lowest lunar tide (about 1 foot) of any place on the Chesapeake, and this twice daily flow sloshes existing water back and forth more than mediating significant water exchange between the mid-upper Severn and the Bay.  Wind driven changes in water height cause more water movement than tides, but strong winds driving these are rare during the summer.  Thus nutrients entering the Severn from the adjacent Bay and from the local watershed are expected to be trapped in the Severn, where they promote phytoplankton growth, driving the summer hypoxia we observed.  Tide-driven water exchanges with the Chesapeake probably account for the better condition of the benthic habitat in the lower Severn, as described in our discussion of the dissolved oxygen results.
   Our salinity and temperature measurements show that in the summer, the Severn's water shows the expected layering, with saltier and colder water near the bottom, and fresher and warmer water near the top.  Both these temperature and salinity differences create a density gradient, with lighter water near the surface and denser water on the bottom.  However, this density gradient is much less pronounced that found in the Chesapeake, where a sharp density change known as the pycnocline occurs in the deeper parts of the Bay.  The pycnocline is a strong barrier to vertical mixing, cutting off the supply of atmospheric oxygen to deeper water layers, and it has been associated with formation of the Bay's "dead zone".  The milder density gradient we found in the Severn is less of a barrier to vertical mixing, but the forces promoting such mixing may be much less in its relatively protected waters with little horizontal water flow.

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