Last week's HRECOS story described the impact of winter temperatures on the annual cycle of water levels in the Hudson River Estuary. Cold temperatures contract water causing sea levels to decline and, consequently, lower water levels in the Estuary. Conversely, when temperatures climb in the spring, the water expands and the sea levels and Estuary water levels increase. This is known as the steric effect.
I received multiple emails this week from people who felt that seasonal changes in precipitation and runoff should also affect the annual cycle of water levels. Winter runoff is usually lower because precipitation is locked in snow and ice. These are unlocked in the spring melt and large quantities of water are flushed through the watershed. The readers who emailed me felt that the winter drought and spring flush could also explain winter low and spring high water levels.
To answer their question, I interviewed Nickitas Georgas who produces the forecasts for HRECOS and for the New York Harbor Observing and Prediction System.
According to Mr. Georgas', long term changes in water levels in the Hudson River Estuary are affected more by sea levels than by seasonal changes in precipitation and run off.
This comparison can be observed by plotting average monthly HRECOS high water levels against sea levels collected by NOAA at the Battery in NY City and discharges from the USGS Station at Green Island. The USGS station at Green Island monitors the discharge of all the precipitation and water runoff collected by the Upper Hudson River before it spills over the Troy Dam and into the Hudson Estuary.
Average monthly sea levels at the Battery in New York City lowergradually in the autumn and rise gradually in the spring due to thesteric effect. This sloping pattern is reflected at all the HRECOSstations.
In comparison, the plot of average monthly discharge from the upper Hudson does not resemble the data from the HRECOS stations. Discharges, for example, are lowest in the summer, not the winter (this is true for all data collected since 1946). Also, dramatic spikes in discharge are not reflected in data from most HRECOS stations. The largest jumps in discharge were observed during the spring flush and during an unusually wet December in 2008. These spikes are seen in the December and March high water levels at Schodack Island, the most northern HRECOS station, but are not observed at any HRECOS station further downstream.
According to Mr. Georgas, high discharges over the Troy Dam "can, at times, obscure the month-to-month steric cycle at Schodack Island but the long-term Estuary-wide influence is the sea level change at its southern boundary."
The Hudson River watershed collects precipitation from roughly 13,000 square miles. Because of this, it's not surprising our readers felt seasonal changes in precipitation and run off should have a significant impact on the annual cycle of water levels in the Hudson Estuary. The Hudson River Estuary, however, is by definition the portion of the river that rises and falls with the sea. It is most vulnerable to the sea even compared to precipitation collected from an area larger than Connecticut and Massachusetts combined.