Rapid River Water Temperature Fluctuations

by Harold F. Maybeck

Meteorologist, Holderness, NH
September, 2004


A Cursory Examination of the Dramatic Decline in Water Temperature on the Pemigewasset River
at the Plymouth, NH River Stage Station on August 30-31, 2004

Abstract

Moderate to heavy thunderstorms occurred across central and northern New Hampshire during the late evening of August 30, 2004. These thunderstorms dumped copious amounts of cooler water into the Pemigewasset River and the close-in drainage areas of its watershed. This sudden influx of cooler water caused the river water temperature at the Plymouth, NH, River Stage Station (USGS #01076500), to drop by 7° Celsius in six hours.


The water temperature of the Pemigewasset River at Plymouth/Holderness, New Hampshire, usually fluctuates through approximately 1°C to 2°C on a diurnal cycle. As expected, the water temperature increases during daylight hours and declines again during the night. Cloudy days and nights make for smaller daytime increases and smaller nighttime decreases. Clear days and nights with increased insolation and radiational cooling make for greater daytime increases and greater nighttime decreases. (Fig. 1).

Just before midnight on August 30, 2004, the water temperature made a dramatic rise of 1.1°C in 30 minutes. Immediately afterwards, the water temperature dropped by 7.0°C in six hours. (Fig. 2 and Table 1). This rapid rise is shown on Figure 2 by the red portion of the graph and the rapid decrease in temperature is shown on Figure 2 by the green portion of the graph. During the late evening of August 30th only .24" of rain fell in Holderness. But, due to the nature of shower form precipitation, between .80" and 1.10" of rain fell upriver in the Woodstock and Lincoln area. In the Pemigewasset River basin, north of the Holderness/Plymouth area, the river runs fairly shallow. With stage heights of only 1.00' at Plymouth before the onset of rain, there were many shoal areas north of the area where the water depth was considerably less. This shallow water depth on the Pemigewasset River and on its tributary, the Baker River, gave rise to the high water temperatures during the afternoon of August 30th.

The initial rapid rise in water temperature can be attributed to the sudden influx of this warmer water, due to the surge of upriver rainfall pushing the shoal area water downstream into Plymouth. The continued flow of water from upriver, after 0100EDT of August 31st, began pushing the now cooler upstream water into the Plymouth area. This upriver water was basically the new showerform precipitation falling into the river and the drainage from the immediate watershed area adjacent to the river. This rain was from cumulonimbus clouds with great vertical extent. This rain was considerably cooler than the ambient river temperature.

Prior history has indicated that a river surge at Plymouth usually occurs approximately 6 to 8 hours after the river crests at Woodstock, NH. In this case, the abundance of precipitation surged the water downstream in a matter of a few hours causing the dramatic drop in river water temperature. After the initial temperature decrease, which lasted until 0700EDT of August 31st, the temperature began to again rise into a diurnal regime that was 4°C to 5°C lower than the regime before the thunderstorm event.

Conclusion

All data, including all tables and graphs, in this paper are a product of the United States Geological Survey (USGS). Additional data and tabular information is from the Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory (CRREL) of the United States Army Corps of Engineers (USACE). This new data of the river water temperature is new. It has only been included at the Plymouth, NH site since August 25, 2004. This is invaluable information and the USGS and CRREL of USACE should be congratulated for including the area of Plymouth/Holderness, New Hampshire with this scientific tool.

Future research using this river water temperature data can lead to a better understanding of the many facets of "a river". This can include, not only the geologic and hydrologic aspects of a river, but also, these water temperature fluctuations can be useful in examining the various aspects of plant, fish and other aquatic life in the river.

Reference: U.S. Geological Survey, Natural Streamflow Information Program. River Gage Station USGS #01076500 Pemigewasset River at Plymouth, NH

Figure 1
Figure 1

Figure 2
Figure 2

Table 1
Table 1






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