Scientists who study river streamflow do not have an easy job. Unlike many weather measurements like temperature, pressure or humidity that change with more predictable variation throughout the course of a year, streamflow is more closely correlated with major rain and snow events. These events occur sporadically throughout the year, often in large doses.
The graph above is a good example of this complexity. The data shown was measured from a stream gauge on the Delaware River in Trenton, New Jersey. The daily mean streamflow (also referred to as “discharge”) from the most recent year is plotted in red, ending April 11, 2013. The long-term average streamflow statistics are calculated from a 99-year data set collected between 1913 and 2011.
The mean daily streamflow includes many large peaks over the course of the year. These peaks correspond to major precipitation events that occurred in the upper Delaware river basin. On the other hand, the lines showing the long-term average streamflow change more gradually throughout the year, with the highest streamflows observed in the spring (especially this week), and lower streamflows in the summer and fall.
Looking at this data, several interesting observations can be made.
- In October 2012, Hurricane Sandy hit New Jersey causing extensive damage, however the observed streamflow during this time was not as high as some of the other events. As it turned out, most of the damage was caused by coastal flooding and high winds and not from flooding rivers.
- Over the course of the winter, several large events were observed. These correspond to the major nor’easter’s that passed through the area, including the (strangely-named) winter storms Nemo and Saturn.
- Last month was unseasonably cold and, while there was a nor’easter early in March, the month also ended up unseasonably dry. However, following today’s rainstorm, it is likely that the streamflow will rise to the more typical values expected during this time of year.
Ultimately, when studying rivers it’s important to remember that individual events will not always neatly line up with long-term averages, but over time, the trends should match.