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Streamflow and Conductance on the Delaware

Conductance vs. Streamflow on the Delaware River at Trenton, NJ

Rivers play an important role in our ecosystem. They provide water for drinking and irrigation of crops, a habitat for fish and other organisms, and routes to easily transport goods. For these reasons and more, it is important to monitor the quality of river water, including its physical, chemical, and biological characteristics.

One often measured parameter is specific conductance. Conductance is a measurement of a substance’s ability to conduct electricity and is related to the amount of ions, like salt, that are dissolved in the water.

Where rivers meet the ocean, the salt typically comes from seawater flowing upstream into the river. How far upriver the saltwater can reach (often called the salt front or salt line) depends greatly on an estuary’s type and the current streamflow.

Further upstream, where the ocean doesn’t have as much influence, the amount of dissolved salts in river water is generally related to how much precipitation there has been. When rainfall is light, more water on land can evaporate before it reaches the river, which concentrates the amount of dissolved salts in the water that remains. When rainfall is heavy, water tends to flow more quickly into rivers and streams, with smaller concentrations of dissolved salts.

The image above shows the relationship between river flow (discharge) and conductance over a 3+ year period on the Delaware River in Trenton, NJ. In general, the conductance is quite low, and well below accepted salt front cutoffs of ~400-1,000 micro-Siemens per centimeter, which correspond to chloride concentrations of 100-250 milligrams/liter. However, there is clearly an inverse relationship between conductance and discharge. When discharge is strong, water conductance is low, though it never gets below ~100 µS/cm. Likewise, when discharge is light, conductance is 2-3 times higher. While the salt concentration on the Delaware River near Trenton is generally low, it does depend on the streamflow.

Knowing the location of the salt front is important, especially on rivers where water is drawn for human consumption or irrigation, and for protecting riverine infrastructure like ships that corrode more easily in salty water. The Delaware River Basin Commission regularly monitors the salt front location in order to control its location by storing and releasing water in reservoirs upstream.


RTD Activity Idea: Monitoring Streamflow

Streamflow over the course of 2012 from the USGS streamgage in Trenton, NJ

The task of monitoring the nation’s numerous streams and rivers falls to the United States Geological Survey. The USGS maintains a large network of instruments that record streamflow, water height, temperature, conductivity, water quality and several additional environmental variables. One of the chief uses of this network is to monitor the occurrence of floods and droughts.

Thanks to USGS’s National Water Information System, this data is easily accessible for students to access and visualize, allowing them to investigate river conditions at nearby locations or from across the nation.

Real-time Data Project

Here is a quick activity students can use to investigate current streamflow conditions and compare them with historical norms.

  1. Go to the Current Streamflow Map on the USGS WaterWatch site and select a state.
  2. Click on a station and note whether it is currently above, at, or below normal conditions, as denoted by the color of the dot.
  3. Click on the station ID number. This will take to you the station’s summary page.
  4. In the pull-down list, select “Time-series: Daily data
  5. The top of the page includes some basic information on the station, including its location, a photo of the station, and (for some stations) the upstream drainage area. Underneath this information is a box to customize the graphs appear on the page.
  6. Select “Graph w/ stats” as the output format and enter a date range you’d like to visualize. (Here’s an example that displays data from Trenton, NJ for all of 2012.)
  7. Find the graph for discharge or gage height and compare how the measurements (shown in blue) compare with the historical average (shown in yellow).
  8. If you choose a full year (i.e. from January 1st to December 31st) you can quickly get a good idea of the annual differences and seasonal cycle at a station. (For example, for the Trenton station above, the highest average streamflows are typically seen in March and April, while the lowest occur in the summer and early fall.)
  9. Now that you have the hang of it, you can create graphs for individual years to compare them with each other, or you can look up data around specific events you know of, like major rain storms or droughts.

Engaging Questions

Here are a few questions students can think about before they start their research.

  • What do you know about floods and droughts? What causes them? What impacts do they have on the environment, ecosystems and people?
  • How do you think scientists study river flow? (The two primary methods are gage height and streamflow.)

Suggested Research Questions

Here are several questions students can try to answer by looking at the data.

  1. For your selected station, how does the most recent measurement compare with the current streamflow status (i.e. percentile class) shown on the map?
  2. How long has the station be at that state?
  3. What times of year have the highest streamflow? Which times have the lowest?
  4. Were there any times during the year where the streamflow was exceptionally high or low? Do you have any idea what may have caused these observations?

Relevant References

The WaterWatch web site, also maintained by the USGS, aggregates data from the entire stream gage network into a great set of maps, highlighting the current streamflow, drought and flood conditions around the country. If students are interested in looking at current or past conditions across the nation as a whole, this is a great place to start.


Streamflow on the Delaware

A 1-year graph of river flow on the Delaware River at Trenton NJ

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.


USGS WaterWatch

USGS WaterWatch Web SiteIt’s April. And while the cold temperatures here in New Jersey make it feel like spring hasn’t quite yet arrived, the flowers are starting to poke through the ground, reminding us that spring is coming, and with it, a steady stream of springtime showers should be on their way.

Of course, when rain falls on land, much of it ends up in rivers and streams. And thanks to a network of over 3,000 stream gages monitored by the U.S. Geological Survey, we can easily study how precipitation, including rain and snow, impacts local streams, rivers and estuaries.

All of this data is available on USGS’s WaterWatch web site, which features several easy to use maps, providing a great  way to introduce streamflow data to students and the public, while showcasing how it can be used to monitor floods and droughts – critical issues related to human health, safety and well-being.

Here are a few great places to start.

Current Streamflow Map: This map displays the real-time conditions from all of the streamflow stations across the country.  Some stations measure streamflow discharge or flow rate, while other stations measure gage height, that is, how high the water level is. A few stations even measure other things like temperature, pH and dissolved oxygen. From this map, you can select an individual station to view in detail, access raw data or create custom graphs.  

On the map, each station is represented as a colored dot, whose color is based on how the current streamflow or gage height compares with past records.  Reds designate those stations that are below average while blues are above average, and green dots represent those stations that are in line with historical norms. You can also view historical streamflow maps.

Drought Map: This map highlights which areas of the country have below normal streamflow conditions, typically due to long periods of time with limited rainfall or, in mountainous areas, low levels of wintertime snowpack. Below normal streamflow is generally a good indicator of whether a drought exists, though precipitation, ground water and reservoir levels are also taken into account when declaring an official drought. (See for example, New Jersey’s Drought Information Site.)

Flood Map: Sometimes, you can have too much of a good thing. This map shows those stations stations that are currently reporting conditions drastically higher than their historically normal levels. This can often happen after severe storms with large amounts of precipitation (which is especially true after tropical storms and hurricanes), but it is also common in the spring when mountain snowpack melts. And of course, some rivers are susceptible to a springtime a double whammy.

Personally, I’ve always wondered if a more appropriate phrase for this time of year might be “April flowers bring May showers,” but to make that case, I need to dig through this data some more.


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