Historical Changes to Potomac Streamflow

Introduction to Streamflow

Changes to streamflow can affect many estuarine processes in Chesapeake Bay. What is streamflow (also called discharge)? Streamflow is the measurement of how much water passes through a fixed point (where the gauge is placed) and is typically measured as cubic feet per second (volume per unit of time).

Why do we care? Streamflow is directly connected to nutrient inputs to Chesapeake Bay. The Susquehanna, Potomac, and James Rivers combined contribute 95% of the nitrogen load and 87% of the phosphorus input to the estuary. Additionally, streamflow affects the turbidity (how clear the water is) and local salinity (fresh water). Changes to river discharge, which can change day to day, can also influence habitat suitability. For example, the brook trout needs high flow conditions during their spawning season; decreases in flow will increase sedimentation, often resulting in a lowered egg hatching success.

Figure 2: The 21-year rolling mean annual trend of Potomac streamflow (black) and precipitation (red).

Figure 2: The 21-year rolling mean annual trend of Potomac streamflow (black) and precipitation (red).

Changes in streamflow are affected by both natural and anthropogenic processes. For example, summertime flow is typically lower simply due to the warmer temperatures causing more evaporation. However, streamflow can also be decreased because of industrial use (power generation, water supply, etc.). One of the most important factors which influence streamflow, which makes it of particular interest for us, is precipitation (Figure 2).

Annual Streamflow Patterns

Figure 3: The mean annual streamflow within the Potomac River. A few notable peaks have been highlighted.

Figure 3: The mean annual streamflow within the Potomac River. A few notable peaks have been highlighted.

We looked at the Susquehanna, Potomac (discussed here), and James Rivers for historical changes in streamflow. The first thing I did was look at the overall annual mean to see if there are any changes. Hopefully, Figure 3 demonstrates that on an annual basis, we have gotten a few huge peaks in the more recent history. In fact, the 3 greatest streamflow years have all occurred since 1996 (or in the last 18 years of this 78 year record)! What is also interesting, is the huge streamflow change from 2002 to 2003. Just think of how resilient an organism must be to undergo the lowest streamflow year on record followed by the second greatest event the following year!

Figure 4: The correlation between streamflow and annual precipitation.

Figure 4: The correlation between streamflow and annual precipitation (as a 21-year rolling mean).

Streamflow is influenced by precipitation integrated over the entire watershed of the river. So, it was not surprising that the annual total precipitation (using the HadEX2 gridded product) was significantly correlated to the Potomac River flow. This loosely can demonstrate how important it is to assess changes in precipitation…since those changes can impact streamflow! This is certainly a trend we will be keeping in mind!

Seasonal Streamflow Patterns

Figure 5: The seasonal 21-year rolling mean for the Potomac River.

Figure 5: The seasonal 21-year rolling mean historical patterns for the Potomac River.

But, the annual streamflow trend does not show the seasonal variation. Is streamflow increasing across all seasons, or is it having a large increase in just one month, which is influencing the annual mean? We deconstructed the annual trend into a seasonal trend: winter is the mean streamflow for December, January, and February, spring for March, April, and May, summer for June, July, and August, and autumn for September, October, and November. (Note that we also can deconstruct this trend by month!).

Generally speaking, streamflow has significantly increased, with some sinusoidal wiggles, in all seasons except for summer in the Potomac River. (This same analysis found that summer flow actually decreased in the Susquehanna River).

What do these historical patterns imply for the physical characteristics (salinity, turbidity, etc.) and organisms in Chesapeake Bay? Share your ideas in the comments!

Kari Pohl

About Kari Pohl

I am a post-doctoral researcher at NOAA and the University of Maryland (Center for Environmental Science at Horn Point Laboratory). My work investigates how climate variability and extremes affect the diverse ecosystems in Chesapeake Bay. I received a Ph.D. in oceanography from the University of Rhode Island (2014) and received a B.S. in Environmental Science and a B.A. in Chemistry from Roger Williams University (2009). When I am not busy being a scientist, my hobbies include running, watching (and often yelling at) the Boston Bruins, and taking photos of my cat.
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2 Responses to Historical Changes to Potomac Streamflow

  1. Jacqueline Tay says:

    Cool to see the historical streamflow record. The changes (decadal patterns?) are striking! Low winter-spring streamflow is a pretty good predictor of a big sea nettle bloom (and high streamflow of a small bloom). I think I see some upward “steps” from the 60s to 70s and then 90s? To me, this visually agrees with downward “steps” in the sea nettle record (Breitburg and Fulford 2006). The hypothesis in their paper was that sea nettles have decreased from 60s to present because of loss of oyster habitat. However, the decline in sea nettles may be easily be explained by this change in streamflow?

    • Kari Pohl Kari Pohl says:

      That is a fascinating observation! We can investigate streamflow a little more in depth to see if the timing matches.

      I reanalyzed the streamflow data to check those steps..and they are real!

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