The Journey of the American Eel

American Eel

American Eel. Source:

The only fish you are likely to find in Arlington’s Gulf Branch is the American Eel. This has little to do with the water quality of Gulf Branch and more to do with heavy runoff after rains. The scouring action of rushing water after a storm makes it difficult for fish populations to maintain a foothold (or fin-hold) in the stream. Other streams in Arlington, such as Four Mile Run and Donaldson Run, also have eels, but Gulf Branch is probably the only one where the eels are the only fish you will encounter. This is also a result of the large, steep, rocky waterfall where Gulf Branch dumps into the Potomac, which acts as a barrier to most fish moving upstream. However, one could not ask for a more interesting or unusual fish to be found in Gulf Branch.

American Eels (Anguilla rostrata) have small pectoral fins and long, continuous dorsal and anal fins. Their long, slender bodies look far more snake-like than fish-like. A layer of mucus covers the eel’s skin, and its scales are small and well embedded in its epidermis, making them hard to see. The eel itself may also be hard to spot as they are typically nocturnal. American Eels are a generalist species as a result of their life cycle, and they can survive in a variety of freshwater different habitats including streams, rivers, lakes, estuaries and coastal bays. Their diet is just as varied and includes most aquatic organisms, living or dead, found in any habitat they may occupy.


Eel in its natural habitat

The life cycle of the American Eel is an amazing story that spans miles and decades. These fish are catadromous, meaning they migrate from freshwater to the ocean to spawn. These eels hatch in the Sargasso Sea as small, nearly transparent and flattened larvae that look quite different from adults. The larvae begin a long, passive migration as currents guide them to the coastal waters of Eastern North America over the course of seven to twelve months.

Eels go through several morphological changes as they grow. As the eels grow larger and approach their new freshwater homes, they begin to produce pigment and darken in color. While their transparency made them difficult to spot drifting in currents along the open ocean, this new darker color provides camouflage against the dirt and rocks at the bottom of estuaries and rivers.

While most fish get flushed out as a result of stormwater in Gulf Branch, eels are accomplished, upstream swimmers. Natural barriers like waterfalls, and even human-made barriers such as smaller dams, are not always enough to stop the upstream migration of eels. Eels have been reported moving across short distances on land in wet or rainy conditions to get around waterfalls or dams. American Eels have been spotted in Gulf Branch upstream beyond Military Road. Their elongate body also helps them find shelter in cracks and openings too small ornarrow for larger fish.

Here they may spend 10-25 years maturing before beginning the return journey to the Sargasso Sea. As their body gets ready for the long journey, their pectoral fins enlarge for better swimming capability, their eyes enlarge to adapt to the oceanic environment, and their skin thickens to protect against the saline environment.

American Eels are semelparous. This reproductive strategy is also called “big bang” reproduction because it happens once, in a large quantity and is typically fatal. Female eels may produce 400,000-2,500,000 eggs. Adults die after mating and spawning in the Sargasso Sea, completing an odyssey that lasts more than a decade and covers hundreds of miles. Thankfully, the completion of their journey means the beginning of another, ensuring that Arlingtonians will continue to enjoy encountering this curious fish in our local streams.

Water-Loving Trees and Their Adaptations

Tree roots stream side.Trees grow in all types of environments, and stream side or wetland areas can be some of the most challenging environments for trees to grow. While trees do need water, they face several challenges in a wet environment:

  • While they also use water and sunlight, trees need oxygen. Trees share at least 60 percent of genetic information with humans, and breathe and use oxygen for energy. A flooded or very wet soil has little oxygen, so this makes root growth and function difficult.
  • Wet environments are perfect for certain decay organisms to break down and damage trees.
  • Periodically flooded or flowing streams and lakes face more disturbance, with wet and dry periods during the year increasing the stress to the surrounding trees. While a tree in a flat field may suffer from temperature changes, the environment around water changes more drastically, requiring much hardier trees.

Trees growing near water with a view of Rosslyn.In Arlington, we have a lot of species that work well in this environment. Trees native to our County and in the region have adapted to these stresses in various ways:

  • Some trees adapt to wet soils by having very superficial roots. These roots exchange oxygen at the top layer of soil and help the tree survive in a high water table. Species that characterize this include maples, sweetgum, magnolias and elms. Great examples of these superficial roots can be found in Bon Air Park along Four Mile Run, near the shelter, where we have some of our native red maple stands.
  • The most common strategy to wet soils is to grow faster than any decay that may form, and heal over wounds caused by disturbance quickly. Eventually, these trees lose this battle, but they often pair the strategy by reproducing early in their age. These trees can be structurally unsound and often contain significant decay. Examples of trees like this are silver maple, American elm, and green ash. Barcroft Park is a great place to find many of these species along Four Mile Run throughout the park.
  • Slow-growing, rot-resistant wood is one of the more long-term strategies to deal with the high disturbance of a waterlogged environment. The species using this strategy tend to be found in older environments where they have waited patiently, growing slowly, to take over the shorter-lived species over time. Some of these trees are the longest-living species in eastern North America, and include black tupelo, black locust, baldcypress (longest-living tree species in the east, up to 2,000 years) and arborvitae. While baldcypress and arborvitae aren’t directly native to Arlington, you can find blackgum and black locust growing along Ballston Pond.

Trees growing in water.Because of all their adaptations and their resistance to disturbance, many species that can handle tough environments like this can be appropriate trees for urban, compacted planting as well. Arlington uses many species commonly found along streams to beautify our streets and improve our tree canopy. We also plant many of these species to restore existing streams to improve retention of soil and clean our water in the short- and long-term.

To learn more about trees in Arlington, including our tree programs, proper practices and recommended trees for planting, visit

Mayflies Make a Summer Splash

The summer season for Arlington’s Volunteer Stream Monitoring Program came to a close on Aug. 3. We were blessed with beautiful weather during the entire monitoring period and had ample bugs to count at every monitoring site. The stream monitoring program studies macroinvertebrates  the small organisms that live underwater in our streams, lack a backbone and can be seen with the naked eye. These “stream bugs” live a portion of their life cycle in streams and depend on the stream ecosystem for habitat and food. Some macroinvertebrates are more tolerant of stream pollution than others, which makes them good indicators of water quality. All of the photos below were taken by volunteer photographer Nick Leach.

Stream monitors with nets, boots and buckets.

Stream monitor looking at macroinvertebrates.

A summer stream monitor.

Stream monitor chemistry tests.

Volunteers perform several water chemistry tests during each monitoring session.

Stream monitors net picking.

The bugs often cling to the net, making “net picking” an important step.

Right about the time we were completing our summer monitoring, mayflies were making national news for their massive numbers in the Upper Midwest. Mayflies were the most abundant bug at each of our nine monitoring sites, but our numbers were nothing like what the Midwest has been experiencing.

Mayflies in a tray.

Can you find the two mayflies in the ice cube compartment above?

There are more than 130 species of mayflies in the U.S. according to J. Reese Voshell’s guidebook, A Guide to Common Freshwater Invertebrates of North America. Only one type of mayfly has been documented in Arlington, the small minnow mayfly. While mayflies are generally considered to be sensitive organisms and indicators of good water quality, the small minnow mayfly is an exception. The small minnow mayfly is able to survive the impacts of stormwater runoff and the pollutants stormwater carries, making it a more tolerant mayfly than some of its more sensitive relatives.