Where the river meets the sea

A guest blog by Christie Hegermiller

Google Earth aerial imagery of Corte Madera Bay, in San Francisco Bay. True color reveals the quantity of mud that moves through the marsh and embayment.

Google Earth aerial imagery of Corte Madera Bay, in San Francisco Bay. True color reveals the quantity of mud that moves through the marsh and embayment.

At the confluence of fresh, muddy rivers and salty, cold ocean waters, estuaries connect interior lands to the vast ocean. Estuaries are valuable ecosystems, hosting great diversity and large populations of fish, birds, and shellfish. They are also valuable to people, providing food, water, and acting as a first defense against coastal storms and river flooding. Large cities have thrived along the lower reaches of rivers and at the ocean coastline for centuries or, at some locations, millennia. As a result, estuaries are some of the most human-impacted waters in the world. The three largest estuaries in the United States - Chesapeake Bay, San Francisco Bay, and Puget Sound – are surrounded by huge population centers. Even smaller estuaries, such as the Hudson River Estuary, Penobscot Bay, or Morro Bay, are surrounded by major cities and towns that were built on the breadth of natural resources available via the waters.

Estuarine habitats sustain many animals, including the osprey seen here in Chesapeake Bay. Photo: Bob Quinn.

Estuarine habitats sustain many animals, including the osprey seen here in Chesapeake Bay. Photo: Bob Quinn.

While people have benefitted from estuaries for generations, we have also had extensive impacts. Since the Industrial Revolution, water has been harnessed by dams for drinking, sand and mud have been dredged and removed for shipping channels, wetlands have been filled to increase habitable space, and fish stocks have been depleted by industrial fishing practices. Foreign materials have been introduced to estuaries by other practices, such as inland mining and logging. Early pollution from industrial water usage and poor sewage systems rendered estuarine waters particularly dirty. Fortunately, local and state-wide measures and the federal Clean Water Act have largely improved the health of estuaries. Nearly every estuary in our country is the focus of an effort to restore these waters to the bounty they once were. However, many estuaries remain heavily impacted. Many estuaries are still sediment-starved from dredging and upriver damming, meaning that less mud is available to build marshes and wetlands, and less sand exits estuaries to sustain coastal beaches. Despite regulations that decrease modern pollution, relic contaminants, such as mercury or polychlorinated biphenyls (PCBs), are chemically bound to the mud and, therefore, transported or deposited with the sediments. Understanding the physical processes at play in estuaries can focus restoration efforts to increase effectiveness and economic feasibility.

Relic contaminants from early, unregulated industry are often bound to muddy sediments, so they persist for a long time in estuaries. This image is from the Petitcodiac River Estuary, which feeds in to the Bay of Fundy in Canada.

Relic contaminants from early, unregulated industry are often bound to muddy sediments, so they persist for a long time in estuaries. This image is from the Petitcodiac River Estuary, which feeds in to the Bay of Fundy in Canada.

I am interested in how tides and currents distribute sand and mud throughout estuaries. Though many scientists have observed these processes in estuaries across the planet, measurements are limited to single points over discrete time periods because of the difficulty and expense of spending time out on the water. Instead, my work uses a numerical model that represents an estuary, with fresh water flowing in from the river and salt water flowing in or out with the tide from the ocean. A numerical model uses physics equations to simulate how water flows around the estuary. Models like mine help fill in the gaps in our collective knowledge gained from decades of observational studies and, in this case, can be used to generalize how tides and currents affect mud residence time in estuaries. Residence time refers to how long something stays in a system. For mud with pollutants bound to it, the residence time allows me to identify areas that might be hotspots for relic contamination. Using the model, I am also able to identify conditions under which mud and sand could be deposited on top of existing marshes, helping them to grow vertically to combat sea level rise. Furthermore, I can anticipate how changes in river flow or other physical conditions over time will impact mud distribution in the estuary.

My undergraduate advisor, Gail Kineke (Boston College), and I pump sediment-laden water off the top of a core of mud along the Louisiana coast. This early research inspired me to pursue estuary dynamics further. 

My undergraduate advisor, Gail Kineke (Boston College), and I pump sediment-laden water off the top of a core of mud along the Louisiana coast. This early research inspired me to pursue estuary dynamics further. 

As children, we’re often kept from playing in the mud. My research has put me in situations that would horrify any mom: I’ve walked through marshes where the mud comes above your knees; I’ve pulled buckets of mud with worms and algae onto decks of rocking boats; I’ve even smelled mud to approximate how oxygen-depleted an estuary was. By combining these observations with a numerical model, my research helps us better understand the forces that transport sand and mud in estuaries to predict the fate of relic contaminants or marsh growth. My research feeds into the extensive restoration efforts occurring around the country to return these valuable waters to healthy ecosystems that can sustain both human and wildlife populations for centuries to come.

Christie Hegermiller is a PhD candidate in Ocean Sciences at UCSC. You can follow along with her research at http://coastalchristie.weebly.com/