Offshore mud sources and mangrove development on the coast east of the Amazon River mouth

UFPA (Universidade Federal do Pará) and UW are collaborating on a project to study the tide-dominated estuaries and extensive mangrove plains east of the Amazon River mouth. In a recently submitted manuscript, we examined the Caeté estuary to understand how the longest mangrove belt in the world was built by rivers which don’t supply much mud to the coast. Instead, the Amazon river plume and its shelf deposits are the likely sources of mud to the Caeté and nearby estuaries.

Nils deploying equipment in a mangrove forest in the Caeté estuary.

In the Caeté, complex dynamics in suspended sediment concentrations are related to superimposed variations in rainfall and local fluvial discharge (landward end) and seasonal dynamics of the shelf (seaward end). During the dry season and transitional periods, the estuary is importing mud from the shelf. Tidal processes result in sediment accretion on mangrove mud-flats. During the rainy season, rainfall-related runoff and fluvial discharge increase. Suspended sediment is transported to the lower estuary, where flooded barrier-island trap sediment in a newly-formed back-barrier environment. Our results reinforce the concept that sediment-transport convergence and turbidity maxima are distinctive features of tide-dominated estuaries, and these features result in sediment entrapment. For tropical coasts, sediment trapped by the ETM can be buried in adjacent mangrove forests. Our results may also show that local fluvial sediment sources are of secondary relevance compared to offshore sources.

Our lab has an ongoing research project examining tidal river hydro- and sediment dynamics in the Ayeyarwady (Irrawaddy) River delta of Myanmar (Burma).  The Ayeyarwady River is likely the third largest source of sediment to the global ocean after the Amazon and Ganges-Brahmaputra Rivers, but relatively little is known about this large river system compared to other rivers of its size.

A group of us (Andrea, Chuck, Hannah, and Aaron)  just returned from three weeks of fieldwork on three distributaries of the Ayeyarwady- the Yangon, Bogale, and Pathein Rivers.  This was the second of two initial field campaigns designed to capture the two dominant seasonal conditions in this monsoonal system.  Last September we undertook a similar field effort to capture high discharge of the river during the rainy season, which also coincides with more energetic marine conditions.  This most recent trip was timed to capture low-flow of the river and quiescent marine conditions.

Together with an enthusiastic group of scientists and students from two Myanmar universities (Pathein University and Myanmar Maritime University), we investigated river dynamics and water-column properties using a variety of Instrumentation (e.g., boat-mounted and fixed ADCPs, CTD casts) as well as river-bed properties (e.g., grain size, sediment-accumulation history) using grab samples, augers, and x-radiography.

Below are some photos from our time in the field highlighting some of the work we accomplished, challenges we faced, and fun we had!

Hannah takes notes while logging ADCP data on the Bogale River.	Aaron discusses CTD data collected in the Yangon River with Colleagues from Myanmar Maritime University.
The CTD comes back on deck.	An improvised towed ADCP platform on the Yangon River (thwarted by VERY high SSC).
A 6-m saltwater crocodile reminds us why we minimize the amount of time we spend in the water near Meinmahla Island.	Our research vessel on the Bogale River. Note the ADCP pole midship.
An Aquadopp is deployed in a tidal channel on Meinmahla Island.	Some of the colonial architecture in downtown Yangon.

Dan C. spent the end of August in Saigon organizing logistics for the September field work. The scientific equipment has arrived, and we are excited to start in the field in a couple weeks. Here’s a view of the Saigon River from downtown Saigon (Ho Chi Minh City), about 130 km northwest of our study site on the tidal Mekong River:

August 17-21 saw the completion of another successful Elwha cruise. After an exciting (i.e. rainy!) March, the summer was relatively quiet in terms of river discharge. The primary goal of the cruise was to remove, refurbish, and re-deploy the primary seabed instrument system, which we did in record time (36 hours!) after an initial failure of one of the release mechanisms (thanks APL for loaning us a backup control unit!). Meanwhile, ship-based survey efforts focused on collecting grab samples from the Freshwater Bay deposit to track any changes in extent throughout the summer. On the final day of the cruise, we also recovered 6 box cores from Freshwater Bay, which will be x-rayed, analyzed for grain size, and measured for radioisotopes in the lab.

Many thanks to everyone who helped make this a successful cruise! Bob and Todd of the Barnes, Aaron, Dan N., Wenhua, Julia, and Brianna from our lab group, Maggie McKeon from UW Civil Engineering, Bethany Nagid from UCSC, and Jacob Melly from Peninsula College provided invaluable assistance and no shortage of positive, creative thinking. We also enjoyed hosting guests from UW Advancement and the Seattle community on the box coring & and re-deployment portion of the cruise, and appreciated the opportunity to share the exciting research happening at the Elwha.

Dan Culling, Research Scientist joined the lab in March, and spent an intensive spring and summer coordinating field logistics for upcoming Mekong Delta field work. Together with Aaron and Emily, Dan organized and packed a 20’ container, which arrived in Ho Chi Minh City in late August. The field work, due to start in September, will focus on sediment-transport process and deposits in the tidal river, mangroves, and continental shelf. This work is an exciting collaboration between UW, Vietnam National University, VAST Institute of Marine Geology & Geophysics (Vietnam), NC State University, Washington State University, Tulane University, Boston University, University of Waikato (NZ), University of Miami, and UNSECO-IHE.

Members of the Sediment Dynamics Group are slowly arriving in Brazil for our fifth tidal Amazon River cruise. Yesterday along with our Brazilian collaborators we measured discharge at the town of Obidos, the farthest downstream gauging station on the Amazon. Today we will measure discharge on the Tapajos River, a major tributary that meets the Amazon at the city of Santarem. Tonight we meet the rest of our UW and Brazilian colleagues and continue our research farther downstream.