Volcanic isles, like those found in Hawaii and the Caribbean, are encircled by coral reefs that form a complex, living loop around an island. A coral reef is interrupted at intervals by reef passes — broad channels that slice through the coral and act as pathways for ocean water and nutrients to circulate in and out. These aquatic corridors facilitate movement across a reef, aiding the vitality of corals by expelling freshwater and delivering essential nutrients.
Recently, MIT researchers have discovered that river systems emanating from islands help shape reef passes. In a study published today in the journal Geophysical Research Letters, the team demonstrates that the positions of reef passes along coral reefs correlate with where rivers discharge into an island’s shoreline.
Their results offer the first empirical proof of rivers creating reef passes. Scientists and explorers have theorized that this could be true: where a river on a volcanic isle reaches the shore, the freshwater and sediment it transports flows toward the reef, where a sufficiently strong current can erode into the adjacent coral. This theory has been suggested previously but has not been quantitatively validated until this study.
“The findings from this research enhance our understanding of how the wellbeing of coral reefs relies on the islands they surround,” states study author Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric, and Planetary Sciences at MIT.
“Much of the current discourse regarding rivers and their effects on reefs has been negative due to human influence and the repercussions of agricultural activities,” adds lead author Megan Gillen, a graduate student in the MIT-WHOI Joint Program in Oceanography. “This study illustrates the potential long-term advantages rivers can offer reefs, which I hope shifts the paradigm and emphasizes the natural relationship of rivers with reefs.”
Another author on the study is Andrew Ashton from the Woods Hole Oceanographic Institution.
Mapping the Connections
The new research is grounded in the team’s examination of the Society Islands, a group of isles in the South Pacific Ocean, including Tahiti and Bora Bora. Gillen, who joined the MIT-WHOI program in 2020, was interested in investigating the links between coral reefs and their surrounding islands. Facing limited opportunities for on-site research during the Covid-19 pandemic, she and Perron utilized satellite imagery and maps of island topography to glean insights. They conducted a quick search on Google Earth and focused on the Society Islands for their distinctly visible reef and island characteristics.
“The islands in this chain boast iconic, stunning reefs, and we consistently observed these reef passes that seemed to correspond with deeply indented sections of the coastline,” Gillen comments. “We began to question whether there was a relationship here.”
From an aerial perspective, the coral reefs surrounding certain islands exhibit what appear to be indentations, resembling fissures that run straight through a ring. These voids in the coral are reef passes — extensive channels that plunge tens of meters deep and can be wide enough for some vessels to navigate through. Upon initial observation, Gillen remarked that the most apparent reef passes seemed to line up with inundated river valleys — depressions in the coastline that have been eroded over time by rivers originating from the island. She pondered whether, and to what extent, island rivers might influence reef passes.
“Researchers have studied the flow through reef passes to comprehend how ocean waves and seawater circulate in and out of lagoons, yet claims regarding the formation of these passes have been scarce,” Gillen explains. “Reef pass formation has been infrequently discussed in the literature, and researchers have not delved deeply into it.”
Understanding the Reefs
To gain a comprehensive understanding of the terrain in and around the Society Islands, the team employed data from the NASA Shuttle Radar Topography Mission — two radar antennas that traveled aboard the space shuttle in 1999 and mapped the topography across 80 percent of the Earth’s surface.
The researchers leveraged the mission’s topographic data to generate a map of every drainage basin along each island’s coast, providing insight into where significant rivers flow or have previously flowed. They also identified the locations of every reef pass in the surrounding coral reefs. They effectively “unwrapped” each island’s coastline and reef into a straight line and compared the positions of basins with reef passes.
“By examining the unwrapped shorelines, we discovered a marked correlation in the spatial relationship between these substantial river basins and the alignment of the reef passes,” Gillen notes. “Thus, we can assert that statistically, the arrangement of reef passes and large rivers does not appear random. The major rivers contribute to the formation of passes.”
Regarding how rivers shape the coral conduits, the team proposes two concepts, termed reef incision and reef encroachment. In reef incision, they suggest that reef passes can emerge during periods of comparatively low sea levels, exposing the reef above the water’s surface and allowing a river to flow directly over the reef. The water and sediment transported by the river may then wear away the coral, gradually carving a path through the reef.
When sea levels are relatively higher, the team theorizes that a reef pass can still develop through reef encroachment. Coral reefs naturally thrive close to the water’s surface, where light is available for photosynthesis. As sea levels rise, corals naturally expand upward and inward toward an island to “keep pace” with the water line.
“Reefs migrate toward the islands as sea levels rise, attempting to maintain alignment with changing average sea levels,” Gillen explains.
However, parts of the encroaching reef may end up in ancient river channels that were previously carved by significant rivers and that are lower than the rest of the island’s coastline. The corals within these riverbeds become submerged deeper than light can penetrate into the water column, eventually suffocating, thereby creating a void in the shape of a reef pass.
“We don’t believe it’s strictly one or the other,” Gillen states. “Reef incision occurs when sea levels lower, and reef encroachment takes place when sea levels rise. Both processes, happening over numerous cycles of sea-level changes and island evolution, likely contribute to the formation and upkeep of reef passes over time.”
The team also examined potential differences in reef passes surrounding older versus younger islands. They found that younger islands were accompanied by more reef passes that were situated closer together, while older islands had fewer passes that were spaced further apart.
As islands mature, they subside or sink into the ocean, diminishing the amount of land that channels rainwater into rivers. Eventually, rivers become too weak to keep the reef passes open, at which moment the ocean likely takes precedence, and incoming waves may seal off some passes.
Gillen is exploring concepts for how rivers or river-like flows might be engineered to create pathways through coral reefs in ways that would enhance circulation and benefit reef vitality.
“Part of me wonders: If we had a more consistent flow in regions lacking natural rivers interacting with the reef, could that potentially enhance health by reintegrating that river aspect into the reef system?” Gillen contemplates. “That’s something we are considering.”
This research received partial funding from the WHOI Watson and Von Damm fellowships.