{"id":26716,"date":"2025-06-24T15:48:10","date_gmt":"2025-06-24T10:03:10","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=26716"},"modified":"2025-06-24T15:48:23","modified_gmt":"2025-06-24T10:03:23","slug":"island-rivers-carve-passageways-through-coral-reefs","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/island-rivers-carve-passageways-through-coral-reefs\/","title":{"rendered":"Island rivers carve passageways through coral reefs"},"content":{"rendered":"\n

Research shows these channels allow seawater and nutrients to flow in and out, helping to maintain reef health over millions of years.<\/strong><\/em><\/p>\n\n\n\n

\"\"
Pictured is a shallow reef flat channel on the atoll of Tetiaroa, located north of Tahiti in the Society Islands. MIT researchers have found evidence that island rivers may carve out paths in surrounding reefs over time, helping to maintain their health over millions of years. IMAGE: Remi Conte, Tetiaroa Society<\/sup><\/em><\/figcaption><\/figure>\n\n\n

Jennifer Chu<\/p><\/div><\/div>\n\n\n

Cambridge, Mass.\u2014 Volcanic islands, such as the islands of Hawaii and the Caribbean, are surrounded by coral reefs that encircle an island in a labyrinthine, living ring. A coral reef is punctured at points by reef passes\u2014wide channels that cut through the coral and serve as conduits for ocean water and nutrients to filter in and out. These watery passageways provide circulation throughout a reef, helping to maintain the health of corals by flushing out freshwater and transporting key nutrients.<\/p>\n\n\n\n

Now, MIT scientists have found that reef passes are shaped by island rivers. In a study<\/a> appearing in the journal Geophysical Research Letters<\/em>, the team shows that the locations of reef passes along coral reefs line up with where rivers funnel out from an island\u2019s coast. <\/p>\n\n\n\n

Their findings provide the first quantitative evidence of rivers forming reef passes. Scientists and explorers had speculated that this may be the case: where a river on a volcanic island meets the coast, the freshwater and sediment it carries flows toward the reef, where a strong enough flow can tunnel into the surrounding coral. This idea has been proposed from time to time but never quantitatively tested until now.\u00a0<\/p>\n\n\n\n

\u201cThe results of this study help us to understand how the health of coral reefs depends on the islands they surround,\u201d says study author Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric and Planetary Sciences at MIT. <\/p>\n\n\n\n

\u201cA lot of discussion around rivers and their impact on reefs today has been negative because of human impact and the effects of agricultural practices,\u201d adds lead author Megan Gillen, a graduate student in the MIT-WHOI Joint Program in Oceanography. \u201cThis study shows the potential long-term benefits rivers can have on reefs, which I hope will reshape the paradigm and highlight the natural state of rivers interacting with reefs.\u201d<\/p>\n\n\n\n

The study\u2019s other co-author is Andrew Ashton of the Woods Hole Oceanographic Institution. <\/p>\n\n\n\n

Drawing the lines<\/strong><\/p>\n\n\n\n

The new study is based on the team\u2019s analysis of the Society Islands, a chain of islands in the South Pacific Ocean that includes Tahiti and Bora Bora. Gillen, who joined the MIT-WHOI program in 2020, was interested in exploring connections between coral reefs and the islands they surround. With limited options for on-site work during the Covid-19 pandemic, she and Perron looked to see what they could learn through satellite images and maps of island topography. They did a quick search using Google Earth and zeroed in on the Society Islands for their uniquely visible reef and island features.<\/p>\n\n\n\n

\u201cThe islands in this chain have these iconic, beautiful reefs, and we kept noticing these reef passes that seemed to align with deeply embayed portions of the coastline,\u201d Gillen says. \u201cWe started asking ourselves, is there a correlation here?\u201d<\/p>\n\n\n\n

Viewed from above, the coral reefs that circle some islands bear what look to be notches, like cracks that run straight through a ring. These breaks in the coral are reef passes\u2014large channels that run tens of meters deep and can be wide enough for some boats to pass through. <\/p>\n\n\n\n

On first look, Gillen noticed that the most obvious reef passes seemed to line up with flooded river valleys\u2014depressions in the coastline that have been eroded over time by island rivers that flow toward the ocean. She wondered whether and to what extent island rivers might shape reef passes.\u00a0<\/p>\n\n\n\n

\u201cPeople have examined the flow through reef passes to understand how ocean waves and seawater circulate in and out of lagoons, but there have been no claims of how these passes are formed,\u201d Gillen says. \u201cReef pass formation has been mentioned infrequently in the literature, and people haven\u2019t explored it in depth.\u201d<\/p>\n\n\n\n

Reefs unraveled<\/strong><\/p>\n\n\n\n

To get a detailed view of the topography in and around the Society Islands, the team used data from the NASA Shuttle Radar Topography Mission \u2014 two radar antennae that flew aboard the space shuttle in 1999 and measured the topography across 80 percent of the Earth\u2019s surface. <\/p>\n\n\n\n

The researchers used the mission\u2019s topographic data in the Society Islands to create a map of every drainage basin along the coast of each island to get an idea of where major rivers flow or once flowed. They also marked the locations of every reef pass in the surrounding coral reefs. They then essentially \u201cunraveled\u201d each island\u2019s coastline and reef into a straight line, and compared the locations of basins versus reef passes.\u00a0<\/p>\n\n\n\n

\u201cLooking at the unwrapped shorelines, we find a significant correlation in the spatial relationship between these big river basins and where the passes line up,\u201d Gillen says. \u201cSo we can say that statistically, the alignment of reef passes and large rivers does not seem random. The big rivers have a role in forming passes.\u201d<\/p>\n\n\n\n

As for how rivers shape the coral conduits, the team has two ideas, which they call, respectively, reef incision and reef encroachment. In reef incision, they propose that reef passes can form in times when the sea level is relatively low, such that the reef is exposed above the sea surface and a river can flow directly over the reef. The water and sediment carried by the river can then erode the coral, progressively carving a path through the reef. <\/p>\n\n\n\n

When the sea level is relatively higher, the team suspects a reef pass can still form through reef encroachment. Coral reefs naturally live close to the water surface, where there is light and opportunity for photosynthesis. When sea levels rise, corals naturally grow upward and inward toward an island to try to \u201ccatch up\u201d to the water line.\u00a0<\/p>\n\n\n\n

\u201cReefs migrate toward the islands as sea levels rise, trying to keep pace with changing average sea level,\u201d Gillen says. <\/p>\n\n\n\n

However, part of the encroaching reef can end up in old river channels that were previously carved out by large rivers and that are lower than the rest of the island coastline. The corals in these river beds end up deeper than light can extend into the water column, and inevitably drown, leaving a gap in the form of a reef pass. <\/p>\n\n\n\n

\u201cWe don\u2019t think it\u2019s an either\/or situation,\u201d Gillen says. \u201cReef incision occurs when sea levels fall, and reef encroachment happens when sea levels rise. Both mechanisms, occurring over dozens of cycles of sea-level rise and island evolution, are likely responsible for the formation and maintenance of reef passes over time.\u201d<\/p>\n\n\n\n

The team also looked to see whether there were differences in reef passes in older versus younger islands. They observed that younger islands were surrounded by more reef passes that were spaced closer together, versus older islands that had fewer reef passes that were farther apart. <\/p>\n\n\n\n

As islands age, they subside, or sink, into the ocean, which reduces the amount of land that funnels rainwater into rivers. Eventually, rivers are too weak to keep the reef passes open, at which point, the ocean likely takes over, and incoming waves could act to close up some passes.<\/p>\n\n\n\n

Gillen is exploring ideas for how rivers, or river-like flow, can be engineered to create paths through coral reefs in ways that would promote circulation and benefit reef health. <\/p>\n\n\n\n

\u201cPart of me wonders: If you had a more persistent flow, in places where you don\u2019t naturally have rivers interacting with the reef, could that potentially be a way to increase health, by incorporating that river component back into the reef system?\u201d Gillen says. \u201cThat\u2019s something we\u2019re thinking about.\u201d<\/p>\n\n\n\n

This research was supported, in part, by the WHOI Watson and Von Damm fellowships. <\/p>\n","protected":false},"excerpt":{"rendered":"

Volcanic islands, such as the islands of Hawaii and the Caribbean, are surrounded by coral reefs that encircle an island in a labyrinthine, living ring. <\/p>\n","protected":false},"author":2,"featured_media":26717,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[15,60],"tags":[],"class_list":["post-26716","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-environment","category-earth-science"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0.webp",900,600,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-200x200.webp",200,200,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-675x450.webp",675,450,true],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-768x512.webp",750,500,true],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0.webp",750,500,false],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0.webp",900,600,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0.webp",900,600,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0.webp",900,600,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-870x570.webp",870,570,true],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-600x600.webp",600,600,true],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-600x600.webp",600,600,true],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-760x490.webp",760,490,true],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-550x360.webp",550,360,true],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-95x65.webp",95,65,true],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-640x600.webp",640,600,true],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-96x96.webp",96,96,true],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/06\/MIT-Reef-River_Tetiaroa01-press_0-150x100.webp",150,100,true]},"author_info":{"info":["Jennifer Chu"]},"category_info":"Environment<\/a> Earth Science<\/a>","tag_info":"Earth Science","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/26716","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=26716"}],"version-history":[{"count":1,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/26716\/revisions"}],"predecessor-version":[{"id":26718,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/26716\/revisions\/26718"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/26717"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=26716"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=26716"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=26716"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}