A Terrain-Based Method for Mapping Coastal River Outlets and Contributing Areas
This paper presents a reproducible, terrain-based workflow using a 10~m digital surface model to systematically map and rank coastal river outlets and contributing areas across 19 Faroese islands, successfully resolving challenges posed by steep topography and engineered diversions to produce an auditable catalogue for future hydrological studies.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
On small, steep islands, the landscape often refuses to behave like the neat, branching river systems found on large continents. Here, a single hill might shed its rainwater through dozens of short, unnamed channels that vanish into the sea within a few hundred meters. For scientists studying how fresh water moves from land to ocean, or for managers trying to protect coastal ecosystems, this creates a difficult puzzle: how do you count and map something that is everywhere and nowhere at once? Traditional maps often rely on naming rivers or drawing lines where water is deep enough to be seen, but on rugged archipelagos, these lines are arbitrary. A method that works for a major river valley fails when applied to a cliffside where every drop of rain finds its own path to the sea. To understand the true flow of water in these places, researchers need a way to see the entire drainage network without forcing it into a pre-existing category of "river" or "not a river."
To solve this, a team of researchers from the University of the Faroe Islands developed a new, computer-based method to map every single place where land drainage reaches the coast. They focused on the Faroe Islands, a rugged archipelago in the North Atlantic where the terrain is steep and the coastline is deeply indented. Instead of trying to identify named rivers, they treated the entire land surface as a grid of tiny squares. They placed a virtual "seed" at regular intervals across every square of land on nineteen different islands. From each seed, a computer algorithm traced the path a drop of water would take, following the steepest downward slope until it hit the sea. This process generated a massive list of potential outlets, capturing even the smallest, unnamed streams that conventional maps ignore.
The researchers then had to make sense of this overwhelming amount of data. Because many seeds from the same small valley would end up at slightly different points along the rocky shore, the team grouped these endpoints together. They decided that any two endpoints within fifty meters of each other belonged to the same outlet. This consolidation reduced the 92,259 individual paths into a manageable list of 6,535 distinct coastal outlets across the nineteen islands. Crucially, the method did not just find the outlets; it ranked them by importance. By counting how many seeds flowed into each outlet, the team could determine which coastal spots received the most water. They found that the number of outlets needed to represent 90% of the total drainage varied wildly from island to island. On the tiny island of Lítla Dímun, just twenty-two outlets were enough to capture most of the flow, while on the larger island of Eysturoy, it took 546 outlets to reach the same level of coverage. This proved that there is no single rule for how many "main rivers" an island has; the answer depends entirely on the shape of the land.
The study also had to account for human-made changes to the landscape. On Eysturoy, a hydroelectric power plant at Eiði captures water from several natural valleys and redirects it through a reservoir to a single engineered outlet. The researchers built this specific diversion into their computer model, ensuring that the water was counted at the power plant's exit rather than at the natural valley mouths. This allowed the model to reflect the actual water reaching the sea, rather than just the theoretical path of rain falling on the ground. When they tested their method against a list of 279 known drainage points from existing maps, they found that 86.4% of the known points were within 200 meters of one of their modeled outlets. While this might sound like a large gap, the researchers explained that many of the reference points on the old maps were not precise coastal locations but rather inland markers or general areas. The new method provided a consistent, reproducible way to locate every possible entry point for fresh water, regardless of whether it had ever been named or surveyed before.
The researchers were careful to distinguish what their map could and could not do. They emphasized that their results show the area of land that drains to a specific point, but they did not measure the actual volume of water flowing there. Rainfall varies significantly across the islands, with some areas receiving more than four times the rain of others, and factors like evaporation or groundwater storage also change the final flow. Therefore, their map is not a measurement of discharge, but a precise, auditable foundation for future studies. By providing a permanent list of outlets with unique identifiers, the team has created a tool that other scientists can use to add rainfall data later, without having to remap the terrain. The work confirms that on steep, complex islands, drainage is a continuous spectrum rather than a set of discrete rivers, and that understanding the full picture requires looking at the entire landscape, not just the largest channels.
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