Vocabulary:

• droughts—long periods of low rainfall and very dry conditions
• erosion—the movement of broken-down rock, soil, or sand from one place to another by running water or wind
• invasive species—living things that do not naturally belong to an ecosystem
• pollution—anything harmful or poisonous that is put into an environment
• sediment—very small pieces of broken-down rocks
• watershed—an area of land that catches rain and moves it downhill into a body of water

In many Pacific Island cultures, ancient chiefs divided high islands into areas of land that extended from the mountains to the sea. Each area, separated by a hill or ridge, formed a watershed. Within a watershed, the land and water are connected, giving generations of islanders access to natural resources: wood from the forests, fresh water from streams, crops from planted land, and fish from coral reefs.

In a watershed, gravity makes water flow from an area of higher elevation to an area of lower elevation. Rainfall that does not get absorbed by the soil collects in streams. Streams may flow into rivers, which empty into a common body of water, such as a lake or the sea. A high island watershed typically consists of three zones: forest, river floodplain, and coastal marine. What happens in a higher zone can affect everything below it.

Since Pacific islands were first settled more than a thousand years ago, many have been transformed. Over time, farming, construction, invasive species, and growing populations of pigs and goats have loosened the soil, making the watersheds more subject to erosion. When it rains, large amounts of sediment and pollution—including sewage, pesticides, and fertilizers—are picked up from the ground’s surface and delivered to the reefs. Because fresh water is less dense than salt water, this sediment may stay suspended in the ocean for days, blocking the sunlight that corals use for photosynthesis. When the sediment finally does settle, it can blanket the corals and prevent them from feeding and reproducing.

While islanders can do little themselves to prevent climate change, local actions can reduce damage to reefs from sediment and pollution. For example, because most sediment is trapped by mangrove forests before reaching the ocean, protecting mangroves can help reduce the harmful effects of soil erosion on coral reefs. Replanting areas of a watershed that were previously cleared of vegetation and controlling the populations of wild pigs and goats can also help keep the soil from washing away and ending up on a reef.

With flooding, droughts, and major storms likely to occur more frequently with climate change, watersheds will feel even more stress going forward. On many islands, scientists are studying how these events may affect the health of the watershed and coral reefs. Their findings can help community leaders and policymakers decide which actions to take. Traditional knowledge, which understands watersheds as a system of parts that all work together, can play an important role in watershed planning.

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