Trickle-down Effect

A guide to our watersheds.

illustration: stephen schildbach

The water you brushed your teeth with this morning is older than the average college student. That’s right: it takes a raindrop roughly 25 years to travel from the top of the Koolau Mountains and through an island-wide transmission system before it hits your sink. Why does it take so long? Just imagine this: Rain hits the earth, soaks into the soil, then embarks on a long journey through a network of porous volcanic rock until it’s absorbed into the aquifer. For rainwater to be captured, it needs lush, healthy forests.

On Oahu alone, residents use roughly 150 million gallons of water per day, according to the Honolulu Board of Water Supply, and the majority of it comes from the Koolau Mountains. Miranda Smith, coordinator for the Koolau Mountains Watershed Partnership, explains the relationship between water and the forest: “Think of a native forest as the sponge that maintains our water supply … you have multiple levels, from the upper canopy, which includes koa and ohia trees, to the understory, made up of ferns and mosses. When rain hits a healthy forest, plants slow the water down, allowing it to be absorbed. “In an invasive forest, there are few layers, and little diversity. The water is going to hit the side of the mountain and run off into lakes and streams,” explains Smith. “So it’s what is on top of the earth slows down the water, and allows it to trickle below.”

Today, a number of factors, such as invasive weeds and feral animals, affect the health of our forests—and consequently our water. And in Hawaii, everyone lives in a watershed—or a region of land that catches and collects rainfall and distributes it to an outlet, like the ocean. (Think of the ahupuaa system.) These days, however, multiple landowners typically own separate parts of a watershed, making it difficult to manage the area as a whole.

The solution? In 1991, the first Watershed Partnership was established in East Maui and connected federal, state and private landowners seeking to manage land for conservation and fresh-water supply. Now, nine Watershed Partnerships exist on six islands, encompassing a total of 1.6 million acres across the state and bringing together more than 45 private landowners and 24 public agencies—from Kamehameha Schools and Bishop Museum to the state Department of Land and Natural Resources and the U.S. National Parks. It’s a unique example of environmental stewardship, which shows that property lines can be overlooked when it involves the common good.

What exactly do the partnerships do? Projects include constructing fences, removing invasive species and restoring native habitats after fires.

What can you do to help keep our watersheds healthy? Never plant invasive plants in your yard; clean your boots before a hike to prevent seeds of invasive plants from spreading; contact your representative to share your support for watersheds; and volunteer your time. For more information, see the Hawaii Association of Watershed Partnerships at