We all depend on water every day, we expect it every time we turn on the facet or hose, and yet many of us don't know where it comes from or the characteristics of our island that provide us with this priceless resource.

What is a Watershed?

A watershed is a diverse and highly organized ecosystem which specializes in the capturing of rain and atmospheric moisture from the air and sky. They are usually located in high mountains or valleys, which then divert the water into rivers and streams. Watersheds are composed of several layers of dense vegetation. Oahu's watershed can also be described as a Hawaiian rain forest, which captures and saves large amounts of water.

What are the Characteristics of a Watershed?

Watersheds contain multiple layers of vegetation that soaks up rainfall and retains moisture. Each layer of plant life plays a unique and integral role based upon its location in the watershed.

watershed layers

Emergent Trees, the tallest in the watershed's rainforest, are the first trees to have contact with water from rain, fog, or heavy moisture laden air. Rainwater trickles down its limbs and leaves to feed plants in the lower levels of the rainforest.

Tall Canopy trees receive the most rain and condensation. Moisture forms on its leaves, which then runs down their trunks to the levels below.

Sub-canopy trees and shrubs collects water from the taller trees above. These short, dense trees slow evaporation by keeping the air near the ground saturated.

The Understory layer consists of ferns and low shrubs. These plants absorb water drops from taller trees and help cushion the impact on the soil.

Lastly, Groundcover plants such as moss and grass shield soil from sunlight, keeping the soil moist by minimizing water lost to evaporation.

Where are these Watersheds on Oahu?

Oahu's watersheds are located in two distinct areas. The Koolau mountain range is Oahu's primary watershed area, located above the windward shore and on the eastern side of the central Oahu plains, creating the back drop of the Honolulu metropolitan area. The Waianae mountain range is located in Leeward Oahu on the west side of the island.

How Oahu's Mountain Ranges Influence the Rain

watershed illustrationOahu's unique topography, particularly its steep mountain ranges, is the key to our island's ability to attract clouds and abundant rainfall. As ocean water evaporates into the atmosphere, northeast tradewinds drive moisture laden clouds inland. The high mountain ranges then force clouds over its peaks, causing precipitation and condensation over watershed areas. As rainwater slowly percolates into the earth, water is naturally filtered by our volcanic soil and stored by dike rock compartments, which overflow and fill our underground water sources.

watershed young mountains

On young, high mountains such as the Big Island's, clouds drop their precipitation before they are pushed to the highest elevations, leaving the upper reaches dry and desert-like.

watershed older islands

On older, eroded islands such as Oahu and Kauai, rainfall is heaviest on the windward slopes and mountain peaks, allowing lush vegetation to cover even the highest ridges.

watershed flat island

A relatively flat island such as Niihau has very little rainfall because it lacks the high elevation slopes. Without the slopes, winds cannot push moist air upwards to produce clouds and precipitation.

How to Protect the Watershed

Today, the watershed is at a greater risk than ever. Invasive plant and animal species, deforestation, and excessive human foot traffic are just a few things that threaten the watershed. Everyone should take personality responsibility to preserve and protect watershed areas and water resources for future generations to enjoy.

To help protect against invasive species, support agriculture quarantine laws and participate in invasive species removal projects. To prevent forest degradation, stay on designated trails and respect the wildlife when out and about in the watershed. Also, join community watershed partnership groups that organize activities for watershed restoration and education.

Additional Resources

The Water Cycle
Water for Life: Rain Forests and the Water Cycle
Oahu's Water Cycle and You

For More Information

To learn more on how you can join the effort to protect our wetlands and forests, please call the BWS at (808) 748-5000.