How Wetlands Work

The critical role of Wetlands: Coastal wetlands in Maui play a critical role in protecting beaches and other valuable near-shore natural resources by providing a number of important ecosystem services, including regulating and attenuating sediment flow in nearshore waters, creating a nutrient sink that prevents nitrogen and phosphorous pollution from entering the ocean and harming coral reefs, provisioning of habitat for indigenous and endemic Hawaiian waterbirds, and providing for groundwater aquifer recharge, and flood mitigation. The four main beneficial services Wetlands provide include:

  1. Wetlands as biological filters, the kidneys of our water cycle.
  2. Wetlands play an important role of wetlands in flood mitigation,
  3. Wetlands are an important part of coastal hazard reduction.
  4. Wetlands help protect coral reefs that are vital to securing our shorelines.

The mechanical action of Wetlands: Wetlands have both a mechanical function, and a biological one (as well as a cultural, spiritual, and educational function). From a purely mechanical point of view, wetlands are huge filters. That can absorb dirty rainwater and filter it before it reaches the ocean.

Freshwater flows from the wetland through the sand into the ocean. Wetlands and sand dunes filter stormwater before it reaches the ocean.

Wetlands also function as detention basins, which store and hold back vast amounts of water, and slowing the rate at which stormwater flows into the sea. Also, because where there is a widening of the streambed around wetlands, it increases their holding capacity. This allows the wetland creates a buffering effect that can de-energize fast-moving water, so it creates less erosion and becomes less destructive to the landscape.

Wetlands expand and contract: Wetlands have the ability to change their size both in their depth and in width. Wetlands are naturally responsive to changing conditions. They can overflow onto the surrounding land expanding their surface area, and also increase their depth. Wetlands can also rise up from underneath the ground through underground connections and saturate relatively dry areas. Wetlands can suddenly appear even if there is no water visibly flowing into them. Wetlands that appear after rainfall or temporarily are sometimes called “ephemeral” wetlands.

Emergent Wetland fed by underground springs

Wetlands are not always wet: Wetlands come in many shapes and sizes. Some wetlands have deep pools of permanent water, we call these emergent wetlands. Some have no visible water because they are choked up with plant life. These wetlands are sometimes only distinguishable by the types of vegetation growing there. Some wetlands are only muddy bogs, and some can appear as “moist ground” because the water’s level may just be slightly below the surface. Some wetlands become fully dry between rain events and come and go on a temporary basis. All wetland types have an important role in the watershed and the ecosystem. 

Salt pond in the wetlands.

Integrated Watershed System: Wetlands are features of an integrated watershed system, that transports water from Mauka to Makai. Just as your own body’s circulatory system transports life-giving blood around your body, so does the water cycle transport lifegiving water through our island’s watershed. Whereas your body has kidneys to filter the blood, our water cycle has wetlands that function as kidneys do, filtering the water to keep it clean from dirt and contaminants. In short, wetlands are nature’s kidneys.

Wetlands help filter the sediment from stormwater runoff

Maui’s Waterscape: Wetlands are one of the most visible features of South Maui’s waterscape. Wetlands often occur when underground springs come close to the surface, creating a wet zone that is fed by underground springs. Water levels in wetlands are also affected by seasonal rainfall variations in the local drainage basin, as well as rainfall events in the upper watershed. Some nearshore wetlands are affected by tides and the ocean. Coastal wetlands are connected to the ocean through an underground nexus of waterways. Tidal effects from the ocean can sometimes affect wetlands miles inland.

Hidden Wetlands: Natural wetlands get covered over and buried. By their very nature, as “dirt catchers”, wetlands can accumulate large amounts of mud and soil. This is where polluted stormwater runoff usually stays while the sediment (dirt) it carries settles out. Often, we see a new layer of silty mud in these wetlands after a flash flood or heavy rain event. Over time these silt deposits build up the floor of the wetlands and may even bury them. You can see many wetlands that have become narrower and turn into streams surrounded by mudflats. These mudflats are the perfect place for new vegetation to grow, so we often see a vibrant and dense area of vegetation. Sometimes all we can see of a wetland is the vegetation, but just below the surface, the same water cycles continue to operate. Unfortunately, many “hidden” wetlands and functional floodplain areas surrounding wetlands have been mistaken as vacant land by developers.  And these kinds of areas are built over, or even deliberately filled in to be used as building sites. This unfortunate trend of building closer and closer to wetlands is called “encroachment”. Encroachment into wetlands and their surrounding buffer zones, greatly limits the ability of the wetlands to function normally. Wetlands are often prevented from widening or renewing themselves, and they start to diminish in their ability to buffer stormwater.

Scrub shrub wetland vegetation and soil types.

Nature has created these wetland systems: Wetlands are dynamic and responsive water features that nature has designed, built, and maintains. Natural Wetlands are far superior to anything humans can make so it is our kuleana (responsibility) to treat them with respect and acknowledge the great services that they provide.

The Kalepolepo wetlands trap silt and mud coming from Kulanihakoi Gulch.

Encroachment increases flooding: It is inevitable that reducing the number of wetlands or the size of wetlands will lead to more flooding. This is especially evident on the Kihei flood plain. The entire area was known for flooding, but with all the wetlands and open space in the area could absorb most of the rainwater and catch the sediments coming down the mountain. The floodplain soil is a testament to thousands of years of these cycles, and the accretion of silty soils near the shoreline. But as the flood plain started to be built out, wetlands were restricted, and flooding became more prevalent. It is interesting to note that before the current era of development, Kihei was considered one huge swampy wetland that was unsuitable for development. 

This is an example of the silt and mud brought by stormwater. Healthy wetlands can usually capture most of the silt before it reaches the ocean.