Kauaÿi

Niÿihau

Oÿahu

Maui

Molokaÿi

Kahoÿolawe Hawaiÿi

Hawaiian Islands

Länaÿi

25 MILES

 50 KILOMETERS

0

0

N

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3049 

May 2011

From Ridge to Reef—Linking Erosion and 

Changing Watersheds to Impacts on the Coral Reef 

Ecosystems of Hawaiÿi and the Pacific Ocean

Coral reef ecosystems are

threatened by unprecedented 

watershed changes in the United States 

and worldwide. These ecosystems 

sustain fishing and tourism industries 

essential to the economic survival of 

many communities. Sediment, nutrients, 

and pollutants from watersheds are 

increasingly transported to coastal 

waters, where these contaminants 

damage corals. Although pollution 

from watersheds is one of many 

factors threatening coral survival, it 

is one that local people can have a 

profound influence on. U.S. Geological 

Survey scientists are using mapping, 

monitoring, and computer modeling to 

better forecast the effects of watershed 

changes on reef health. Working with 

communities in Hawai‘i and on other 

U.S. islands in the Pacific, they are 

helping to provide the science needed to 

make informed decisions on watershed 

and coral reef management.

Coral reef ecosystems are in decline worldwide, and their survival 

is in doubt in some locations. In response to this ongoing threat, 

scientists from the U.S. Geological Survey (USGS) are collaborating 

with local communities and agencies in Hawaiÿi and on other U.S. 

islands in the Pacific Ocean to understand the sources and impacts 

of sediment, nutrients, and pollutants from watersheds. A watershed 

is all of the upslope land area that could contribute water and 

sediment to a stream. Identifying and monitoring effects of landbased contaminants from watersheds on nearshore ecology helps 

stakeholders evaluate costs and benefits of management actions. 

Such informed watershed management protects the health of coral 

reefs, while preserving water and biological resources on land. 

The goal of USGS scientists is to quantify watershed changes 

and their impacts on coral reef ecosystems so that resource 

managers can make informed decisions. To do this, the USGS is 

linking measurements of human-induced changes in terrestrial 

ecosystems with models that estimate erosion. The scientists 

are using vegetation and surficial mapping, field experiments, 

sensor networks, and monitoring to create a “sediment budget” 

for the landscape. By tracking sediment and water movement 

from upland areas (the “ridge”), down transport paths to coastal 

This image shows the close link between onshore watersheds and nearshore coral reef ecosystems. 

U.S. Geological Survey scientists are working to understand how watershed and land-use issues are 

affecting coral reef health in Hawai‘i and on other Pacific islands. This scientific information will help 

policymakers and local communities to make informed land-management decisions. (Image created

from USGS photos by Mike Field.)

waters (the “reef”), 

they are developing 

forecasts of the likely 

consequences of climate change 

and land-management decisions on 

watersheds and reefs. 

Science on the Ridge

High above the reef, erosion of steep volcanic 

slopes moves mud and silt into drainage networks. 

The USGS is quantifying erosion and watershed changes in 

these steeplands using remote sensing and field data to create very 

high-resolution geomorphic and vegetation maps. Scientists are 

combining these maps with measurements of rainfall, infiltration, 

and the strength of the roots of both native plant communities and 

invasive species to show how these factors affect changes in soil 

resistance to landslides and overland flow erosion. They are also 
monitoring changes in vegetation composition and structure in 

response to management actions to restore native-dominated plant 

communities (for example, efforts to control feral pigs and goats).

To better understand the mechanics of erosion on high islands 

in the Pacific, USGS scientists have deployed an advanced 

network of sensors on some of the most severely degraded 

landscapes on the Hawaiian island of Molokaÿi. During storms, 

this network measures the intensity and duration of rainfall, the 

water content of the soil, the depth of water flowing over the 

hillslope, and the amount of water and sediment runoff. After 

storms, scientists resurvey the area to detect small changes 

in topography from erosion using ground-based lidar (light 

detection and ranging). This highly accurate laser surveying 

technique can be used to generate centimeter-scale topographic 

maps for change detection. Scientists are using these maps and 

sensor readings to quantify erosion rates for hillslopes, the source 

for fine sediment deposited on nearby reefs. To extend these 

erosion rates to the whole watershed, they are using satellite and 

airborne imagery and airborne lidar to create maps of surface 

processes (for example, rockfall, soil creep, overland flow, and 

gullying). The scientists are also conducting field experiments 

to characterize soil infiltration rates and resistance to overland 

flow erosion, using measurements from infiltrometers and 

To better understand how erosion and sediment 

transport affect the health of coral reefs, the 

U.S. Geologial Survey has installed instruments 

in the Kawela watershed on the island of 

Molokaÿi. At this experimental site, scientists 

are using a rain gage to measure the intensity 

and duration of rainfall, soil moisture sensors to 

measure how much water is in the soil, overland 

flow meters to measure how deep water flow 

is over the hillslopes, and a flume and pump 

to measure the amount of water and sediment 

that run off from the small catchment. When 

rain falls, the network records data at intervals 

from 5 seconds to 5 minutes. Erosion rates 

and susceptibilities measured in detail at such 

sites can be projected across the rest of the 

watershed using careful geomorphic mapping.

Inset shows a scientist downloading data from an 

overland flow sensor. (USGS photos by Jonathan 

Stock and Kevin Schmidt.)

During heavy rains, hillslope and stream-bank erosion provide fine sediment to flood flows (left). At the coast, flood flows enter the nearshore marine 

environment (center). Sediment-laden freshwater is less dense than seawater, so plumes of muddy water containing sediment from uplands may last 

for days in the nearshore environment (right), blocking much-needed sunlight that corals use for photosynthesis. These plumes eventually deposit their 

sediment on the seafloor, smothering corals and preventing them from feeding and reproducing. (USGS photos left/right by Mike Field; middle photo 

courtesy of Lyndon Dela Cruz.)

cohesive-strength meters. Radiometric dating methods allow 

them to estimate longer-term rates of erosion and deposition. By 

combining mapping, monitoring, and experimental data, USGS 

researchers are creating sediment budgets to forecast the supply 

of fine sediment to the reef. In some landscapes, they have found 

that about 1% of the watershed area contributes almost half of the 

annual sediment load.

Science Along the Transport Path

Mud and silt erode from steep hillslopes during storms and move 

down toward the reef. To estimate the amount of this fine sediment 

delivered from eroding uplands to coral reef ecosystems, the USGS 

has installed stream gages with suspended-sediment collectors 

on Kauaÿi, Molokaÿi, and Kahoÿolawe. These gages measure the 

mass of sediment delivered to the reef in stream water, providing 

information that forms the beginning of long-term datasets on 

sediment flux to reefs from steep, volcanic highlands. Scientists 

are also measuring erosion using networks of permanent markers, 

or pins, on Molokaÿi, Kahoÿolawe, and the island of Guam. These 

erosion pins are driven into the ground, and periodic measurements 

are made of the length of pin exposed by soil erosion, especially 

after heavy rainfalls. These simple measurements complement 

sedimenttransportpathtoreef

hillslope sediment source
The “ridge-to-reef” concept is rooted in the traditions 

of many Pacific island cultures. Ancient chiefs divided 

islands into parcels of land running from the mountains 

to the sea. A landowner, household, or village received 

a parcel with a supposedly equal share of available 

environmental resources—a system that is still in use 

in many Pacific island nations today.

In Hawaiÿi, these traditional land divisions are called 

ahupuaÿa. Ahupuaÿa boundaries typically form a 

watershed, running from ridgeline to ridgeline across 

a valley and from the uplands to the sea. Ideally, each 

provides access to resources, including wood from 

forested areas, water from streams, land for cultivating, 

and fishing from the nearshore coral reef. As shown in 

this painting, stream water is diverted to narrow taro 

fields in the upper valley, and broader taro fields fill 

the wider valley mouth. Coconut trees offer additional 

food and drink for the village. A coastal fishpond and 

the nearshore coral reef further provide resources 

from the sea. (Painting by Marilyn Kahalewai, used 

with permission from Kamehameha Schools.)

Recent watershed changes on remote Pacific 

islands are a legacy of human arrival over the past 

1,500 years. These alterations have increased 

dramatically since substantial European contact in the 

19th century. Habitats have been transformed by 

invasive species, wildfire, agriculture, and grazing by 

feral animals, such as pigs and goats. These changes 

swept across watersheds, and made their soils 

dramatically more vulnerable to erosion. Shrinking 

ground cover and vegetation canopy loss leads to an 

increase in rainfall intensity on exposed soils and a 

potential reduction in the binding force of roots. Both 

effects make soils more vulnerable to erosion. As a 

result, sediment, nutrients, and other contaminants are 

now delivered at increased rates to many coral reef 

ecosystems. By examining the pathways and concentrations of sediment and other contaminants, U.S. 

Geological Survey scientists are learning how these 

land-based pollutants imperil coral reef ecosystems.

Origins of Ridge to Reef—Connecting Polynesian Tradition to the Present

detailed site explorations and can be used by policymakers and 

local communities to assess the success of mitigation efforts.

Science on the Reef

On the reef, mud and silt that arrive with floods linger as 

they are episodically resuspended, degrading the ecosystem. 

The USGS is using aerial and satellite imagery to map coral 

distribution and density on reefs. In addition, scientists have 

placed underwater instruments on reefs to measure water clarity, 

waves, currents, temperature, salinity, and light penetration. 

Sediment traps and seabed sampling provide the basis for 

quantifying sediment accumulation rates and suspended-sediment 

concentrations. Surveys using portable instruments, as well as 

long-term measurements at fixed locations, provide information 

about different areas of the reef. This information is used to 

characterize sediment deposition rates and size distributions, 

wave energy associated with sediment resuspension, and the rate 

at which sediment leaves the reef. Laboratory and field studies of 

coral response to sedimentation and decreased light availability 

will help set goals for acceptable levels of sedimentation, while 

also calibrating numerical models of wave and current processes. 

These models will help forecast the effects of sedimentation on 

reef health. The models will also be used to begin identifying 

threshold levels for sediment input and water turbidity (lack of 

clarity caused by suspended sediment particles) on coral reefs.

A Look to the Future

Around the globe, coral reefs are now dying at a rate of nearly 

1% every year. Without protection and mitigation, their future is 

in jeopardy. In addition to reduced global biodiversity, the loss 

of these ecosystems will affect millions of Pacific Islanders and 

others who depend directly on coral reefs for food resources, 

coastal protection, and tourism and other livelihoods. Changing 

climate conditions are leading to warmer and more acidic ocean 

waters that can kill corals, but these threats are impossible to 

manage at local scales. 
coral reef coral reef

sediment sediment suspended suspended

Moloka‘i Moloka‘i

USGS hillslope experimental site

USGS stream gage 0 5 KILOMETERS

0 3 MILES

Heavy rains of a Kona storm 

fell over the Hawaiian island of 

Molokaÿi in November 2001. 

Water ponding and flowing 

from eroding hillslopes (see 

U.S. Geological Survey 

experimental site) moved mud 

and silt down through the stream 

network, past the present site of 

a USGS stream gage (triangle) 

and out to the nearshore. Two 

days later, the suspended 

sediment was still evident in the 

water over the reef flat, as seen 

in this Landsat satellite image. 

High sediment concentrations in 

the water column or blanketing 

the seafloor can harm coral reef 

ecosystems by reducing light for 

photosynthesis, impairing coral 

reproduction, smothering and 

abrading coral, and increasing 

the growth of macroalgae.

Local management actions, however, can reduce damage 

to reefs from overfishing, anchor damage, and sediment 

pollution. For example, where small bare areas of watersheds 

produce disproportionately high sediment loads, it may be 

economically effective to mitigate them through revegetation. 

Forecasting the effects of watershed changes on coral reefs 

will require knowledge gathered through a combination of 

scientific disciplines. 

In Hawaiÿi this effort involves many partners, including Federal 

agencies (such as National Park Service, Fish and Wildlife Service, 

and Environmental Protection Agency), State agencies (such 

as Department of Health and Department of Land and Natural 

Resources), other organizations (such as The Nature Conservancy 

and Hanalei Watershed Hui), and local communities. 

Results from ongoing USGS studies will provide partners 

with crucial scientific information needed to assist coastal 

resource managers in prioritizing decisions for sound ridgeto-reef management. This information can also be used to 

guide approaches such as adaptive management and to help 

understand how ecosystems will respond to changes in climate 

in Hawaiÿi, U.S. Pacific islands, and on high islands elsewhere 

in tropical seas.

Jonathan D. Stock, Susan A. Cochran, Michael E. Field,

James D. Jacobi, and Gordon Tribble

Edited by James W. Hendley II

Graphics and layout by Jeanne S. DiLeo and David R. Jones

For more information contact:

U.S. Geological Survey

400 Natural Bridges Drive

Santa Cruz, CA 95060

(831) 427-4745

http://coralreefs.wr.usgs.gov/

See also:

The Coral Reef of South Moloka‘i, Hawai‘i; Portrait of a 

Sediment-Threatened Fringing Reef, U.S. Geological Scientific 

Investigations Report 2007-5101 (http://pubs.usgs.gov/

sir/2007/5101/)

and

U.S. Coral Reefs—Imperiled National Treasures, USGS Fact 

Sheet 025–02 (http:// pubs.usgs.gov/fs/2002/fs025-02/)

This Fact Sheet and any updates to it are available online at 

http://pubs.usgs.gov/fs/2011/3049/

To better understand what happens when sediment from onshore reaches coral reefs, U.S. Geological Survey scientists are using seafloor instruments 

to measure oceanic conditions. Divers install instruments in a sand channel (left) to measure currents, waves, temperature, and turbidity. An underwater 

camera system (middle) collects images at specific times throughout each day to document sediment settling on the coral reef. A sediment trap (right) 

collects suspended sediment as it falls through the water column to the seafloor. The white specimen bottles below the platform are programmed to 

rotate into place underneath the cylindrical collection funnel for a predetermined number of days. These instruments are being used to help quantify the 

transport rate and residence time of harmful sediment on the reef. (USGS photos by Mike Field and Tom Reiss.)