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S  E D I M E N T D  E L I V E R Y R  AT E S

D5

What Is It?

BENCHMARK

Maintain

Maintain (modeled) annual sediment

Sediment delivery rate refers to an estimate of

delivery rates at current (2003/2004)

the amount of suspended solids that reach a river

levels.

levels.

discharge point within a specific period of time.

This rate indicates the extent of erosion occurring

TA R G E T

upstream, mobilizing sediments that will eventually be

A 5% reduction in sediment delivery by

deposited in coastal and marine environments.

2025. Higher reductions (~10% could

Why Do We Measure It?

be achieved if appropriate agricultural

management practices are adopted).

Transport of soil into rivers and eventually coastal

waters is a major concern, particularly for nearshore

RED FL AG

environments. Sediment erosion and deposition

Any increase in modeled or measured

are naturally occurring processes to which marine

sediment

sediment delivery rates.

ecosystems are adapted. However, land disturbances

that increase sedimentation can impact marine

ecosystems in several ways -- some deleterious.

can be modeled for earlier times when the land cover

was in an unaltered, "pristine" state.

Increased water turbidity reduces the amount of light

reaching the seafloor, thereby reducing photosynthesis

Causes of observed or modeled increases in

(and growth) of seagrasses and corals. In severe cases

sediment delivery rates are not readily attributable to

(usually on nearshore reefs), corals and seagrasses

specific activities. It is impossible to determine (solely

can be literally smothered by sediment. In extreme

from a model) whether elevated delivery rates are

cases, sedimentation can completely change the

being caused by (for example) agriculture, ranching,

composition of the marine benthic community and

deforestation, general infrastructure development or

alter shorelines.

climatic changes.

Estimating, or modeling sediment delivery rates

Therefore, it may be difficult to identify a point of

at river mouths provides information about which

intervention to address erosion issues. Repeated

watersheds are likely contributing the majority of

model iterations, incorporating refinements such as

sediments to the coast. This information can be

different agricultural management practices or more

used to guide interventions. Management efforts

specific land cover classes, may prove helpful.

then aim to maintain or reduce sediment discharge

rates to levels that minimize disturbances to marine

In addition, current models do not have the capacity to

ecosystems.

account for reduced sediment delivery for watersheds

with hydroelectric dams (reservoirs of sediments) or

How Do We Measure It?

those that are adopting better management practices

to reduce erosion. Hopefully, future model versions

Ideally, sediment delivery rates would be measured

will be able to accommodate these site-based

directly at each river mouth. Alternatively, sediment

adjustments.

delivery can be estimated using GIS erosion and

sediment delivery models, based on watershed

Modeled delivery estimates must be validated at a

characteristics such as land cover, precipitation, soil

range of river mouth sites (from actual measurements

characteristics and slope.

of sediment delivery rates). Observations of reef

Usefulness

Usefulness

sedimentation rates and community structure, coral

recruitment, and current flow patterns are also needed

Modeled sediment delivery rates are helpful for

to identify potential impacts of sediment delivery on

identifying potentially harmful levels of erosion on a

reef health.

spatially explicit basis. This indicator also provides the

possibility of looking back in time, as delivery rates

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