Sediment generation from land uses and landforms

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Sediment generation from land uses and landforms
Chris Phillips Les Basher Roger Young Mike
Marden
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Outline

• issue
• context background
• objective
• approach

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Issue
Decline in the Motueka trout fishery has been
linked to increased input of sediment, mainly
from the Separation Point Granite terrain. This
is believed to have increased the proportion of
the riverbed that is covered in sand and silt and
affected trout spawning, food supply, and habitat
such as the density of pools in the catchment.
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Background

• decline in trout abundance and biomass in Motueka
  River since 1985
• linked to increased input of fine sediment
• direct and indirect effects of sediment on trout
  habitat and populations
• key property is proportion of sand and silt
• believed to have increased in many parts of the
  river system

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Why were here now

• knowledge base interviews
• topic on everyones lips
• perceived or real
• natural or induced
• as many opinions as there are trout
• specific meetings forestry, FG

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Current knowledge

• previous work (1980s) focused on sediment
  generation from forestry activities on SPG
• earthworks associated with roads and landings
  were the main sediment producing areas
• landslides are the biggest sediment contributors
• on whole-catchment basis natural erosion rates
  higher than those induced by land disturbance

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Current knowledge cont.

• variety of factors influencing present sediment
  characteristics of Motueka River including
• the long-term impact of large storms
• sediment generation from both natural erosion and
  that related to forestry activities on SPG
• long-term impact of past forestry management
  practices
• river bank stability in general

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Knowledge gaps

• causes of decline not well known scientifically
• little data on riverbed characteristics whole
  system
• little data on variation in rates of sediment
  supply
• time scale for sediment movement through system?
• what are the major sources of sediment?
• relative role of natural erosion and land-use?
• relative role of long-term impact of large
  storms?

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Objective
To establish changes in riverbed characteristics
important to maintenance of the trout fishery,
and to determine the major sources of sediment
input and options for managing the rate of
sediment supply.
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Approach

• characterise stream bed particle size composition
• establish stream bed monitoring sites
• reconstruct past stream bed character
• determine major sources of sediment input
• analyse trends in SS concentration
• incorporate sediment sampling strategy into ESN
• link with riparian/in-stream investigation sites

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Kahurangi Nat Park
Dart
Red Hills
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Kahurangi Nat Park
Dart
Red Hills
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Summary

• Trout decline sediment up?
• Trout decline something else?
• Natural vs induced?
• Short term vs long term?
• Whats important - where and why?
• Whats required - ??