Chris Phillips & Les BasherRoger Young & Mike Marden

Sediment generation from land uses and landforms

Outline

• issue• context – background• objective• approach

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.

Issue

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

Why we’re here now

• knowledge base interviews• topic on everyone’s lips• perceived or real• natural or induced• as many opinions as there are trout• specific meetings – forestry, F&G

Current knowledge

• previous work (1980’s) 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

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

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?

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.

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

Dart

Kahurangi Nat Park

Rainy

Motueka

TadmorSherry

Motupiko

Baton

StanleyBrook

DoveOrinoco

Wai-wheroPearse

Graeme

PokororoBrooklyn

Wangapeka

Red Hills

Dart

Kahurangi Nat Park

Rainy

Motueka

TadmorSherry

Motupiko

Baton

StanleyBrook

DoveOrinoco

Wai-wheroPearse

Graeme

PokororoBrooklyn

Wangapeka

Red Hills

Summary

• Trout decline = sediment up?

• Trout decline = something else?

• Natural vs induced?

• Short term vs long term?

• What’s important - where and why?

• What’s required - $$$ ??