This article is based on a paper by Brendan Hicks (Principal Scientist), presented at the 2024 Stormwater Conference. Cover photo by B Crichton, University of Canterbury.

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Woody debris in stream channels is often viewed as an obstacle, but this “obstacle” also plays a vital role in creating diverse aquatic habitats.

Natural processes like landslides and windthrow deliver trees and branches into streams, integrating them into the channel's morphology. This debris can stabilise banks, enhance habitat complexity, and support diverse aquatic life, from fish to invertebrates.

While it can pose challenges in urban areas, where debris may obstruct culverts or damage infrastructure, its ecological benefits in less constrained settings are significant.

Balancing these competing priorities is essential for sustainable stream management.

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Function

Woody debris provides crucial ecological functions that go beyond its appearance as driftwood. When positioned in the stream channel and submerged during low flows, it contributes significantly to aquatic ecosystems in several ways.

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1. Geomorphic Stability

• Debris aligned parallel to stream banks reduces erosion by reinforcing the bank structure.
• Logs spanning the channel can create pools by directing water flow and scouring the streambed downstream. These pools provide critical refuges for fish, particularly during high water velocities.

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2. Habitat Diversity

• Submerged logs create crevices and other microhabitats that support invertebrates such as aquatic insects and freshwater crayfish (kōura) and offer cover for native fish like banded kōkopu and eels. These species rely on the hard surfaces, heterogeneous water velocities and hiding places created by woody debris.
• Kōura, which are nocturnal, often feed on insects and organic matter trapped by logs, highlighting the interplay between debris and the aquatic food web.

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3. Sediment Dynamics

• Debris helps regulate sediment movement and storage, trapping gravels and contributing to the natural evolution of the stream channel that maintains ecological processes.

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Urban Streams

In urban settings, woody debris is often seen as a hazard. Mobile debris can accumulate in culverts, under bridges, or along constrained channels, potentially causing flooding or damage to infrastructure.

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This perception often prompts removal efforts, sometimes at the cost of ecological degradation. Without debris, stream channels lose complexity, leading to reduced habitat availability and simplified flow patterns. In turn, this can result in decreased populations of fish and invertebrates. The removal of woody debris is linked to reduced pool length and depth, which negatively impacts species like banded kōkopu that depend on pools for shelter and food. Additionally, expanded riffle habitats and diminished pool-to-riffle ratios further disrupt the balance of stream ecosystems.

A solution to this issue to incorporate woody debris into stream works and fish passage barrier remediation projects, to create the habitat while removing the mobility of the debris that can cause issues.

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Unconstrained Streams

In contrast, wider, unconstrained streams with ample floodplains can accommodate floods and woody debris without causing significant harm to infrastructure.

Non-functional debris, suspended above the channel, is another consideration. While it doesn’t benefit the stream directly at low flows, it remains part of the debris pool and may move to a functional position during future floods. Strategic management of this debris can maximise its potential ecological benefits while avoiding damage to infrastructure.

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Following heavy rainfall in January 2023, catastrophic floods swept across Auckland, leaving streams, like the Swanson Stream, laden with woody debris. This stream has a partly forested catchment that experienced significant accumulation of wood as a result of landslides and high flows.

In some sections, logs positioned parallel to the bank stabilised the stream edges and created valuable fish habitat without the need for human intervention, creating cover for eels and kokopu. Other debris suspended above the stream channel offered no immediate benefit, but remained a part of the larger debris reservoir that will move during future floods, potentially creating benefits in future.

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A Balanced Approach

Effective management of woody debris in streams requires balancing ecological benefits with the need to protect infrastructure. Rather than defaulting to debris removal, stream managers should evaluate the potential functions of in-stream debris within the context of the specific stream environment.

In more constrained, urban streams, selective removal may be required to prevent blockages and infrastructure damage; however, in wider unconstrained streams, retaining debris can support the stability of the channel and enhance the aquatic habitats.

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Key considerations when using woody debris include:

Geomorphic Context

Understand how woody debris influences the sediment movement and channel form

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Ecological Benefits

Consider the role that this debris plays in providing habitat and supporting aquatic ecosystems

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Flood Dynamics

Allow the river room to move during high flows and minimise risks to nearby assets. Avoiding building on floodplains is a key planning action to reduce asset damage.

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While often considered an obstruction, woody debris is a cornerstone of many of our native healthy freshwater ecosystems. We can support the resilience, stability and biodiversity of our streams despite the challenges by strategically incorporating it into our stream works projects.

Retaining woody debris where feasible is not just a choice for ecology; it is an investment in the long-term health and stability of our freshwater systems.

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