**9. Conclusions**

This paper introduces the characteristics and challenges of a poorly described class of human impact on streams: river relocation channels. The term 'river diversion' has typically been ambiguous, often used for several types of engineering approaches. We sugges<sup>t</sup> that 'river relocation' more accurately describes the permanent or temporary relocation of a river channel into a new course. The new course can be lined or unlined, and cut into bedrock or alluvium. A river relocation channel that does not correctly mimic natural channel characteristics can have a profound impact on the overall performance and success of the river relocation.

Traditionally, relocated channels were designed to carry large floods, but at a minimum construction cost. This means that river relocation channels were typically constructed as short, narrow, and steep as possible. The common result is excessive erosion or sedimentation in the new channel, and hanging tributaries. This has secondary consequences, including headward erosion into the upstream reach, disruption to the sediment flow regime into the downstream reach, loss of vegetation, poor water quality, loss of biodiversity, and in some cases, river channel collapse.

Rivers will continue to be relocated for infrastructure projects, flood protection, and mining operations. From an engineering perspective, it is now increasingly important to be able to design and build a permanent river relocation channel that, for the least cost, eventually has the morphology, vegetation, and dynamics of up and downstream reaches of stream. Ideally, relocated river channels should eventually be indistinguishable from the natural counterparts up and downstream. This will only be possible where managers have good understanding of the geomorphology of the river system, and the mechanisms that control major channel adjustment, such as flooding, vegetation, and sediment supply. Overall, the presence of natural features and geomorphic stability will facilitate the long-term recovery of the river relocation. Improved understanding of these natural features will allow for the identification of a natural state and projected behaviour over time. Recent analyses have identified thresholds of stream power for certain river types that have led to improved design. Poor-performing relocated channels can be a major long-term liability to companies, and once relinquished, to governments. Finally, we need to remember that river relocations will be there for millennia and need to be designed accordingly.

**Author Contributions:** A.F. conceived and conducted the literature review; I.D.R. supervised the project; A.F. and I.D.R. and R.H. wrote the paper; I.D.R. and R.H. provided ongoing reviews to paper drafts.

**Acknowledgments:** The authors would like to thank BHP-Billiton and the George Lansell Memorial Scholarship (the University of Melbourne) for financial assistance for this project. The authors wish to acknowledge the cartographic services of Chandra Jayasuriya for the production of Figure 5.

**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
