**1. Introduction**

The majority of the world's rivers are now modified by humans [1–3], and many of those modifications affect the form and character of the river channels themselves. These changes have been well documented through research on: channelisation [4–6], dams [7–10], flow impoundment [11], and levees [12]. However, there is a type of river channel modification that has not been well documented, and this is the diversion of rivers into entirely new channels for part of their length. The lack of research surrounding this type of river channel change is compounded by the lack of formal description or classification of this type of channel modification.

The term 'river diversion' is commonly used to describe various engineered changes to channels and is routinely used to describe diversions of water out of a channel, such as for irrigation or for inter-basin transfers. For example, the Chinese recently completed the South North Water Transfer, the world's largest water diversion [13]. However, this paper is not concerned with this type of water diversion, where a proportion of water is essentially decanted out of a waterway (thus, we do not consider aqueduct systems, the many canal bypass channels that cross much of Europe [14,15], or the irrigation networks that are so common across the world's lowlands). Instead, here we are concerned

with the physical relocation of a river channel to a new position. For this reason, we refer to these as 'river relocations'. This channel change is distinct from diversion of the water, or channelization of the river in position. Thus, our interests relate to engineering and geomorphology more than hydrology. In addition, the relocation of a river has been described in many ways, including: watercourse diversion [16], river realignment [17], channelization [18], water diversion [19], river deviation [20], and river flow control works [21], which are frequently used interchangeably. In our definition of river relocation, river flow is redirected into a new, purpose-built channel, and returned either to the original channel downstream, a new channel, such as a neighbouring watercourse, or a river mouth in the downstream position. In this definition of river relocation, the water within the channel is typically neither used in any consumptive sense, nor stored with the intention of being used or treated [22,23].

Fundamentally, a relocated channel replaces a natural section of a river with a short section of artificial (man-made) channel. The artificial channel is usually different from the natural channel in several ways: it is often shorter and steeper, has different bed and bank material, has no floodplain, and cuts across tributaries. These differences then lead to secondary effects including erosion, flooding, and barriers to fish passage. Thus, relocated channels are not just engineering problems, as they affect every aspect of river geomorphology and ecology.

Many river reaches across the world have been relocated (see Figure 1 for a small selection), but there is little research into the impacts of their relocation, their construction, or subsequent performance. To some extent this is because rivers are often relocated in places where they receive little scrutiny, such as for mining. This paper (a) classifies different types of river relocations, (b) presents case studies to illustrate key engineering, construction, and performance issues that arise from river relocation, (c) reviews the key consequences and challenges of relocating a natural stream, and (d) suggests guidelines for their design and subsequent rehabilitation.

**Figure 1.** Global map of river relocation case studies considered in this paper.
