For much of the twentieth century, engineering in the river basins of Europe and North America was directed at straightening, deepening and confining the watercourses. Meanders were cut off, banks were strengthened with concrete, and wetlands that absorbed seasonal floods were drained for farmland and housing. The rivers that resulted were easier to shipping, more predictable for farmers and, for a time, less likely to overflow. They also lost a great many of their fish, birds and insects, and their ability to clean their own water declined sharply.

Since the 1990s a contrary movement has grown, sometimes called river restoration and sometimes, less formally, rewilding of rivers. Its premise is that a river works best when it has the space to do its natural work: to spread across a floodplain, to carry and deposit gravel, to swing between channels. Restoration projects set out to give back some of that space, and, in doing so, they often reduce downstream flood risk rather than increasing it.

The river Kissimmee in Florida illustrates this approach at a large scale. In the 1960s, the winding 160-kilometre river was reshaped into a 90-kilometre straight canal, with the stated aim of improving drainage. The wetlands that had lined the original channel dried out, fish populations collapsed and water quality in the downstream Lake Okeechobee deteriorated. Starting in 1992, and continuing for nearly three decades, sections of the canal were filled in and the original meanders were restored. The project cost hundreds of millions of dollars, and its results, measured by populations of wading birds and by water quality, have been strongly positive. The United States Army Corps of Engineers, which had originally built the canal, has since described the restoration as among the most successful of its kind.

Smaller-scale work in Europe has produced similar results. On the river Drau in Austria, embankments were removed in selected sections during the early 2000s, allowing the river to re-establish gravel bars and islands. Trout and other fish populations recovered within a few years, and the area became valuable for kayaking and tourism. Hydrologist Dr. Ivo Petric has shown that the restored sections now absorb peak flows far more effectively than the confined sections, reducing the risk of downstream flooding in neighbouring villages.

Not all restoration projects succeed. The dynamics of rivers are complicated, and a restoration that fails to account for sediment inputs, land-use change or the stability of the adjacent terrain can create new problems. A well-known example comes from a project in the English Midlands in the early 2000s, where a straightened stream was reshaped into meanders. Within a year, a sudden storm overwhelmed the new design, cutting a fresh channel through a field and leaving the intended meanders stranded. The project had not allowed enough space for the river's natural exuberance. Engineers working on subsequent restorations have since treated such cases as instructive, emphasising that meanders cannot simply be drawn on a map; they have to be given room to move.

Public attitudes can also be complicated. A river that has been confined for a century acquires the status of a known neighbour. Residents may oppose the removal of a concrete bank they have seen all their lives, even when hydrological models suggest that the bank makes flooding worse. Dr. Petric's team has found that local support rises sharply when restoration projects are accompanied by usable amenities - a path along the riverbank, a viewing platform, a gentle beach for children - that translate the ecological gains into everyday experience.

River restoration is often presented as a return to nature, but the phrase is slightly misleading. The landscapes being restored were themselves shaped by centuries of human use before the confining engineering began. Truly wild rivers, unaffected by agriculture or settlement, are now rare in most parts of Europe. What restoration typically aims at is not an original state but a more resilient equilibrium, in which the river moves more freely, absorbs more of its own disturbances and provides habitat for more of its former inhabitants. The most striking finding of the past thirty years is how quickly, once given room, rivers tend to reassemble such an equilibrium on their own.