Large Scale Restoration of Channels--
Most of the methods described above can be used in small stretches of river, and especially in comparatively small streams, to produce local improvements in fish populations. Most of the solutions are also of relatively low cost. The methods described in this section are more complicated and require a greater level of investment. The goal is not only to restore channel diversity, but also to allow the river to develop a lateral expansion zone that may have some of the characters of a true floodplain.

The simplest approach to the restoration of larger stretches of straight channel is to restore the meander (winding or curving) structure. In the case of river straightening, this can be done by breaking the levees that contain the channel, allowing it to return to its original river bed. More frequently, the former bed of the river has disappeared, and new channels have to be dug artificially.

A second approach is to construct a multi-stage channel by setting back the levee on one or both sides of the river so that there is a low flow and a high flow channel.

Levees can be set back even further from the main channel allowing space in which the river can re-establish its meanders naturally. Alternatively, engineering solutions can be used to start the river meandering process. The space between the levees forms a floodplain in which many of the features, such as backwaters, channels, floodplain lakes and swamps can develop or be introduced. The plain can also be used for several purposes including wildlife reserves, reforestation, the grazing of cattle or growing of crops during the dry season.

As far as possible, channel diversity should be increased by reconnection of old channels and side arms that were cut off by the old levee structures close to the river.

Restoration of the Floodplain--
The main objective in reconnecting the floodplain is to restore lateral connectivity, especially to former floodplain water bodies that have become isolated from the channel. Restoring lateral connectivity is important for improving wetland ecosystem function, restoration of aquatic plant communities and waterfowl habitat, and restoring migration pathways for fish and other aquatic animals. This usually involves breaking the levee and actively reconnecting the pool, wetland or channel to the main channel of the river. A new flood control structure (levee) may have to be built behind the reconnected waterbody. Sluice gates or submersible weirs may control flow into and out of the waterbody. The waterbody may be connected at the upstream and downstream end or at the downstream end only. The way in which the waterbody is connected affects its ecological characteristics and the types of vegetation, invertebrates, birds and fish that eventually colonize the restored habitat.

In addition to reconnection of existing but old floodplain features, new features can be made by connecting borrow pits, and sand and gravel extraction sites near to the river. Leaving such sites in a suitable condition for connection to the river can be a condition of the water extraction license issued to the developer.

It is equally important to conserve and restore the floodplain itself, especially where it is (or was) forested. Floodplain forest and mangroves have high social and economic value to people, and also high environmental value to the aquatic organisms that depend on them for habitat. They also contribute to the maintenance of biodiversity of many species of animals including fish, some of which have become highly dependent on forest products such as fruit for feeding. The practice of clearing flooded forest and converting this habitat to rice culture is a serious problem in the Mekong Basin, particularly along the fringes of the Great Lake in Cambodia. This loss of habitat has serious repercussions on biodiversity. Therefore, projects designed to reforest cleared areas are a valuable component of rehabilitation programmes.

In areas where the floodplain has been separated from the main channel for a long time, flooding may not occur because the channel bed has been eroded downwards to a point where the bankfull state is never exceeded. Two solutions have been used to overcome this difficulty:

submersible dams can be installed across the river raising the level at a particular point from which it can be deflected onto the floodplain;

lowering the level of the floodplain by scraping off the surface layers of soil. In either case, it may be necessary to install submersible weirs across the floodplain to direct the water and retain it as long as possible.

It is clear that the total conservation of the river system in a pristine state, or even in its current condition, is unrealistic. The needs for economic and social development will continue to place demands on the river and its landscape that are not wholly compatible with the interests of many of the plant and animal species that inhabit natural floodplains. This means that specific, but limited areas should be set aside as reserves along the river for the conservation of aquatic fauna, so long as the linkages between them are preserved. In other rivers, the idea that the structure of the river and floodplain needs to be restored or protected in specific areas along the river is termed the 'string of beads' approach.

It is, however, not enough to simply create reserve areas. These must be selected according to their significance to species, or groups of species, as breeding, nursery, feeding and refuge sites. In addition, the pathways between the various sites must be kept clear so that fish and other aquatic organisms can move freely between them.

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