Absolutely! Trees are Nature's helpers in the task of reducing geomorphological instability and slow down erosion phenomena, and this works for river basins too.

Most of the tree and shrub species that are typical of river banks and levees (riparian vegetation) do absorbe a lot of water (they are called hygrophyle species) and their roots strengthen the shores. In particular in flash floods(http://en.wikipedia.org/wiki/Flash_flood) trees, and vegetation in general, can have a very important role:

• They hold the shores and keep the river bed in place and in shape. This helps the water flush away reducing the length of the flood.
• They serve as a filter for floating objects (such as rocks, trees and debries), that are a major threat during floods because they crush on house and infrastructure or they createlidsunder bridges. When pressure builds up, these lids either crush creating a second flood wave or they tear the bridge down.
• They slow down the water reducing flood energy and therefore the threat for people and buildings.

The best way to go, at least for flash floods, is to create a Retention basin (not sure of the exact term in english) with a dense vegetation canopy made of autoctonous riparian trees upstream the areas you want to protect. You will not only have the best flood protection system you can get but you will also create a highly valuable natural area, very important both for the riverine ecosystem and for the whole valley. These areas can even be used for recreation and tourism, if a good flood alert system is in place.

More in general, maintaining a vegetation buffer of at least 20m along river banks is very effective in controling floods.

Some of the most common hygrophile tree species are being used to control floods, both in ponds and on the levees, are:

• Poplars (Poupulus Nigra & Alba)
  

• Ash (mainly Fraxinus Angustifolia or Fraxinus Excelsior)
  

• Willows (genus Salix, there are hundreds of different species to pick). By far the most used, they have a fantastic root system and can live with their root underwater for months!
  

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• Elms (Ulmus )
  

Among the shrubs the Elder (Sambucus Nigra) is very resistant and very important ecologically.

There are several critical points to be considered in order to use vegetation to control floods:

• Vegetation has to be maintained. Dead trees have to be cleared, tree stability especially on the river front has to be monitored
• Forestation is a critical process that sometimes can fail for no apparent reason. Good knowledge of the local ecosystem and vegetation association is required
• A good variety of species has to be used in order to ensure the regeneration of the plants and the ecological quality of the area. Remember: a healthy environment is the first buffer against any natural hazard, and especially in river basins.

Sì, le loro radici possono essere usate per tenere insieme i terreni di terra nelle zone basse.

Sono ancora più utili sui pendii delle colline per prevenire le frane e l'erosione del suolo. Aiutano a ridurre i sedimenti che rendono i letti dei fiumi e dei laghi a valle meno profondi e più inclini alle inondazioni. Ecco perché dopo un incendio nella foresta, una delle prime cose che il servizio forestale fa è costruire dighe di tronchi per controllare il flusso dell'acqua e l'erosione del suolo.