Managing the effects of riparian vegetation on flooding

~ The major effect of removing riparian vegetation and wood from streams has been the changes in channel form (widening, deepening and straightening) that have occurred. It is important to consider that we are returning vegetation to a channel system that now has a much larger flow capacity. ~ The major hydrological effect of returning vegetation to streams is via its influence on roughness and flow resistance. ~ Revegetating riparian zones, or adding large wood to stream channels, increases the stage of floods at a cross-section and reach scale, although in many cases the effects are likely to be small. The effect will be greatest where the vegetation is planted across the full width of a floodplain. ~ Adding or removing large wood (snags) in streams has little effect on the height and duration of large floods. ~ At catchment scale, the cumulative effect of riparian revegetation is to increase flood stage and duration in headwater streams (where flooding is usually not a problem anyway), but decrease flood stage in larger streams, further downstream, where flooding may in the past have been a problem (local-scale versus network-scale effects). ~ Although the effect of riparian vegetation on flooding is modest in comparison to the effects of dams and river regulation, it should be considered in planning major revegetation works. The effect is largely positive for downstream catchments, where riparian vegetation will reduce the depth of flooding. The decreased flow depth comes at the cost of slightly longer flood durations. ~ Riparian revegetation should be seen as a catchment scale tool that can have a beneficial effect on flooding in lowland areas. Whilst flow regulation and landuse change affect the amount of water available in floods (magnitude and frequency), riparian vegetation affects the velocity of the flood wave delivered to the stream. All of these interacting aspects need to be considered together.