Restoring multi life-stage, connected aquatic habitat via geomorphic forcing in gravel bed river restoration

Direct anthropogenic disturbance to rivers from flood embankments, canalisation, dam construction, and land management have resulted in an altered hydraulic and sediment regime, producing straightened, single-thread reaches, disconnected from floodplains . These impacts have resulted in a loss of diverse in-channel geomorphology and aquatic habitat. Disconnection from the floodplain has also affected the in-channel and riparian food-web, on which aquatic species rely. We present examples of two Scottish Highland restorations where a straightened, single-thread gravel channel has been allowed to reconnect with its floodplain, and where large wood placements have been used as the primary hydromorphic forcing. One case is presented for a perched channel , the other on an incised channel , both now realigned on their floodplains. Morphodynamic modelling of the effects of large wood placements, and yearly topographic surveys of both sites , are used with hydraulic models to predict growth of complex geomorphology and resultant Atlantic Salmon spawning habitat (a function of Froude number and sediment size) . In addition, we predict a growth of juvenile Atlantic S almon habitat using empirical fry abundance models (validated on Scottish rivers) . We find that fry habitat is located close to spawning habitat (typically fry are observed to utilize habitat close to their redds ) , and both habitat s increase markedly post restoration . The natural development of these rivers has led to multi-thread channels with wetland areas . I t has been an item of discussion at several meetings whether Atlantic Salmon fry utilize the wetland areas- this habitat type being absent in the data-set used in most empirical fry-abundance models. We demonstrate first that the wetland areas provide resilence in drought and flood . Then we couple an invertebrate drift model with the channel hydraulic models to map the invertebrate food supply for fry. We demonstrate that the drift availability correlates very closely with the empirical fry abundance models . B oth predict a lack of fry in the wetlands. However, we show that the wetland areas, and abundance of large wet wood, results in a greater overall drift supply and hence improve the fry habitat in adjacent areas of the channel.