Eco-morphodynamic restoration and the development of aquatic habitat in a gravel bed river.

Natural gravel bed rivers typically contain diverse salmonid habitat: pools and wetlands are refugia for fish in both flood flows and drought events; riffles and the margins of bar-forms often provide a substrate size and hydraulic conditions that are suited to salmonid spawning. In addition, natural fallen wood in the riparian zone, or in-channel, and riparian vegetation, acts as habitat for invertebrates that are used as food by juvenile fish. Human impacted rivers are often straightened, canalized, and disconnected from their floodplain. The desire to prevent in-channel wood, which is often seen as a risk to infrastructure, has also led to many impacted rivers having their riparian zone felled of trees. We describe a river restoration on a straightened, perched, treeless reach of a Scottish gravel bed river with high sediment supply. This human-impacted reach had almost no salmonid habitat for spawning, high or low flow events. The restoration re-aligned the channel, reconnected it to its floodplain, developed wetland habitat, and introduced large wood in the design trapezoidal channel to promote rapid, but controlled, eco-morphodynamic change, natural sediment storage on bar-forms, and a growth in salmonid habitat. We describe a series of eco-morphodynamic models of the interaction between the large wood and river gravels, and the development of geomorphic units over a time scale of two years. The eco-morphodynamic modelling we present is a combination of 2D morphodynamic modelling, habitat-suitability index modelling and geomorphic unit interpretation. The models are compared with detailed topographic surveys and sediment sampling on the reach, and surveyed salmonid redd locations over three spawning seasons. It is shown that the large wood causes rapid geomorphic change, leading to habitat growth, and that this change is relatively predictable when wood is used as the restoration tool.