Stream restoration
[2][3] Form-based restoration techniques include deflectors; cross-vanes; weirs, step-pools and other grade-control structures; engineered log jams; bank stabilization methods and other channel-reconfiguration efforts.
[4] Despite the significant number of stream-restoration projects worldwide, the effectiveness of stream restoration remains poorly quantified, partly due to insufficient monitoring.
[13] In the past decades, stream restoration has emerged as a significant discipline in the field of water-resources management, due to the degradation of many aquatic and riparian ecosystems related to human activities.
[3] Targeted outcomes can include improved water quality, enhanced fish habitat and abundance, as well as increased bank and channel stability.
These alterations affect the dissipation of energy through a channel, which impacts flow velocity and turbulence, water-surface elevations, sediment transport, and scour, among other characteristics.
They can limit bank erosion and generate varying flow conditions in terms of depth and velocity, which can positively impact fish habitat.
[28] Because of channelization and removal of beaver dams and woody debris, many streams lack the hydraulic complexity that is necessary to maintain bank stabilization and healthy aquatic habitats.
Depending on the scale of a project, a channel's cross-section can be modified, and meanders can be constructed through earthworks to achieve the target stream morphology.
[33][34] This method involves a classification of the stream to be restored based on parameters such as channel pattern and geometry, topography, slope, and bed material.
The method relies on the construction of the desired morphology, and its stabilization with natural materials such as boulders and vegetation to limit erosion and channel mobility.
Common criticisms are that the scale at which form-based restoration is often much smaller than the spatial and temporal scales of the processes that cause the observed problems and that the target state is frequently influenced by the social conception of what a stream should look like and does not necessarily take into account the stream's geomorphological context (e.g., meandering rivers tend to be viewed as more "natural" and more beautiful, whereas local conditions sometimes favour other patterns such as braided rivers).
[15][36][37] The NCD method has been criticized for its improper application in the Washington, D.C. area to small-order, interior-forested, upper-headwater streams and wetlands, leading to loss of natural forest ecosystems.
[2] Similarly, removing dams and grade control structures can restore water and sediment fluxes and result in more diversified habitats, although impacts on fish communities can be difficult to assess.
[3] In streams where existing infrastructures cannot be removed or modified, it is also possible to optimize sediment and water management in order to maximize connectivity and achieve flow patterns that ensure minimum ecosystem requirements.
Although this concept is often restricted to the lateral mobility of streams (related to bank erosion), some systems also integrate the space necessary for floods of various return periods.
[15] This situation appears to result from limited data on the restored streams' biophysical and geochemical contexts, to insufficient post-monitoring work and to the varying metrics used to evaluate project effectiveness.
[4][23] In general, project effectiveness has been found to be dependent on selection of an appropriate restoration method considering the nature, cause and scale of the degradation problem.
As such, reach-scale projects generally fail at restoring conditions whose root cause lies at the watershed scale, such as water quality issues.
[55] Although often viewed as a challenge, public involvement is generally considered to be a positive factor for the long-term success of stream restoration projects.
[43][44] Although this evolution is generally viewed positively by the scientific community, a concern expressed by some is that it could lead to less flexibility and less room for innovation in a field that is still in development.
