The use of in‐stream wood is one of the most commonly employed natural flood management (NFM) techniques. The effectiveness of NFM wood structures in reducing flood risks (i.e., their “primary” effect) has been relatively well documented. However, their additional or “secondary” effects on other natural processes have not been fully evaluated. These secondary effects can be inferred by reviewing previous studies that scrutinized natural wood accumulations or artificial wood structures constructed for purposes other than NFM. The degree of contact with base flows and the stream bed provides a broad classification of NFM wood structures. Having considered the similarities between NFM wood structures and other in‐stream wood types, it is suggested that the following geomorphic effects are common to all types of NFM wood structures: pool formation; accumulation of clasts immediately upstream; buffering against stream bed coarsening; and bank erosion, causing channel widening and the formation of floodplain channels. These geomorphic changes contribute to stream bed heterogeneity, potentially creating new niches for aquatic organisms such as macroinvertebrates. Moreover, NFM wood structures may retain benthic organisms accidentally flushed away during flood events, serving as sources of colonists during phases of recovery. Geomorphic changes induced by NFM wood structures may also contribute to spatial variation in rates of biogeochemical processing. Accumulation of fine sediments in some areas may provide more surfaces for the attachment of organic matter and micro‐organisms, hence increasing benthic metabolic rates. Stream bed scouring in other areas may lead to sediment instability, suppressing the growth of micro‐organisms and benthic metabolic rates. This article is categorized under: Water and Life > Conservation, Management, and Awareness Science of Water > Water Extremes Engineering Water > Sustainable Engineering of Water

One popular Natural Flood Management (NFM) technique involves the construction of channel-spanning woody dams in low-order streams that maintain a clearance height above base flows. While extensive research has examined the geomorphic effects of natural wood accumulations, little has been documented of NFM woody dams, which are structurally distinct from natural accumulations and may produce different patterns of erosion and deposition. This consideration is crucial because changes in physical habitat characteristics have implications for flood management objectives as well as ecosystem structure and functioning. This study adopted a Before-After Control-Impact (BACI) design to assess the geomorphic effects of NFM woody dams in the upper River Cover catchment, United Kingdom. One baseline survey prior to and three monitoring surveys up to 2 years following dam construction were conducted. Structure-from-Motion (SfM) photogrammetry was employed to capture topographic change, supplemented by bathymetric surveys.Results highlight that where the dams remained secure in place, they promoted instream habitat diversity by creating underflow pools. Sediment storage was observed only where the dams had clearance heights <0.3 m from the stream bed.Additionally, the dams commonly led to bank erosion, likely enhanced by inherent bank instability in the study catchment as observed along the control reaches. However, volumes of sediments eroded and deposited were not statistically different between the control and woody dam reaches. Longer monitoring is required to determine whether these effects on channel morphology and habitat diversity will persist, amplify, or diminish over time, and to better understand the longevity of NFM woody dams.