Table 1
Key findings on impacts from flow regulation.
| Group | Impact | References |
|---|---|---|
| Riperian vegetation | Flow regulation cause shifts in riparian vegetation composition. | Bejarano et al., 2018; Bipa et al., 2023 |
| Flow regulation reduces the extent and cover of riparian vegetation. | Jansson et al., 2000; Widén et al., 2022 | |
| Prolonged low flows can result in more drought-tolerant species replacing the typical riparian species. | Poff and Zimmerman, 2010 | |
| Encroachment of the inundation sensitive Norway spruce (Picea abies) due to lack of flooding. | SEPA, 2011; Vlahakis, 2023 | |
| Unnatural flooding displace or injure plants, and can drown roots causing oxygen deficit, resulting in lower growth and mortality among plant species. | Johansson and Nilsson, 2002; Nilsson and Svedmark, 2002 | |
| Flow fluctuations related to hydropeaking reduce growth and survival in many, but not all, plant species. | Baladrón et al., 2022 | |
| River sites adjacent to regulated rivers have lower riparian plant diversity and cover than similar sites in unregulated rivers. | Nilsson and Jansson, 1995; Jansson et al., 2000; Nilsson and Svedmark, 2002 | |
| Hydropeaking intensity negatively correlated with summer decomposition rates | Nordström et al., 2025 | |
| Flow regulation can replace the natural riparian zonation with a narrow band of tolerant species, leaving barren soil in the most flow/level variation exposed section. | Nilsson et al., 1991; Bejarano et al., 2018 | |
| Species rich riparian forests and the willow shrub zones are disproportionately negatively affected by flow regulation as these are the zones created by seasonal high flow events that often are lacking in regulated rivers. | Malm Renöfält and Jansson, 2023 | |
| Macro-phytes | Flow is an important determinant for the presence and composition of macrophyte species in the river, both directly and mediated through effects on erosion and sedimentation | Bunn and Arthington, 2002 |
| Excessive flow velocities, as well as drying, cause the reduction or disappearance of macrophytes. | Chambers et al., 1991; Biggs, 1996 | |
| Reduced flow velocities can result in substantial increase in macrophyte occupancy in the river. | Rørslett et al., 1989 | |
| Different species have different flow velocity preferences, for example aquatic mosses preferring higher water velocities, and modified flow velocities can change the composition of macrophytes. | Biggs, 1996; French and Chambers, 1996 | |
| Discharge fluctuation influences the macrophyte community, with some species being more, and other less, tolerant to water level variation. | Walker et al., 1994; Mjelde et al., 2013 | |
| Benthic algae | In reaches subject to hydropeaking, diatom abundance is substantially reduced in the desiccation zone, although re-wetting can quickly increase this abundance. | Bondar-Kunze et al., 2015 |
| Different diatom species react differently to the flow regulation related to hydropower with both seasonal flow regulation and short term hydropeaking influencing diatom composition. | Truchy et al., 2022 | |
| Diatom species richness is lower after compared to before dam construction and in regulated compared to more natural river reaches. | Wu et al., 2009; 2010 | |
| Macro-inverte-brates | Flow regulation has an important impact on the macroinvertebrate community with hydropower production often resulting in altered species richness and diversity downstream of dams. | Mihalicz et al., 2019; Wang et al., 2020 |
| Reduction or other alteration of naturally fast flowing water results in the disappearance or decline of species adapted to this environment as flow interacts with, and forms, substrate conditions that shape the macroinvertebrates community composition. | Englund et al., 1997 | |
| Large variation in discharge associated with hydropeaking can cause flushing and drying of the individual macroinvertebrates present. | Robinson et al., 2004; Gibbins et al., 2007; Bruno et al., 2013 | |
| Hydropeaking causes changes the composition of the macroinvertebrate community. For example, species sensitive to disturbance and emerging insects (as compared to non-insect macroinvertebrates) reduced with hydropeaking intensity and closeness to the dam. | Kennedy et al., 2014; Abernethy et al., 2021 | |
| Flow regulation often causes diversity to decrease while abundance can even increase, when sensitive groups disappear and individuals of tolerant taxa increase in numbers. | Poff and Zimmerman, 2010; Mihalicz et al., 2019; Wang et al., 2020; Jones 2013; Holt et al., 2015 | |
| Reduced flow increases the proportion of fine substrate (sand and silt) and reduces the availability of coarse substrate and with this important habitat and food resources for macroinvertebrates. | Wang et al., 2020 | |
| Mayflies (Ephemeroptera), stoneflies (Plecoptera), caddisflies (Trichoptera), and true bugs (Hemiptera) are particularly negatively affected by hydropower. | Krajenbrink et al., 2019; Mihalicz et al., 2019; Wang et al., 2020 | |
| Mussels | Dams can modify the composition of the mussel community in impacted catchment areas and also generally reduce their abundance, sometimes resulting in local extirpations of certain species. | Layzer et al., 1993; Randklev et al., 2016; Sousa et al., 2020 |
| Mussels are impacted by the change in sediment characteristics imposed by flow regulation, such as the accumulation of fine sediment at reduced flows or the sediment armouring caused by temporary high flows, and can suffer from reduced oxygen levels in impounded low flow reaches. | Layzer et al., 1993; Wegscheider et al., 2019; Sousa et al., 2020). | |
| Lowered water levels can cause general mussel mortality from drying or increased predation pressure. | Sousa et al., 2018 | |
| Individual mussels may be flushed away at high discharge. | Sousa et al., 2020 | |
| Fish | When fast-flowing lotic environments are lost or reduced, so are the associated rheophilic fish species. | Liew et al., 2016 |
| Lower diversity and density of shallow water fish in regulated than unregulated river reaches. No such effects among fish inhabiting deep waters. | Travnichek and Maceina, 1994 | |
| Reaches downstream of dams with reduced discharge can have lower fish abundance, as well as altered species composition, favoring generalists, limnophilic species, or species adapted to more stable environments compared to free flowing reaches. | Mims and Olden, 2013; Benejam et al., 2016 | |
| Hydropeaking and related flow variation frequently causes stranding related mortality and downstream displacement of fish. | Schmutz et al., 2015 | |
| In relation to hydropeaking, even if all life stages can be flushed away at high discharges, eggs and larvae run a particularly high risk of being stranded or freeze at low winter water levels | Young et al., 2011; Bartoň et al., 2023; Pander et al., 2023 | |
| In relation to hydropeaking, stranding rates are typically higher at more extreme down-ramping rates but are also affected by environmental conditions. | Young et al., 2011; Führer et al., 2022 | |
| In relation to hydropeaking, the likelihood of stranding is higher over lower sloping shores, at colder temperatures, in otherwise stable discharge conditions, and in environments with shelters and potholes. | Saltveit et al., 2001; Nagrodski et al., 2012; Auer et al., 2017 | |
| Stranding rates in relation to hydropeaking can vary between day and night; this effect seems to vary both among and within species, and is likely dependent on specific local habitat characteristics. | Young et al., 2011 | |
| In relation to hydropeaking, variable channel morphology (e.g., side-channels) and in-stream structures (larger-sized sediments or large woody debris) can function as flow-shelter, protecting fish from downstream displacement during high flows, but may also constitute ecological traps if fish choose to remain sheltered instead of following the receding water. | Heggenes, 1988; Young et al., 2011; Harby and Noack, 2013; Cousin et al., 2025 | |
| Abrupt flow variation may contribute to reproduction failures through dewatering of spawning habitat, disruption of migratory cues, and disturbance of spawning behavior. | Schmutz et al., 2015; Bartoň et al., 2022; Pander et al., 2023 | |
| Through repeated hydropeaking events accumulation of relatively modest effects can lead to transformation of the affected fish community. | Young et al., 2011 | |
| River reaches subject to hydropeaking was associated with higher habitat overlap and more use of deep pools compared to more natural controls in Cypriniformes fish. | Leite et al., 2025 | |
| Flow regulation can impact the overall fitness of fishes, through direct or indirect effects on growth and survival. As habitat availability is under frequent change under hydropeaking, more mobile species and life-stages are forced into repeated movements in search of suitable habitats, which may result in wasted energy and lost foraging opportunities. For territorial species, like juvenile salmonids, it may also lead to repeated loss of territory access and costs related to competition to regain good territories. In addition, increased mobility can increase exposure rate to predators. | Bätz et al., 2024; Daufresne et al., 2015; Puffer et al., 2015; Schmutz et al., 2015 | |
| Birds | Breeding success, female condition, and breeding timing of pied flycatchers (Ficedula hypoleuca) were lower in regulated rivers compared to natural rivers. | Strasevicius et al., 2013 |
| Construction of small hydropower stations reduced the number of white-throated dippers (Cinclus cinclus) at the sites, but this could be compensated for by the placement of nest-boxes. | Walseng and Jerstad, 2011, 2014 | |
| Water regulation effects on downstream wetlands can impact bird reproduction in riparian habitats. | Kingsford and Auld, 2005; Graf, 2006 | |
| Terrestrial and amphibious animals | Changed flooding regime changed the composition but not the taxonomic richness of terrestrial arthropods. | Ellis et al., 2001 |
| Total abundance of terrestrial invertebrates was lower along regulated rivers compared to unregulated controls. | Jonsson et al., 2013 | |
| Southern dune tiger beetle (Cicindela maritima) and giant riverbank wolf spider (Arctosa cinerea) are threatened due to river flow regulation and artificial armouring of riverbanks, preventing floods that causes habitat loss. | Åström et al., 2017 | |
| River breeding frogs can be negatively affected by flow regulations through both desiccation and flushing. They were more likely to be found in free flowing than in regulated rivers, and early life mortality correlated with flow variability. | Kupferberg et al., 2012 | |
| In semi-aquatic mammals, hydropower production has been reported to affect distribution, fitness, movement, nutrition and reproduction, mainly through its effects on available habitat. | Altanov et al., 2025 | |
| Marsh deer (Blastocerus dichotomus) and hippos (Hippopotamus amphibius) have been reported to decline following damming and associated flooding of habitats. | Andriolo et al., 2013; Bempah et al., 2022 | |
| Catches of caddisflies (after emergence) in light traps correlate with river regulation regime. | Kennedy et al., 2016 | |
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