WFD requires that impounded river reaches achieve Good Ecological Status (GES) based on fish community, macbenefitsnvertebrates, macrophytes and phytobenthos. The fish community Biological Quality Element (BQE) is almost always the limiting factor in impounded rivers, directly linked to dissolved oxygen, temperature, fish passage and habitat quality in the impounded pool.
Water quality management for impounded river reaches, navigation weirs and low-head dams. Stratification, nutrient control, DO management and WFD ecological compliance.
Thermal and oxygen stratification engineering for weir pools and navigation impoundments. Aeration system design, cascade weir enhancement and WFD DO compliance.
Phosphorus internal loading, algal bloom management and nutrient cycling engineering for impounded river reaches and weir pools. WFD Good Ecological Status compliance pathway.
WFD and EA consent compliance for fish ecology at hydropower headponds. DO, temperature, fish pass assessment and minimum residual flow management.
NATFISH / Environment Agency Fish Index (EFI+): The UK fish BQE assessment compares observed fish community (species richness, EQR-weighted abundance of reference species) against expected community under natural conditions. Impounded reaches typically score Moderate or Poor status because: (1) rheophilic species (chub, barbel, dace) are absent or at low densities; (2) lentic-tolerant species (bream, roach, carp) dominate; (3) passage-dependent species (salmon, sea trout, eel) absent if weir has no fish pass. EQR < 0.6 = Moderate; < 0.4 = Poor.
| Fish Species / Group | Preferred Habitat | Response to Impoundment | Remediation Required |
|---|---|---|---|
| Atlantic salmon (Salmo salar) | Fast-flowing, clean gravel | Blocked by weir; absent without fish pass | Fish pass + downstream DO >7 mg/L |
| Brown / sea trout (S. trutta) | Gravel riffles, cool water | Blocked; hypolimnion too warm in summer | Fish pass + temperature management |
| Barbel (Barbus barbus) | Fast gravel run, high DO | Declines; replaced by bream in slow pool | DO > 7 mg/L; riffle habitat restoration |
| Chub (Squalius cephalus) | Moderate flow, submerged roots | Moderate; can tolerate pool but prefers flow | Flow restoration; woody debris habitat |
| Bream (Abramis brama) | Slow-flow, turbid, deep pools | Increases; can dominate; tolerates DO 3 mg/L | Control eutrophication; increase flow |
| European eel (Anguilla anguilla) | All habitats; migratory | Blocked by weir; critically endangered | Eel pass (WFD, Eel Regulations 2009) |
| Macbenefitsnvertebrates (EPT taxa) | Clean riffles, high DO | Replaced by chironomids, oligochaetes in pool | DO > 7; restore lotic habitat patches |
Annual single-pass electrofishing (June–September) to 150–200 m reach above and below weir using CEN EN 14011 standard. Calculate EFI+ (Fish Index) using EA online tool. Record species, life stage, density (fish/100 m). Compare to WFD reference conditions for the river typology. If fish BQE EQR < 0.6: Good Ecological Status not being achieved.
Review weir fish pass against EA Fish Pass Manual (2019): type (pool-and-weir, rock ramp, Larinier, bypass channel), attraction flow (≥ 5% of Q95), pool dimensions, maximum velocity (< 1.5 m/s for salmon; < 0.9 m/s for coarse fish). Commission EA-approved hydraulic model of weir and fish pass flows. Upgrade or construct fish pass to comply with Eel Regulations 2009 and EA requirements.
Install aeration to maintain DO > 7 mg/L (salmonid) or > 5 mg/L (cyprinid) at the mid-pool monitoring point. Size aerator for worst-case summer DO deficit (depth profile survey at 14-day intervals June–August). Operate aeration automatically on DO threshold: activate at DO < 5 mg/L, deactivate at DO > 8 mg/L to minimise power consumption.
Introduce instream habitat diversity to the impounded pool: large woody debris (LWD) as fish shelter; stone berms to create localised faster-flow riffle patches; gravel introduction (D50 20–40 mm) on sheltered margins to provide rheophilic macbenefitsnvertebrate habitat. These measures partially restore lotic character and increase EFI+ score.
Under the Eel Regulations 2009, all obstruction to eel migration requires authorised passage or exemption. Install eel pass (brush pass, tile pass or bristle pass) on or alongside weir face. Monitor eel passage annually (downstream fyke nets in October for silver eel downstream migration; upstream traps in May–July for elvers). Report to EA eel data portal.
Include fish ecology management in the River Basin Management Plan (RBMP) Measures Programme submission to EA. Demonstrate year-on-year EFI+ improvement. If Good Ecological Status not achievable due to hydromorphological alterations (weir) under WFD Article 4(3): apply for Heavily Modified Water Body (HMWB) designation with Good Ecological Potential (GEP) target instead — requires demonstration that all feasible measures are implemented.
Fish pass design, minimum residual flows and downstream DO standards for run-of-river hydropower schemes.
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Read MoreShare your EFI+ score, fish pass assessment and EA RBMP objective. We will design an integrated fish ecology improvement programme addressing DO, habitat and passage.
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