Adverse ecological effects of small hydropower stations in the Czech Republic: 1. Bypass plants

Adverse effects of 23 bypass- or mill stream-type small hydropower stations (SHPSs) have been studied by traditional benthological and ichthyological techniques. Four locations, if available, were distinguished for each SHPS: the natural flow sections (NFS) above and below the SHPS area, the mill stream or diversion channel bringing water to the turbine, and the reduced discharge section (RDS) of the former stream course. Most SHPSs took a high percentage of the stream discharge (turbine intake capacity was usually bigger than average yearly discharge of the stream at a given point). Despite this, the decrease in the stream-wetted area was relatively low (usually 0–30%). The changes of benthic communities did not show any distinct trend explicable by the SHPS operation. Mechanical damage of fish by turbines was found rather rarely with SHPSs. The diverting weir (which usually distributes the discharge between the RDS and the mill stream), in combination with the abstraction of water for SHPSs, was an important migration barrier for resident fish in 30% of SHPSs. Water abstraction causes succession from large-bodied fish species (adult brown trout, chub, dace, grayling, etc.) towards small-bodied fish (trout fry, minnow, bullhead, stone loach, gudgeon). Average individual weight and fish biomass decreased four times in RDSs. Fish biomass losses exhibit significant negative relationships to the ratio of turbine intake capacity and stream discharge. Several mitigation measures are suggested. © 1997 John Wiley & Sons, Ltd.