I think for large hydro power generation the is a possibility of CO2 emission from large presence of algae. You may need to consult more litterateurs.

That depends on the scale. Large-scale hydropower schemes have significant environmental and social impacts (large amounts of methane production, large impact on local wildlife, ecosystems, displacement of local residents). In contrast to large-scale hydropower systems, small-scale hydropower (SHP) has much lower environmental impact on wildlife or ecosystems, mainly because the majority of SHP plants are run-of-river or implemented in existing water infrastructure. Nevertheless, the reduction in water flow rate between the point of diversion and the tailrace downstream of the powerhouse may in some cases affect spawning and fish migration, and therefore appropriate fish-pass should be implemented. Finally, although not “really clean”, even large scale hydro power generation is certainly cleaner than nuclear or fossil-fuelled generation.