I’ve worked on about 30 different run of river hydro electric projects over the last 15 years. Yes there is some environmental impact but the ones I have worked on have had a team of biologists and years of environmental studies and they would never result in a dead fish any more often that the river would have naturally by itself. If any habitat is destroyed they have to create new habitat such as spawning channels. They probably spend 10%-20% of the entire budget on environmental. These run of river plants don’t flood more than 1000m^2 area, use a weir instead of a dam, leave water in all sections of the river channel, and don’t cause unnatural changes in the river level at a rate greater than 1” per hour which is a very conservative rate compared to what the rivers of this size and nature naturally do by themselves when it rains or melts.
I know you said Dams and not weirs and not run of river plants, and big dams have their issues, but not on the same scale as the oil industry.
I have studies hydropower effects at my university programme, and a vocational aquaculture program, and as you say, there are definitely ways to lower the impact. In Sweden, all the major rivers except for two have large scale dams installed. Most of that was built between 1900 and 1950 and not much after 1970. Not much consideration for biodiversity or environmental impact was taken into account. At some point hydropower supplied 95% of Swedish electricity. Important during the country’s industrialisation.
There is a nice reference list, including many English language reports at the end of this report from the Swedish Agency of Marine and Water Management. [1]
I think they summarise it nicely by saying “Hydropower is one of the biggest impacts on the ecological status of a river. At the same time it is one of our most important energy sources.” [in Sweden]
I'm curious how well this applies to the short-term effects of filling the Grand Ethiopian Renaissance Dam? [1]
As I understand it a lot of the downstream ecological effects rely entirely on the fill rate the Ethiopians choose (3 years, 5 years, 10 years, 15 years, etc). A fast fill rate (5 years or less) could have significant short to medium term effects on the arable land downstream in Sudan and especially Egypt.
The dam is oversized for its power output (to ensure it can meet peak output demands during the wet seasons) and will operate as a fish trap, cutting the number of fish able to head downstream (and ultimately to the sea / mouth of the Nile) by a significant amount.
I know you said Dams and not weirs and not run of river plants, and big dams have their issues, but not on the same scale as the oil industry.