(for those unfamiliar with what I affectionately call the "Perma-cult": https://en.wikipedia.org/wiki/Bill_Mollison)
>So, it seems as though hydropower is not as clean as we once thought. But that’s isn’t a good reason to be ripping out dams. Hydropower generation often replaces much dirtier sources of energy, such as coal and even natural gas. In addition, dams provide us a wide range of public benefits such as flood control, recreation, and water management.
It sounds like the Permaculture strategy of disfavoring singular large dams (huge concrete construction projects) and favoring smaller "farm dams" (cheap earthen construction) is the way to go. The smaller scale means more surface area:volume, increasing the equilibrium oxygen level in the water and reducing methanogenesis.
Also, the typical Permaculture dam construction practice is to scrape off and conserve all the topsoil from the inside of the dam. This should further reduce methane production (which also represents squandered biomass, ie stored carbon). This technique is practical on the scale of a farm dam, but totally impractical for a huge reservoir.
Farm dams have cost advantages in flood control and water management over monolithic dam projects, since they're near to the big consumers of water (farms). So the largest diversion canals and tunnels never need to be built. Simple gravity-fed water run through plastic pipes serves for irrigation purposes, so it improves on-farm water security.
>groundwater storage may be a ‘cleaner’ alternative. Groundwater recharge basins are regularly maintained to have less organic matter at the bottom, since it can clog the percolation of water into groundwater aquifers beneath. In addition, recharge basins are shallower than man-made reservoirs and periodically become empty during dry periods when less water is available to recharge groundwater. This periodic drying out helps keep soils aerated, whereas man-made reservoirs can stay inundated with water for longer periods of time.
I wonder how "groundwater recharge basins" differ from swales (long on-contour ditches)? They also intercept large quantities of runoff and divert it to recharging groundwater. They also require minimal earth movement, since it's a small surface feature compared to farm dams. Swales can also feed into dams, dramatically increasing their total catchment and infiltration capacity (since before the dam overflows, it floods and soaks the length of the swale).
https://www.youtube.com/watch?v=E79C9tdcgEA
http://permaculturenews.org/category/earthworks-earth-resour...