Hydropower

As of 2010, 5.8% of power supply in the United States, projected 5.9% by 2035. Largest renewable source EEI, 40
In-conduit hydropower: turbines inside of the water pipes used to source water for a city– minimize environmental impact. Portland, Oregon uses this (3.5' turbines) Hawken, 27

How does it work?

Water flow moves a primary mover such as a turbine or water wheel
Run-of-the-river does not interfere with the flow of water; dammed hydropower involves blocking the water flow to constrict flow points (increasing pressure) and/or to build up hydrostatic head for more convenient timing
Flow can be moderated with sluice gates

One of the oldest forms of energy storage is behind a dam– including using the energy of water flow at times where capacity is higher than demand to pump water back up into a reservoir

Hydropower is subject to disruption by drought and flooding (Kainji Dam in Nigeria is a good example, missing production targets by up to 70%)[Brown, 121][Brown]
Traditional dams disrupt river flow, create new reservoirs, and displace people– drastically impacting ecosystems and communities in hard-to-measure ways [Brown, 121][Brown]
Dam building is not climate neutral. Large dams use large amounts of concrete, which has a large carbon footprint, and methane from decomposing plant material in new reservoirs is also nontrivial. Some studies show that lifetime emissions from these factors can exceed lifetime emissions from some fossil fuel-burning plants [Brown, 122][Brown]
Large dams have a lot of negative environmental impact, and take a great deal of up-front capital. Smaller hydro projects such as run-of-the-river plants appear to be the future of hydropower [Brown, 132][Brown]

[aggarwal]: https://www.sciencedirect.com/science/article/pii/S1040619013001917 "Aggarwal, Sonia and Harvey, Hal. 'Rethinking Energy Policy to Deliver a Clean Energy Future.' Energy Innovation, 2013."