It's 19% of worldwide electricity generation.
http://ga.water.usgs.gov/edu/wuhy.html
Furthermore, hydropower may be throttled up and down, such that the combination of hydro + wind and solar can hit penetrations near 50% before more energy storage is needed.
Also, wind power is less expensive on a kwhr delivered basis than coal. http://www.stanford.edu/group/efmh/jacobson/Ia.html
Most of the cost-effective places to build hydroelectric dams are already utilized. Also, the lakes created behind the dams displace people.
By choosing a more expensive form of power generation, the economy is impacted. This is an often-overlooked external cost of renewables.