I've an old stream culvet which runs directly under my garden

... but I bet the amount of electricity from it would still be pointlessly tiny.
There is a formula for converting water power to electrical, it goes something like; Flow (in Litres per second), multiplied by Height/Head (in metres), multiplied by Gravity (about 9.6) = Watts. Take off about 20% to allow for conversion losses. Navitron.org sell water turbines if you don't fancy making one.
The result may not be enough to directly power anything, but trickle charging 12v batteries for emergency use can be good. There was a guy stateside who owned a log cabin up in the wilds. He found a tiny spring and ran an old hose pipe from it to a bicycle wheel with plastic spoon heads glued to the rim. This ran an alternator and charged up his battery during the week. Now his cabin has lights at weekends.
I have three little 10W solar panels in the window that each keep a 90Ah deep cycle leisure battery charged.
When there's a power cut, we can run SMD LED lights, watch a portable TV, record a show, watch a DVD. The main batteries can run an intelligent charger for AA and AAA batteries for torches. We can also charge mobiles. The mobiles have 5,337 mp3 tracks and can be boosted by AA powered speakers. 10 AA rechargeable batteries in a holder give 12v. This can be used to power the router for broadband. A DC-DC converter running off another pack will run a Raspberry Pi using the TV for output.
We bought a 12v camping shower. Four days later our hot water died. Two thirds cold water to one third boiling water from the kettle gives a nice hot shower from two clean builder's buckets. My wife said it's one of the best showers she has ever had. Four long showers and the battery voltage only dropped by 0.4v.
At Christmas, all the lights run from rechargeable AAs, although, unless there's a power cut, we usually charge them from the mains. If the power cut is in Winter, we have the gas stove running from two large bottles (no mains gas here) and the woodburner keeps us from freezing.
No, I'm not a Survivalist, just that we live on the top of a very windy Cornish hill!