EREN :Hydro Energy
http://www.doe.gov/html/eren/13.html
It is a subset of the Energy Science and Technology Database (EDB).You can search articles about microhydro here.
Impulse Turbines
http://toffee.snt.utwente.nl/~wot/water/impulse.htm
Its subjects:basic principle of impulse turbines ,the cross-flow turbine ,the pelton wheel,it shows you many details about them.

Alternative Energy Systems Inc.
http://www.altenergysys.com
provides solar, wind and micro-hydro energy system design.
Evans Engineering
http://www.microhydro.com
maker of small water turbines that generate power, especially suitable for remote or rural needs. Site has informative overview of waterpower basics.This site is good in microhydro i think.
Small Hydropower Systems
http://solstice.crest.org/renewables/re-kiosk/hydro/index.shtml
It includes overview,thoery,applications,case studies and environment of small hydropower systems.