The leading cause of climate change today is the burning of fossil fuels related to energy production. One approach to reducing greenhouse gas emissions, therefore, is to more actively switch to renewable technologies in the production of electricity, and reduce the use of fossil fuels in electricity production. This is the goal of renewable portfolio standard (RPS) legislation, currently in effect in 28 states across the country. In this paper we discuss the potential for water power development as one method to reduce US greenhouse gas emissions. We look at the potential from (1) new small/micro hydropower dams, (2) uprating facilities at existing large hydropower dams, (3) new generating facilities at existing non-hydropower dams, and (4) hydrokinetics. We analyze this potential by type, by state, and by its ability to satisfy current RPS goals. Finally, we consider the cost-effectiveness of developing these sources of water-based energy. We find that while water power will never be the complete answer to emissions-free energy production, a strong case can be made that it can be a useful part of the answer. (C) 2008 Elsevier Ltd. All rights reserved.