Piedmont Hydro Technologies, LLC considers hydropower to be a renewable source of energy. No fuel, including water, is consumed to make hydropower. All hydro requires is water flowing through a turbine to make electricity; only energy is removed from the water (that, and trash from the river that could clog up or damage the turbines). Water exits a hydropower facility slower and cleaner, but otherwise unaltered.  There are no significant changes to temperature, pH, or dissolved oxygen due to water passing through the turbine.

Energy that would have otherwise been wasted by tumbling down the riverbed is instead turned into clean electric power; electricity produced with zero emissions. That power would otherwise have to have been produced by other methods. In the U.S., that likely means burning fossil fuels, a process that produces environmentally harmful by-products. (Fossil fuels provide about 81% of all the energy  consumed in the United States.) (Source: http://www.eia.gov/)

Reduction of Greenhouse Gases

For example, a 1 MW (1000 kW) hydropower plant, operating at a 50% plant factor (a reasonable assumption, although many plants, if sized correctly, have a much higher plant factor), would produce 4.38 GWh (4,380,000 kWh) of green energy in a year. This is enough energy to power roughly 400 average U.S. homes.
(Source: http://www.eia.gov/)

In order to produce this much electricity, 3,547,800 pounds of coal would have to be burned. Burning this much coal produces the following greenhouse gases and pollutants:

CO2 - 9,110,400 pounds of CO2 per year
NOx - 11,388 pounds of NOx per year
SO2 - 28470 pounds of SO2 per year

Or, this annual reduction in CO2 emissions is equivalent to 700,800 trees planted, 11,095,295 miles not driven, or 269,985 days not driven in a year.

(Source: http://www.ncgreenpower.com/elements/pdfs/Calculator%20Methodology.pdf)

Hydro in the US

Unlike some parts of the world, the United States cannot meet its entire energy demand with hydropower. The US does not have enough suitable hydro resources, and we choose not to use some resources for other environmental reasons. However, PHT believes that hydropower is an important part of the energy mix. Most of PHTs projects are for upgrades to existing plants, or new facilities at existing dams.  There are many, many thousands of existing dams in the US that are either unpowered, or need aging equipment replaced to bring the facility back online, or to make more power by using modern technology to more efficiently use the resource already in place.

In the United States, hydropower produces more renewable electricity than all other renewable sources combined, including wind and solar. (Source: http://www.eia.gov/)