The surge in local and state mandates to derive ever-increasing amounts of energy from “clean” sources appears to be spurring a return to the nation’s rivers and other waterways for power.

In the short term, this translates into upgrades and expansions of existing hydroelectric plants, or adding turbines to dams that were not designed to produce electricity with turbines.

From The Associated Press on Monday:

American Municipal Power-Ohio is a nonprofit wholesale power supplier for 123 municipal systems in Ohio, Kentucky, Pennsylvania, Virginia, West Virginia and Michigan. It already owns a hydro plant on the Ohio River and is involved in developing five more. … There are 20 navigation and flood control dams on the Ohio River along its 981 miles from Pittsburgh to Cairo, Ill. Hydro plants at six of the dams already are producing electricity, with a generating capacity of more than 300 megawatts; four more that have been licensed would double that perhaps be on line in 2013.

In October, USA Today called the uptick in hydropower licensing and construction, “the biggest hydropower expansion since the 1980s. Utilities are proposing more than 70 projects that would boost U.S. hydroelectric capacity by at least 11,000 megawatts, or 11 percent, over the next decade.”

Currently, conventional hydropower accounts for about 7 percent of electricity generation in the United States, according to the Energy Information Administration.

Still, the “clean” credentials of hydropower have long been in dispute. While the particulate and greenhouse gas emissions of a typical hydropower facility tend to be far lower than those of comparably sized fossil fuel-based plants, there are demonstrable environmental trade-offs — including effects on fish and other wildlife — that have put power companies and conservationists at odds.

The release of methane, too — a potent greenhouse gas — when areas behind a dam are flooded and left to decay, has also tarnished hydropower’s image.

But other methods for harnessing energy from rivers are continually being explored and developed — including riverbed turbines like those recently tested in the East River, often called kinetic hydropower. Here, the ebb and flow of a rivers tides spin underwater turbines in much the same way wind turns the giant rotors on land-based turbines.

A separate and potentially more invasive form of tidal energy capture involves sequestering water in lagoons and other basins during high tides, and then distilling it back through a system of turbines as the tide recedes.

Earlier this year, Britain started a two-year feasibility study into both technologies for potential deployment at Bristol Channel.

Smaller “run-of-river” projects, which make use of gravity by channeling the natural downward flow of water from highlands to lower lying areas without the need for dams or reservoirs, are also gaining renewed interest.

Two such projects, costing roughly $3.4 billion, have been proposed in Canada. General Electric’s financial-services unit, along with Plutonic Power of Vancouver, submitted bids to help build two facilities last week.

Another technology appearing on the horizon, which has the potential to take advantage of much slower currents, involves a system of cylinders inspired by the way fish swim. From Britain’s Guardian newspaper on Monday:

As water flows past, the cylinder creates vortices, which push and pull the cylinder up and down. The mechanical energy in the vibrations is then converted into electricity. Cylinders arranged over a cubic metre of the sea or river bed in a flow of three knots can produce 51 watts. This is more efficient than similar-sized turbines or wave generators, and the amount of power produced can increase sharply if the flow is faster or if more cylinders are added. A “field” of cylinders built on the sea bed over a 1km by 1.5km area, and the height of a two-storey house, with a flow of just three knots, could generate enough power for around 100,000 homes. Just a few of the cylinders, stacked in a short ladder, could power an anchored ship or a lighthouse.

Systems could be sited on river beds or suspended in the ocean. The scientists behind the technology, which has been developed in research funded by the U.S. government, say [it] … would require up to 50 times less ocean acreage than wave power generation.

Questions of scalability for any of these dam alternatives, of course — as well as the environmental trade-offs — remain to be worked out.