Then there's the energy embodied in all those square yards of concrete used to build the dam. I don't know exactly how much energy that is, only that it's got to be a huge amount.

Dammed if we do, dammed if we don't..

I recall running across a stat that making concrete is responsible for 7% of greenhouse gas emissions, but I can't find it right now.  

Also, whatever the absolute value of post-construction emissions are, it's worth keeping in mind that methane is 20 times more potent in trapping heat than an equivalent molarity of CO2 . . .

>Also, whatever the absolute value of post-construction emissions are, it's worth keeping in mind that methane is 20 times more potent in trapping heat than an equivalent molarity of CO2 .

Sure, but no one is arguing that methane and CO2 are equivalent. The argument IS over net methane. That is X amount of methane is emitted from dams. (There may be some dispute over X, but not a lot.) The question is how much methane would have been emitted if the dam was not there.

In other words, would most of the methane have been released in any case? Or would it have stayed in the water long enough for bacteria to convert it to CO2 - as you say a greenhouse gas 1/20th the  intensity of methane.

In short the question is not total emissions, it is total METHANE. Do dams result in LESS methane being converted to CO2 than natural rivers, and thus emit MORE methane?

So far we have answers from Fearnside and from the Hydroelectric industry. While I'd back Fearnside any day as the more credible source, science works based first on peer reviewed work then on replication. Both Fearnside and the industry have published peer reviewed papers on the subject. What we need are third parties to do independent studies - so we can see which set of results are replicated.

I wonder if this is a moot point, in terms of new dams, since as I understand it, most of the sites for hydroelectric power have already been exploited. The dams have been built, the concrete manufactured and the end of the dam's lifetime is in sight (due to silting up).

Also, if the climate effects of dams are comparable to burning natural gas, then that would argue for maintaining or even extending hydroelectric power, since natural gas is one of the cleaner sources of power.  (Not taking into consideration the other environmental effects of dams.)

I await your corrections!

While undeveloped Hydropower represents an extremely tiny percent of current world consumption, there are some extremely large projects under consideration - especially in Africa. Many of these projects would displace large numbers of people. One of the big arguments for going ahead with them is carbon neutrality. So it remains an important question to answer. As far as existing dams go, the question is not quite so clear. But in many cases they have really  horrible additional effects.

Also, though natural gas is less carbon intensive than coal or oil as an electricity source, ultimately we are going to have to phase out all major carbon emitting sources. If existing dams are part of this, we may not have to shut them down right away, but it is not good news for the future - especially since dams are use for more than electricity. We depend on them for drinking water and flood control. (I'm not clear on whether it is dams in general or only those producing hydroelectricity which have this affect. I hope for better information in the coming week.)

Also hydro-electric power is one of the few forms of renewable energy that is fully dispatchable. It is useful for base power, load following and peaking. It can be used to shape wind electricity and make it dispatchable (up to a point).

Ouch. :)

I forgot, by restoring wetlands the carbon sink effect will actually sequester a huge amount of greenhouse gases.

That problem of methane emission from organic matter in silt is due mainly to high nitrogen concentrations in lakes and rivers from agrichem and manure runnoff from farms, lawns, golf courses, and feedlot farming.

The manure can be digested and the methane consumed in fuel cell/turbine generators (75% efficient)to back up the grid that eventually will be mainly supplied with renewable energy.  The cO2 recycled through algae solar systems that make more fuel.

The chemical fertilizer can be entrapped out of the watershed by filtering algae from the lake or river into bioreactors that float on the water and produce extra methane to feed the solid oxide fuel cell/turbine generators.  

This water bourne algae is a huge energy source, and using it would allow the removal of pollutants along with the algae.  Heavy metals can be separated from the bioreactor sediment using renewable energy.

The bioreactor filter would allow everything but the algae to escape digestion into biogas, clean water, and fertilizer.

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Is there an environmentally friendly way, for the short-term, that we might capture the methane and use it for energy, such as in a digester?

But unless all the silt is run through a digestor, the methane already dissolved in the water will be released at the dam from the turbulence.  It might be too difficult to extract the methane and trap it in the dam itself.  

That would take a huge structure over the dam outflow.  And gas separation, normally a very energy intensive process.  It might be possible with nanotech filtration though.

That's why I think stopping the nitrogen from fertilizer runnoff is the way to go.  Digestion of organic material and release of methane is facilitated by the proper ratio of carbon to nitrogen.

Cellulose, the main carbon containing element will not break down into methane without enough nitrogen.  Wetlands and lakes with naturally low levels of  nitrogen tend to store carbon underwater.  When fertilizer runnoff increases the nitrogen the methane is released through bacterial digestion.

Algae and other plants in the water ecosystem incorporate nitrogen into their structure. If that algae were filtered out of the water and digested the nitrogen from the fertilizer is removed from the ecosystem, along with a lot of pollutants like heavy metals.

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