The number I was given for Lake Powell, behind Hoover/Boulder Dam,
is that it will become hydroelectrically unproductive in 200 years
due to silting. Pondage is necessary because rivers, though they
will still drop by the same height difference regardless of
pondage, flow at vastly different rates with changes on a short
time scale. Pondage acts as a buffer and energy storage medium.
The demand on energy production also varies on a diurnal basis,
and a river's energy is hard to store during the hours of low
demand without pondage. A hydroelectric facility operating
directly from a river cannot be operated nearly so efficiently as
one operated from pondage.
The Colorado River is, I believe, a particularly silty river, but
dammed rivers here in British Columbia (and we have many of them*)
will also eventually silt up on a time scale that is less than
millenial. Hydroelectric power is not a "green" technology if one
sees the wilderness devastation produced by damming rivers as
significant. Up here very few people actually see these effects
because we are so sparsely populated. If you have a recent atlas,
look at BC. Pay particular attention to Williston Lake and Ootsa
Lake. Both are huge, and both are creations of hydroelectric
projects. There are many other, smaller, lakes that have been
created by dams in the province.
Hydroelectric energy is not free. Calculating its true cost is
highly problematical because one must calculate the very large
environmental cost as a highly significant contribution. I just
wanted to point out that fact because many of us city kids with
physics educations think hydroelectric energy is, essentially,
free. It ain't so.
Leigh
*British Columbia receives one tenth of all of the precipitation
that falls on North America. There is a good reason we are known
as the wet coast of Canada!