I dunno, the British had some very serious ordnance experts work on the
problem
of dam removal during WW2 and concluded that a special depth charge sunk
right
against the upstream face and detonated at the bottom, would be the
easiest and
surest way to take out a big dam.

For the Hoover Dam, I think it's more likely that terrorists would try a
truck
bomb. I understand they've closed the road across the top of the Hoover
Dam,
though.


No, the road is still open across the dam. Just drove across it last
November on a trip to Vegas. US Highway 93, which is a major highway, runs
right across the top of the dam. Guess it seemed like a good idea in 1933.
For a while after 9/11, commercial truck traffic, RV's and (GASP!)
trailered boats with enclosed areas were not allowed to traverse the dam.
That restriction didn't last long, but all vehicles pass through an
inspection point before being able to proceed to the dam since 9/11. The
good news is that construction has started on a bridge immediately
downstream of the dam (similar to Glen Canyon), that will carry all traffic
that now crosses the dam. Unfortunately, it is very challenging terrain,
and will take years to complete. Should have been done years ago, but very
big $ required to get it done.

I'm no where near qualified to understand the physics required to breach a
major dam. The depth charge scenario while plausible, would likely have to
be pulled off by divers to get the charge in position. The dams have always
had standoff boom lines that prevent vessels from getting too close to the
dams. I would imagine there is security personnel stationed on or near the
dams watching with high-powered optics any and all marine traffic activity
in the vicinity of the dam, but have never actually heard of such. Who they
would call on the radio to intercept suspicious marine activity is another
story. On-the-water law enforcement on the lakes is primarily handled by
National Park Rangers, and they are spread very thin.

There is a high volume of commercial air traffic that crosses directly over
Hoover every day and night (the dam is well lit) either taking off or
landing from McCarren Intl. in Las Vegas, or cross country traffic to and
from LAX. Also quite a bit of smaller aircraft traffic doing sightseeing
tours to the Grand Canyon out of Vegas. Wouldn't take much extrapolation of
the 9/11
scenario to imagine how easy it would be to destroy most of the water supply
and power generation capability for the entire southwest US, including
Southern California. Very scary, indeed.