Traffic over the Hoover Dam is clogged and slow in summer.
I used to live in Vegas many years ago, was a truck mechanic, had to go work on trucks stuck on the switchbacks coming and going to the bridge, its a real bottleneck, and yes its also a very easy target for a terrorist.
All one would need is a type of trailer that could have a track that would drop several underwater mines over the water side of the dam spaced apart and on consecutive timers.
Dambusters by an 18 wheeler. A very determined intelligent radical can do it, trick is to research at what depth is optimum and at what point they need to be dropped.