Water Levels

I'm planning out and buying supplies for my first rdwc grow. I have one question that has been bugging me. I usually see others running a multiple 5 gallon bucket under current system with a larger reservoir.

My question is, isn't the water level in the res going to be the exact same as the water level in the 5 gal buckets? My thinking is that unless the res is wider, it's not going to hold a higher volume of water.

Yes, the depth of water in each bucket will be the same until you start pumping/recirculating. The easiest way to explain it is what goes in must match what goes out...equal flow in both directions. The water being pumped is moving faster than the water flowing from the control...in the case of an UC system. So the pump lines are 3/4" and the pipe connecting all the buckets is much larger...2-3"

Not necessarily, if you're doing a DIY you can make it with an overflow.