Silverbullet, first off, thanks for your service sir!
I prefer to look at the Discharge (cubic feet per second) rate on the Walnut gauge, and don't really pay much attention to depth. But depth and discharge generally rise and fall at the same rate.
Here's what I've found, and what I prefer. Very subjective, as each fisherman prefers different conditions:
1. Hard to say exactly what will "trigger a run." Often has as much, maybe even more, to do with water temperature and amount of daylight than the amount of water depending on who you talk to. But when Walnut is at about 100 CFS and rising, you can reasonably expect some fish movement on it and Elk - whether it's fresh fish moving in from the lake or existing fish moving upstream or both. When the Walnut gauge is up around 175-200 CFS and rising, I personally wouldn't really consider fishing either Walnut or Elk as I don't have the skills to do well in those conditions.
2. For ideal fishing conditions for typical drift fishing, take a look at the pics in what FishinGuy posted. My preferences: When Walnut is at 200 CFS and DROPPING, smaller streams are usually prime. When Walnut is at around 150 CFS and dropping, conditions are usually OK there and on Elk. When Walnut is at about 70 CFS and steady or dropping, those are my favorite conditions. Nice flow, a little bit of color, but generally clear enough to see fish pretty well.
3. Would imagine that this weekend will be among one of the most crowded of the year, with many having a long weekend with Monday off for Columbus Day. Check out the Angler Maps here,
http://fisherie.com/Maps, and check out different access points higher on Elk and Walnut and you should be able to find a little elbow room the further you get upstream.
4. Skip16503 on here (Skip Hughes in real life) runs the Erie Project Healing Waters chapter and does a great job. Send him a PM or see if he chimes in here.