I spend the entire summer on the Yellowstone River in Paradise Valley a few miles upriver from Depuy's. Although I drive by it dozens of times every summer, I've never fished it, or any of the Paradise Valley Spring Creeks for that matter. Just too cheap to pay the fee to fish them, I guess, when there are already too many other places to fish in that part of the world for free.
However, that being said, the most comprehensive information about Depuy's that I'm aware of is a series of articles that Tom Travis wrote for Fly Anglers Online in 2013. I believe Tom still resides in Livingston, has been a long time guide there, and has fished and guided on Depuy's for years. If you go the FOAL website, do a search for "Tom Travis Depuy's" and you will find these articles and more detailed info on Depuy's than you'd ever need.
A couple other bits of info related to your trip. The beginning of July is right on the cusp of being the perfect timing, in my opinion, or it could be too early in the season for the best fishing, depending on where you plan to be and fish. I would suggest that you monitor the local fly shop's fishing reports covering the streams and watersheds you plan to fish from now until your departure dates, and be prepared to adjust your plans accordingly, if needed. There are a number of these area fly shops, but 2 in particular are Parks Fly Shop in Gardiner, and Blue Ribbon Fly Shop in West Yellowstone.
The Madison and Henry's Fork should fish well in early July, but keep in mind that a number of places inside Yellowstone Park do not open until mid-July. The Yellowstone River downstream from Gardiner (including its tributaries, such as Mill Creek, previously mentioned) is unfishable at the beginning of July in a normal snowpack year, but could be fishing great then depending on the snowpack - as of this morning, the snowpack in the Upper Yellowstone watershed (which is basically everything upriver from Livingston) was 83% of normal.
One other thing to keep in mind, in driving into and through the Park, is road construction. Gardiner was pretty torn up last summer, and I believe it will also be this summer, and the road from Mammouth to Norris also had major construction last summer, and will still have some this summer. I know you said you are looking forward to getting into a buffalo jam, but I wouldn't think you'd want to spend most of your vacation stuck in traffic jams -- talk to the locals about road conditions before heading out and across YNP. Normally, you can minimize travel delays by heading out very early, plus see more wildlife, or taking an alternate route. (If you want to act like a tourist, and gawk at buffalo, please pull off the road so us locals who are on our way to go fishing somewhere don't get held up!)
In any case, I'm sure you will have a ball.
John
