I have recently worked with a gentleman from Utah who was fishing the Provo & Mammoth Creek and looking for advice on kebari and other flies. A lot depends upon the type of fishing that you enjoy. For subsurface, there is no doubt that I would start with the copper weighted Utah Killer Bugs size 14. I like to pick out the wool a bit with a needle to make them buggier looking.
My own personal favorite fly is the size #12 black killer bugger, that being an good imitation of a lot of things, especially your Western stoneflies. I would take Skye's killer bugger too for a caddis imitation. Speaking of stone flies, I sell a few of patterns that Rob at Tenkara guides developed that are also good stonefly imitations. The Red Ass Monkey and the Grave Digger (which I will have in the store) are well proven on your streams. The reason that these flies work is that they are incredibly buggy and work on the bottom, dead drifted through pocket water. Being larger flies they get down deep. For the toughest pockets, I also have the RA Monkey jig version. My own Skye's patterns offer a similar effect in a wide range of colors (great for emerging caddis) and I can weight them or tie smaller versions for you as well.
However, the Provo is known for it's tiny flies, and anything on a size #16 to 20 hook in brown, yellow, gray or black should serve you well. The Glass Bead Midge will perform for you. For dry flies, try tan, gray, cream, yellow or brown. Variegated hackle like the Grizzly works great. Don't forget to try a larger fly if things slow down. Sometimes that will be the ticket. It's important to move carefully and keep a low profile. These fish often move in toward shore to feed so fish the closer water first.
For Tenkara kebari I would want the floating Dr. Ishigaki & Ishigaki Grizzly (for grays), both size #16. I would tie these with 2-4 turns of Whiting rooster hackle on light wire hooks for good floating characteristics, but also carry a few tied sparsely on a heavier hook. Speaking of these sparse kebari, I have a customer who likes them with an olive body and dark dun hackle, with peacock herl collar. He claims great success. I sell this as the Blue Wing Olive.
Streams also vary greatly depending upon the geography, so it really does make sense to keep a number of patterns on hand rather than relying on a single pattern. Personally, if I wanted to limit myself to a very few flies for nymphing, which (along with the small streamer) is arguably the most effective fly type in existence, I would choose the Killer Bug, Killer Bugger and Killer Kebari, in a variety of colors and a couple of sizes, such as #12 and 16 with and without copper under weight and bead heads. You will want to have both light and dark shades, maybe in black, gray, brown, tan, olive. But don't overlook a bright shade like red, yellow or green for tough conditions.
John Vetterli - our friend from Tenkara Guides has experience on the Provo. He recommends the Utah Killer Bug, Grave Digger and small White, Grey and Black Kebari as top patterns for that water. Thanks John!
Just a general note here: The most popular commercial Western nymphs are Blue Wing Olive, Prince, Tellico, Hare's Ear, Pheasant Tail, Brassie, Caddis Pupa imitations, Stone fly and Stenonema nymph imitations. #20 Baetis, Copper John, San Juan Worm and Zug Bug (not necessarily in that order). They are popular for a reason, they work. There are also many, many newer flies that have proven their worth.