Fall River is almost 1500cfs below it's normal flows for this time of year. This is putting pressure on water storage on the Henrys Fork and will cause higher than normal flows for the Harriman Ranch opener.
The Salmon Fly hatch has come and gone on the lower sections, but with water levels so low, it's made it difficult to float to take advantage of the hatch. It's purely speculation, but the big bugs may be higher on the river towards Cave Falls. You need to have a Yellowstone Park fishing license to fish there so be prepared.
Of course, fish will be in the deeper holes so try stripping a streamer or tossing a big bug with a dropper in these key spots. Fish will be eating small drys mostly towards the evenings, smaller fish will be more widely distributed throughout the river.
The water situation for 2015 will make things interesting as summer approaches.