More experienced fishermen might have more to add, but here's what I know about flows near Ellensburg.
Since it's a managed flow river the runoff season never really ends, It gets down to about 2000cfs near the end of June and they start releasing water for irrigation that brings it up to around 3000cfs. Around the beginning of September they allow the flow to decrease. Based on my two summers fishing I have noticed that there is some good wade fishing before they raise the flows, but it's not as easy as in the fall.
It's an irrigation canal in the summer - 3000+ cfs and full from bank to bank. Float fishing only, pretty much. In April/May it can be fishable (ca. under 2000 cfs at Umtanum), if there isn't too much rain and they have been able to gauge the filling of the reservoirs properly. However, the late seasonal snow in recent years has meant that they have had to fill the reservoirs early in order to be sure to have enough water for the summer, only to have to spill more of the spring runoff, meaning wadable levels have been rare the past few springs.
However, the late seasonal snow in recent years has meant that they have had to fill the reservoirs early in order to be sure to have enough water for the summer, only to have to spill more of the spring runoff, meaning wadable levels have been rare the past few springs.
In 2012 the heavy pack created the reverse situation for a time. I think there was plenty of natural runoff so for a while they weren't spilling as much water. I remember some great fishing that July. I caught my first unguided Yakima Trout then.
If there has been any silver lining in our recent cool, wet early summers is that the demand for irrigation water has not started early in the summer. This has allowed the irrigators to keep the flows lower than normal. But there are irrigation contracts to meet and their demands determine (largely) the flow patterns.
There are really two answers here. The first is the effect of the run-off from the uncontrolled tribs (some too small to mention) like the Teanaway, Swauk and Wilson Creek just to name three.
Temps, both during the day and night, either accelerate the run-off or keep it slow and steady. Warming in the Stuarts and other higher ridges cause a colorful run-off as early as March. More likely the color and extra water from these (while the lakes are still filling) comes all at once if we get a warm spell in the high sixties and seventies, or slower and steady if it is in the fifties during the day and it cools to near or below freezing at night.
Thennnnn.. , when all this is over, the growers will start taking water and calling for more in June. The flows accelerate to 3,000 cfs, and better if it gets hot early and stays that way. They stay there until about labor day when they flip-flop the lakes at the pass for those at the head of the Naches.
It's mostly hoppers tight against the bank, and almost all fishing from a boat. For wade fishing you can look for side channels or high stick from the bank. I've generally avoided the Yak during Summer flows and fished our local freestone rivers and creeks.
Don't let the summer flows stop you from fishing the Yak without a watercraft. The fish are tucked in close to the bank. There are lots of areas that have rip rap and access that you can effectively fish without even needing to get wet while fish on foot. Look of soft seams, behind structure ect.
The more you fish it and explore you'll find more places like this.
It won't be easy like after the flip flop but if you put some miles on your boots you can have some excellent summer fishing.
The best days for me on the Yak are when we drop anchor and fish the side channels. Its nice to wander off and play jedi mind tricks with myself as i stalk the trout in the skinny water that we cant get to from the DB.