Hi there! I have a permit to hike Mt. Whitney in mid-September. I've been carefully tracking the snow conditions, and if the snow hasn't melted out by my date, I'll be releasing my permit. But I have another concern, which is stream/river crossings. My understanding is that river crossings are always part of the hike (even in normal weather years), but that they're more treacherous right now because rivers are running high and fast due to all this snowmelt. Do I have that right? In normal weather years, is there a certain time of year (e.g., early August) when the rivers tend to be more manageable? I'd appreciate anyone's insight into this issue.

I can't speak to specifics on the Whitney Trail...
But, 2 crossing can be avoided by getting to the Portal a little early (daytime) and learning how to use the Old Trail...

Speaking in terms of what I've seen over at Horseshoe Meadows...
(It's where I work out.)
The streams have dropped an amazing amount in the last 2 weeks...I would (almost certainly) state that the streams over there are no longer an issue...

Obviously, the meadows are still a marshy mess...

We went from Cottonwood Pass to Kearsarge a few weeks ago now, starting the 24th, and still had to use alternate crossings for most of the creeks. From the pictures then people said the flow was down quite a bit from early July.

Reports from the last week are that most of the creeks can be crossed at their normal points now. They are still a bit high/fast, but like you said the flow has dropped a tremendous amount over the last 3-4 weeks.