Hi,
Lake Mead might also be pretty low. You can search for Lake Levels and there are websites for each lake that the Government runs.
Lake Mead will be REALLY hot this time of year. Fine if you like it, but when I was there in April several years ago, I decided to go visit Red Rock campground, and it being hot there, went to Zion NP, where it was much cooler.
Some day I plan on spending the whole summer at the North Rim of the Grand Canyon. You can camp for free there in the National Forests, but not within the NP, except for the campground. It is 7,500' elevation and is closed due to snow until mid-May, and is much cooler than the 6,500' elevation south rim area. Bryce Canyon is even cooler in the summer time. I don't know the local lakes there.
You might try Butt Lake that is east of Redding, or perhaps Shaver Lake? It is east of Fresno and I know it is around 5,700' elevation (give or take a few hundred feet). Near Shaver is Huntington Lake (Named after Mr Huntington who owned Southern California Edison and the red lines in LA. Extending the Red line train to Huntington Beach, they re-named the town after Mr. Huntington.
Huntington Lake is around 6,500' and Edison Lake feeds into Huntington, is at a higher elevation, perhaps 7,500'? You can look them all up on Google Maps, then click on Terrain and find the elevations.
Weather.com will give you some idea of the temperatures. Sometimes memorial day at Shaver Lake brings snow or 80F weather.
If you do end up there, remember that fuel up in the mountains is REALLY expensive, while near Fresno is much less. There are Sams Club and Costco in Fresno, both have fuel - I think.
If you like forests, Sequoia National Park is up around 6,500' elevation. It has huge trees, but the entrance of Yosemite has the largest tree, 33' across. You find it by going into Yosemite on highway 41, then turn right, go 2 miles to the Grizzley Tree parking area. Then it is a 1/2 mile walk from there.
Near Yosemite is Bass Lake. Huntington Lake might be fairly full, it has pumped storage from Shaver Lake.
Fred.