As mentioned that is a bit early for larch turning color. That said, it has been a dry summer and I have noticed my aspen already starting to turn due to lack of moisture. Not sure how this will affect the timing of the larch.
There is no Lake Stehekin, It is Lake Chelan and Stehekin is at the head of the lake. It is a long ferry trip to Stehekin, plus an even longer hike to get up into larch elevation. Stehekin is a nice place to visit, just not for larch.
Washington Pass area and the North Cascades Highway (hwy 20) is probably your best bet if your not into backpacking.
Just west of Washington Pass is Rainy Pass that has a fairly short hike to Lake Ann. You need a trailhead pass from the Forest Service to park there. They are $5/day. Lake Ann is fairly high elevation and has some nice larch in the area.
The area is scenic and well worth the visit. Lots of good photography opportunities in the area.
Here is my blog posting on the area.http://usbackroads.blogspot.com/2010/09/highway-20-mazama-washington.html
Here is the Forest Service information on Lake Ann Trail:http://www.fs.usda.gov/wps/portal/fsinternet/!ut/p/c4/04_SB8K8xLLM9MSSzPy8xBz9CP0os3gDfxMDT8MwRydLA1cj72BTJw8jAwjQL8h2VAQAzHJMsQ!!?ss=110617&ttype=recarea&recid=59285&actid=51&navtype=BROWSEBYSUBJECT&position=BROWSEBYSUBJECT&navid=100000000000000&pnavid=null&cid=null&pname=Methow+Valley+Ranger+District+%28North%29+-+Lake+Ann%252FMaple+Pass+Loop+Trail+%2523470
There is also a hiker site that usually starts a Fall Color discussion about this time. Here is their link: nwhikers.net
You can do a search on "larch colors" and you will get a posting of the threads from previous years. Make note of the dates!!
Give a call the Forest Service at the Methow Valley Ranger District in Winthrop for additional information before you leave: http://www.fs.usda.gov/okawen
Hope this helps. Don't worry about catching the peak color. That area is great for wandering around and taking photo's anytime of the year. And if your lucky you just might catch the early color.