I don't like to argue over something that the correct answer can be looked up or tested so I e-mailed the entomology dept at Washington State U. The answer is mixed; there are some hellgrammites along the west coast from BC down to California, but they live in streams and rivers and not in lakes. Without a sample the entomologist did not want to say but a good guess would be an alderfly nymph.
Here is the text of the e-mail:
"At the outset, I must confess that I do not know where Burke Lake is located. Depending upon the area of eastern Washington where Burke Lake might be, my comments may vary.
Am I correct to assume that you have a specimen in hand? There are other insects (especially beetle larvae) that resemble dobsonflies and you may have one of them. Also, I need to know something about the habitat where the specimens was collected.
Dobsonflies belong to a order of insects called Megaloptera. They are related to snakeflies and the lacewings. As you already know, the aquatic larvae are called hellgrammites and they can usually be found under rocks and logs along stream and river courses. As far as I know, they are found in the eastern U.S. as you indicate, but they also occur out west, specifically west of the Cascades in B. C., Oregon and Washington, and in the coast ranges of California. I've never seen specimens or records from east of the Cascades.
Also included in the Megaloptera is a second group, the sialids or alderflies. Adults are much smaller than corydalids and they occur in eastern Washington. Their larvae resemble hellgrammites but they are smaller and typically occur in the muddy substrate of streams, seeps and impounded beaver ponds.
Without looking at the specimen(s), it is difficult to say what you have. Have you checked any insect handbooks and compared the specimen(s) to the illustrations? If not you might check images of the larvae on-line via the Internet. I'm sorry that I cannot be of more help to you at this time."