In the younger portions of the High Cascades, it is becoming apparent to me that surface drainages- that is, streams- are relatively rare. In the younger, rubbly lava flows and tephra falls, the water just runs into the ground and flows whatever direction is "downhill" there. In the case of the upper McKenzie Basin, a great deal of that water ends up resurfacing at Clear Lake.
When I was up there two weeks ago, we decided we didn't want to deal with the congestion at the main tourist area, but decided it might be worth the time to check out the boat ramp and camping area on the southeastern shore. It was a quick drive in, with ample parking, and not the least bit crowded. It is a 5$ day use area, but we were there literally only two or three minutes, so we politely skipped that part. On the other hand, there is a loop trail around the lake that looks interesting, and I think for convenience' sake, this would be the place to park.
The right shoreline is composed of a roughly 3ka lava flow from Sand Mountain that was responsible for creating this lake.