There were 5 terns at Lake Sammamish State Park today, 9/9/08. I watched them fly, fish, and rest for over an hour. My best guess is Least Tern; I might go for a Common Tern if I determined the size is larger that I estimate.
Where were they?
At rest: There is a small roped off section of water immediately to the left of the swimming beach. The area is posted with signs that say something like" No swimming due to underwater hazards." They were sitting on the buoys in the sw corner of that section of water. 4 were on the buoys, the fifth spent more time swimming around.

I first spotted 2 of them heading over to the boat dock off E. Lake Sammamish Pkwy but they returned to the buoys. I was on that small point of land where the slough empties into the lake. I saw that they stayed near the buoys and that there were more than the two, so I walked over to the swimming beach.
My impression is that they are small and probably juveniles.
Size - about the same size as the buoys - 9 inches. I was able to estimate the length of the buoys as they were on land and did not fly!
Light gray back and tops of wings. The one on the water was darker on the mantle.
Forehead: white (not a solid cap)
Feet: red orange
Tail - I never saw the fork, when it hovered it was fan-shaped.
Bill - not a long one. The color was difficult, it was alternately red/ black
Without my scope, I missed some important marks.

Marcy D'Addio
Redmond, WA