A little background about the lake bottom. It is the basin of a melted glacier filled with large granite boulders brought down from the Wallowa Mountains that settled out as the glacier came to a grinding halt and melted. Under those circumstances it took a while to develop a lake bed that would support a food source. Most of the silt rushed into the valley and created very nice farming land, though a bit swampy for quite some time.
On a clear calm day in September you can see the boulders towards the shore. Massive. Near the south and north ends you have sloping terrain that supports food sources. In the middle of this long lake you have very deep water, locals say there is no bottom. Snags are not much of a problem except at the south end. Finding fish is a problem. When I am up there, which isn't very often, most of the boats are at the south end where the river dumps into the lake. That's probably where salmon are. The dam end is likely where the trout are. The top 6 inches warms but the rest stays colder than a witch's hoohah. That's my uneducated guess as I have never fished the lake.