Rough map of the old lake based on old maps and my own walking of the area. The northwest area is very approximate due to grading and filling in the area; The historical lake bed no longer exists.
1880's era river navigation map shown to left.
The north end of the lake drained into the same Lake River back channel as Round Lake still does.
The northern area of the lake and Round Lake were clearly once a larger singular wetland before diking and draining. The southern portion would expand and contract with the water level. It would have mostly flooded to the south during the springtime flooding of the Columbia (before the river was dammed). There is now a cross dike just to the south.
The slough is inside of a peninsula (Chapman Island) of higher ground that leads out to the Columbia. The old channel is mostly still intact, visible as a tree line on the map. It ends at a cattle feed lot right under "501" marker on the map. The area to the north and west appear to be extensively filled in with dredge spoils.