In the early 1960s the Arboretum wetlands were dredged into lagoons to facilitate the construction of S.R. 520. Where did all those dredge spoils go? Much of it west of Foster Island was heaped into a pile that became (still-sinking) Marsh Island.
The east side of Foster Island was a different story. The image above shows a crane bucket dropping a load of lake bottom onto a barge just off the Madison shore. In those days workers were allowed to tow the barges to the middle of the lake and dump the spoils right into the water. This practice stopped later in the project when concerns were raised about a cloudy discoloration in the lake (already heavily polluted by sewer drains). Barges then had to be towed at greater expense through the ship canal and dumped into Puget Sound.