This aerial photograph shows the Arboretum shoreline during the 1930s. It’s worth a close look. Note the Montlake Bridge in the upper right and the Arboretum entrance (Foster Island Road and Arboretum Drive) in the lower left. The photograph is pre-520, pre-MOHAI and pre-Husky-Stadium-grandstands. The lower part of the image shows the reclaimed marshland that was exposed when Lake Washington was lowered nine feet in 1916.
Note the finger of scorched earth that extends across the marshland. That’s the Miller Street Dump, where Montlake tossed its garbage from the early 1900s until 1936. Over time, as trash was thrown from the edge, the dump grew into a peninsula of garbage extending into the marsh.
Now compare the 1930s view with this aerial photograph of 520 freeway construction in the early 1960s:
The vantage point is similar – the Montlake Bridge is in the upper right – but the shoreline is very different. Lagoons were dredged following the peninsula contours of the Miller Street Dump, seen in the finger of land left between the new freeway ramps. This man-made landscape was born out of necessity: construction materials were to be delivered by trucks on terra firma (the dump) or by barges floating on water (lagoons), so the marshy lakebed was scooped up and dumped into a pile that became Marsh Island, seen on the far right.
The Arboretum lagoons were not created as a wetland park so much as they were a consequence of 520’s construction process.