It used to be called Whiskey Flats. A rough and tumble town full of the hard living sailors and others that made their living, legal or otherwise, at this stop on the river. There was a 2 1/2 mile stretch of rapids that made through travel next to impossible and things had to stop here to get portaged around this cascade of water and rocks.
In 1864, the Oregon Pony, the first steam engine in the northwest carried passengers and freight past the rapids. The little engine can be seen at the Marine Park.
Finally, in 1896, the canal and locks were completed to allow passage around this cascade and the town had a new name "Cascade Locks". This island was created as part of the canal and locks. The cache is a medium sized lock and lock located at the north or east end of the island full of goodies of a historical nature.
Be careful going down the bank or you might end up in the river. The easiest way down is to go right to the end of the island and there is a pretty decent path down through the rocks.