According to the official website of the U.S. Forest Service, the Alder Dune Campground boat ramp is located on the Siltcoos Lake in the Oregon Dunes National Recreation Area. The boat ramp is described as a concrete, single-lane ramp that is suitable for small boats and can accommodate trailers up to 20 feet in length. The width of the boat ramp is not specified.

The Siltcoos Lake is a freshwater lake that covers an area of about 3,200 acres and is located on the central coast of Oregon. The lake is known for its fishing opportunities, and various species such as rainbow trout, largemouth bass, and bluegill can be found here.

As per the regulations of the Oregon State Marine Board, the type and size of the watercraft allowed on the Siltcoos Lake are restricted. Only non-motorized watercraft like kayaks, canoes, stand-up paddleboards, and sailboats are allowed on the lake. Gasoline-powered engines are prohibited on the lake, and electric motors are allowed only on boats that are less than 12 feet long.

In conclusion, the Alder Dune Campground boat ramp is a single-lane concrete ramp that is suitable for small boats and can accommodate trailers up to 20 feet in length. It services the Siltcoos Lake, which is a freshwater lake in the Oregon Dunes National Recreation Area. The lake only permits non-motorized watercraft like kayaks, canoes, stand-up paddleboards, and sailboats.