Whenever application is made to the department, by any department of the United States government, for the use of any state-owned tidelands or shorelands, for any public purpose, and the department shall be satisfied that the United States requires or may require the use of the tidelands or shorelands for the public purposes, the department may reserve the tidelands or shorelands from public sale and grant the use of them to the United States, upon payment for the land, so long as it may require the use of them for the public purposes. In such a case, the department shall execute an easement to the United States, which grants the use of the tidelands or shorelands to the United States, so long as it shall require the use of them for the public purpose.