Charles Wilkes headed the U.S. Exploration Expedition that included a lengthy expedition and survey of Oregon Territory in 1841. Wilkes sailed into Puget Sound and then traveled overland from Fort Nisqually to Fort Vancouver. As such Wilkes provides an early description of the southwest Washington prairies as well as other important early observations of the area at the very earliest stages of American and English settlement
Fort Nisqually and Fort Vancouver were Hudson Bay Company forts. Fort Niscually was located near present day Dupont on the slope above and east of the Nisqually River delta. Fort Vancouver was the main Hudson Bay post on the Columbia River at present day Vancouver, Washington. Wilkes described the prairies around Fort Nisqually in very positive terms. Being there in May he had the pleasure of seeing the prairies in fine bloom.
On his overland journey south he noted that at one location the prairie was "covered with strawberries so tempting we were induced to dismount and feast upon them."
Wilkes also provides an account of the Mima Mounds, "We soon reached Butte Prairies, which are extensive, and covered with tumuli or small mounds, at regular distances asunder. As far as I could learn, there is no tradition among the natives relative to them. They are conical mounds, thirty feet in diameter, about six to seven feet high above the level, and many thousands in number. Being anxious as to ascertain if they contained relics, I subsequently visited these prairies and opened three of the mounds, but nothing was found in them but a pavement of round stones."
Hence, Wilkes conducted the first documented scientific investigation of the mounds. Wilkes surmised the mounds had been made by the Indians. His idea of a man made origin was likely influenced by the fact that very similar sized mounds are located in the Midwest and are man made.