[wetibbe]

Ilbegone stated in a recent post: I believe the current population of the US is around 210 million.

Historical populations
Census Pop. %±
1790 3,929,214 —
1800 5,236,631 33.3%
1810 7,239,881 38.3%
1820 9,638,453 33.1%
1830 12,866,020 33.5%
1840 17,069,453 32.7%
1850 23,191,876 35.9%
1860 31,443,321 35.6%
1870 38,558,371 22.6%
1880 49,371,340 28.0%
1890 62,979,766 27.6%
1900 76,212,168 21.0%
1910 92,228,496 21.0%
1920 106,021,537 15.0%
1930 123,202,624 16.2%
1940 132,164,569 7.3%
1950 151,325,798 14.5%
1960 179,323,175 18.5%
1970 203,211,926 13.3%
1980 226,545,805 11.5%
1990 248,709,873 9.8%
2000 281,421,906 13.2%
2010 309,162,581 9.9%

The current population of the USA is 309 million. Refer to the chart for the population in 1800.

The Lewis & Clark expedition occurred for 1804 to 1806. They actually initiated their plans and preparations in Harper's Ferry West Virginia and launched the actual expedition from St. Louis, Missouri.

The Louisiana Purchase in 1803 sparked interest in expansion to the West Coast of the American continent. The United States did not know precisely what it was buying and France was unsure of how much land it was selling. A few weeks after the purchase, President Thomas Jefferson, an advocate of western expansion, had Congress appropriate $2,500 for an expedition. In a message to Congress, Jefferson wrote:

"The river Missouri, and Indians inhabiting it, are not as well known as rendered desirable by their connection with the Mississippi, and consequently with us.... An intelligent officer, with ten or twelve chosen men ... might explore the whole line, even to the Western Ocean..."[2]

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According to books I read the native population was unknown but could have been speculated at 18 million when Columbus discovered America.
Obviously the native population in 1800 and before was unknown so the population figures here above must be for arrivals from Europe and elsewhere.