Fort Three Forks, also known as Old Fort Henry, was one of the first forts or trading ports between the Mississippi and the Pacific Coast. According to history, Fort Three Forks was established early in the spring of 1810 when Andrew and Pierre Menard with a party of about 30 men, and John Colter, as a guide, were sent to the Three Forks by Manuel Lisa to erect a trading post. They planned to trap beaver over the entire area of the Missouri and its branches above the Great Falls. Life was never monotonous at the Fort; there were plenty of things to keep the trappers alert. The Blackfeet troubled the men steadily, and killed eight of the party. When the Blackfeet or Gros Ventres of the prairie failed to put in an appearance, there were always rattlesnakes or grizzly bears that could be counted on to prevent their leading a humdrum existence. Though the main business of the post was to catch beaver, this could not be undertaken without leaving the fort and to do so was perilous. Conditions were extremely discouraging and in the spring of 1811, the post was abandoned.

Time and encroachment of the river have destroyed the greater part of the ground on which the fort stood. However, the actual description of the fort in the handwriting of an anonymous surveyor says, "A double stockade of logs set three feet deep, enclosing an area of 300 sq. feet situated upon the tongue of land about a half a mile wide, between the Jefferson and Madison Rivers, about two miles above the confluence, upon the south bank of the channel of the former stream, now called the Jefferson Slough. The store and warehouse were built on each side of the gates, and on the other side next to the interior of the fort, two buildings were connected by a gate, the space between buildings and stockade filled in with pickets, making a large strong room without any covering overhead. In each store, about five feet from the ground, was a hole 18 inches square, with a strong shutter fastening inside, opening into the enclosed space between the gates."

When the Indians wanted to trade, the inner gate was closed; a man would stand at the outer gate until a number of Indians had passed in and then lock the outer gate. He would then climb through the trading hole into the store. The Indians would pass whatever each one had to trade through the hole into the store and the trader would throw out of the hole whatever the Indians wanted to the value of the article received.

Taken from "Historically Speaking" by Lyle K. Williams