Following excerpt is from Wikipedia
“While there is a tale tells of the famous Bonneville expedition trekking for weeks through rough terrain happening upon the sight of the Boise River Valley. Legend had it that a French guide overwhelmed by the sight of the verdant river exclaimed “Les Bois! Les Bois!” – French for “the trees! the trees!”
But this is more than likely just legend….
Todd Shallat, City Historian and Professor of History at Boise State, suggested that “the story might be legend because the Boise River was so named before Bonneville’s expedition, and the “Les Bois” part was reported by the essayist Washington Irving. The trappers probably did say “Les Bois” or something similar, but that is not how the river, let alone the city, got its name.”
In the 1820’s French furtrappers set traps in the area where Boise now lies and it became a center for trade between the native inhabitants and the new people of the West.
Though mostly an area of high desert, a prominent landmark was the tree lined Boise River Valley, which they called “La Riviere Boise” which means “wooded river.” Though the connection between the Bonneville tale and the naming of the city is dubious, it is clear that the area was referred to as Boise long before the establishment of Fort Boise.
Boise Soldiers Home
The original Fort Boise was 40 miles (64 km) west, down the Boise River, near the confluence with the Snake River at the Oregon border. This fort was erected by the Hudson’s Bay Company in the 1830s. It was abandoned in the 1850s, but massacres along the Oregon Trail prompted the U.S. Army to re-establish a fort in the area in 1863, during the U.S. Civil War.
The new location was selected because it was near the intersection of the Oregon Trail and a major road connecting the Boise Basin (Idaho City) and the Owyhee mining areas, both booming at the time. The area is now the VA Hospital and Military Reserve.
Idaho City was the largest city in the area, but the new Fort Boise grew rapidly (as a staging area to Idaho City) and Boise was incorporated as a city in 1864.
The first capital of Idaho was Lewiston, but Boise replaced it in 1865.