This remarkable attraction was known during the 1880s and 90s as "Big Spring," and sometimes "Wonderful Spring." In 1887 a daring group of eastern tourists steamed thirty miles up the river from Fort Benton to see the falls and the spring. Of the latter, the reporter wrote: "This tremendous outpouring seems to be rather the mouth of a hidden river than a simple spring. Various names were suggested by the visiting party, for it had hitherto borne no other name than the "big spring." Finally, all agreed to christen it "The Giant's Fountain."1 In 1902, when travel writer Olin Wheeler had the above photograph taken, sightseers could view the "Giant Spring" from this boardwalk.
Today it is all part of Giant Spring State Park.
On 18 June 1805, after measuring the "second great Cataract"—the "handsom Fall"—Clark and his five men "proceeded on up the river a little more than a mile to the largest fountain or Spring I ever Saw, and doubt if it is not the largest in American Known." The "emencely Clear" water, which seemed to him "of a bluish Cast," could be seen distinctly for half a mile down the river, which three weeks later he later measured at 270 yards in width.
Rainfall and snowmelt seep into the Madison Limestone on the flanks of the Little Belt Mountains, more than forty miles to the south. Here it flows upward some 700 feet through cracks in the bedrock, emerging at a rate of 220 cubic feet per second (1,645.6 gallons per second, or roughly 144 million gallons per day). Lewis had overlooked it during his reconnaissance up the north side of the river on the thirteenth of June, but he made a special trip from the White Bear Islands camp on the twenty-ninth, and spent twenty minutes studying it.
Laboratory analysis has shown that the water contains calcium, magnesium, bicarbonate, and sulfate. Lewis concluded:
There was another "uncommon object" associated with the spring that escaped notice by Lewis and Clark. William O'Keefe, formerly superintendent of the seven dams the Montana Power Company operated on the Missouri River, explains:
The flow that is visible at Giant Springs is just a portion of its entirety. Looking slightly upstream, a bubble or slight rise in the river surface can be seen, which is evidence of the rest of the spring's subterranean flow. This can be seen very distinctly during the coldest days of winter when the spring water, which is always above the freezing-point, causes a noticeable puff of fog above the river's surface. The comparatively warm water is beneficial to the operation of Rainbow Dam, for it prevents ice from building on the intake screens, as well as on the dam in general.2