Excerpt from Fishery Management Studies on the Madison River System in Yellowstone National Park

The temperature of the water in the Fire hole is influenced to some degree by geysers, but only localized areas become too warm for brown and rainbow trout. Simonian (1952) recorded the influence of Giant Geyser, the largest geyser in the park, on the temperature of the Firehole River. Water temperatures 200 yards downstream from the eruption rose from 64�f. To 75�f. At the point where the thermal water was entering the stream, a temperature of 1is�p. Was recorded. Giant Geyser erupts irregularly with a minimum interval of 48 hours, and the temporary influence of the warm water does not prevent brown and rainbow trout from living under such conditions The distribution of the brook trout in the Firehole was outside of the areas influenced by thermal acti viry; they were principally confined to small trib utary streams and to areas above Old Faithful.

The Firehole River originates in Madison Lake, located about eight miles south of Old Faith ful, and flows northward for about 32 miles where it joins the Gibbon River (fig. The flow in early September 1957 was about 35 cfs at Old Faithful and about 200 cfs where it joins the Gibbon. Many hot springs and geysers enter the river along a 12 -mile section below Old Faithful. The river falls about 700 feet from two miles above Old Faithful to its mouth, an average gra dient of feet per mile. It varies in width from 40 to 100 feet and in depth from six inches to over ten feet. Firehole Falls are 40 feet high and are located about four miles above the stream mouth; for the next two miles below the falls, the Firehole flows through a gorge in a series of cascades. The falls, together with the cascades, serves as a barrier against upstream fish movement.

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