blackfoot-valley

formed many times when an ice lobe up to 30 miles wide advanced into what is now the panhandle of northern Idaho in the present Lake Pend Orielle, dam- ming the Clark Fork of the Columbia River. The waters of what became Glacial Lake Missoula rose to over 4,200 feet above sea level. The site which is now the city of Missoula was under 1,000 feet of water at maximum (and a depth of nearly 2,000 feet behind the ice dam in Idaho). From Missoula, you can see the level marks of the Lakes’ many shorelines etched on the slopes of Mt. Sentinel, which rises steeply above the University of Montana campus; and on Mt. Jumbo, the top 500 feet of which showed as an island above the lake’s highest level. When the ice dam failed, not just once but three dozen times or more over its life span, the sudden and cataclysmic floods that were unleashed were the greatest in the known geologic history of the world. During the same ice age, two even larger lakes existed in the American west. They were Lake Bonneville, which covered most of the western half of present day Utah, and Lake Lahontan in Western Nevada. Both had no removable “stopper” and dried up gradually with the advent of warmer and drier conditions as the ice age end- ed. Utah’s Great Salt Lake and Nevada’s Pyramid Lake are simply their shrunk- en remnants. In the east, even larger Lakes existed. They were Lake Agassiz (currently named Lake Manitoba), which covered most of southern Manitoba and a part of North Dakota and Minnesota, and the current Great Lakes, which formed as the ice sheets retreated. Neither of these drained as catastrophi- cally as Lake Missoula. At its highest level, 15,000 years ago, Glacial Lake Mis- soula was about 200 miles long. Its shoreline was irregular extending up the entire length of the Bitterroot Valley to the south and to present day Polson in the north where it lapped against another ice lobe. To the east, it extended up the Clark Fork nearly to Garrison, and up to the Blackfoot to include the part of its present course through the northern (Wales) section of today’s Ranch. Here it was a narrow arm of still water perhaps fifty feet deep. Into this end of the lake, the Blackfoot would have discharged at a point just a little farther up stream. * Estimates of the highest level of Lake Missoula have ranged from 4,200 to 4,250 feet above its present sea level. That shoreline can be defined by tracing the contour lines on a topo- graphic map. If that level - reached just before the ice dam broke - were known precisely, one could pinpoint the farthest extent of the lake. If it were exactly 4,200 feet, the narrow arm of the lake would have reached to the upper end of the “canyon,” putting its farthest shore in this area on present day Meyer property. If it were 50 feet higher, the lake would have reached a short distance above Nevada Creek’s confluence with the Blackfoot, splitting at that point into two upper arms with the Nevada Creek arm lapping on the present Ranch property line north of Ranch headquarters. Its far end could have been anywhere between the Wales sector and the headquarters area. 19