Abstract
Detailed topographic maps of drainage divides surrounding the Jefferson County, Montana, Boulder River drainage basin were analyzed to determine the nature of drainage systems that preceded today’s Boulder River drainage system and how the Boulder River drainage system evolved from those earlier drainage systems. The Boulder River studied here drains in a north, east, and south direction to the Jefferson River, which at Three Forks, Montana joins the north-oriented Madison and Gallatin Rivers to form the north-oriented Missouri River. The North American east-west Continental Divide surrounds the Boulder River drainage basin western half and mountainous drainage divides with the Jefferson and Missouri Rivers surround the drainage basin’s eastern half. More than 25 deep mountain passes are notched into these drainage divides and provide evidence of the regional drainage system that preceded the present day Boulder River drainage system. Analysis of pass elevations and of orientations of valleys leading in opposite directions from those mountain passes shows that prior to Boulder River drainage system development immense volumes of south-oriented water moving in anastomosing complexes of diverging and converging channels flowed across the Boulder River drainage basin area and that the Boulder River drainage system evolved as deeper channels progressively captured flow from shallower channels. While not documented in detail crustal warping probably raised Boulder River drainage basin areas relative to adjacent valleys and basins as capture events took place. A water source was not determined, but may have been from a large North American continental ice sheet, although Boulder River drainage basin evolution probably occurred while mid Tertiary sediments were filling adjacent valleys and basins.
Highlights
For more than a century since the work of Calhoun [1] and Alden [2] most geologists have believed a continental ice sheet blocked the north-oriented Missouri River in the region north of Great Falls, Montana with the east-oriented downstream Missouri River valley being eroded along that ice sheet’s southern margin
Analysis of pass elevations and of orientations of valleys leading in opposite directions from those mountain passes shows that prior to Boulder River drainage system development immense volumes of south-oriented water moving in anastomosing complexes of diverging and converging channels flowed across the Boulder River drainage basin area and that the Boulder River drainage system evolved as deeper channels progressively captured flow from shallower channels
While detailed interpretations differ the north-oriented upstream Missouri River valley and the abandoned northeast-oriented Missouri River valley segments are usually considered to have been eroded during mid Tertiary time, perhaps in late Oligocene time, long before any recognized North American continental ice sheets and that ancient north-oriented Missouri River is thought to have flowed across Canada and not to the south-oriented Mississippi River as it does today
Summary
For more than a century since the work of Calhoun [1] and Alden [2] most geologists have believed a continental ice sheet blocked the north-oriented Missouri River in the region north of Great Falls, Montana with the east-oriented downstream Missouri River valley being eroded along that ice sheet’s southern margin. Much more recently Sears [4] proposed a major north-oriented river flowing from south of Montana continued along the present day Jefferson and Missouri River valley alignments before continuing in a northeast direction into and across Canada. When in a subsequent discussion Clausen [5] suggested south oriented melt water from a large continental ice sheet was much more likely to have initially eroded what are today north-oriented river valleys Sears [6] replied that mid Tertiary sediments contained in those valleys falsified Clausen’s hypothesis. The study reported here continues that discussion by using topographic map interpretation techniques to determine how mountain passes across drainage divides surrounding the present day Boulder River drainage basin originated and evolved.
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