Analysis of Mountains Passes along the East-West Continental Divide and Other Drainage Divides Surrounding the Boulder River Drainage Basin, Jefferson County, Montana, USA

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.