Big Hole River (Upstream of Jackson to the confluence with the Beaverhead River)
This project was the result of a four-county effort focused on developing a greater understanding of the river dynamics, inundation potential, and channel migration potential for the Big Hole River in Montana. A primary goal of this project was to produce approximate 100-year inundation potential maps and Channel Migration Zone maps for 160 miles of the main stem of the Big Hole River (DTM/AGI, 2005). These maps are being used to help focus management decisions and efforts along the river corridor.
DTM and AGI worked together to identify, scan and georeference two suites of historic imagery; perform a reach-based geomorphic analysis; develop an approximate 100- year inundation potential map based on GIS modeling; and develop CMZ maps for the river corridor The resulting CMZ maps are incorporated into a four-county ordinance for managing the Big Hole River corridor.
The Big Hole River flows for approximately 160 miles from its headwaters on the Montana/Idaho border of southwest Montana to its mouth near Twin Bridges, Montana. The convoluted course of the Big Hole River is attributable to the geologic controls provided by surrounding mountain ranges. The Big Hole River originates as a high-altitude mountain stream in the Beaverhead Mountains south of Jackson, flowing approximately 10 miles northward through the northern flank of the BeaverheadMountains into the Upper Big Hole Valley. From that point it flows northward for approximately 50 miles through a broad high elevation valley that lies on the western margin of the Pioneer Mountains. Between Pintlar Creek and Fishtrap Creek, the river turns northeastward and enters a canyon that wraps around the northern portion of the Pioneer Range. Near Divide, the river turns southward and flows through a series of canyons and open valleys, past Melrose and Glen, until it swings eastward along the south flank of McCartney Mountain, wrapping around its southern and southeastern edges, and following its margin northeastward to its ultimate confluence with the Beaverhead River, near Twin Bridges. Twin Bridges marks the confluences of the Big Hole, Beaverhead, and Ruby Rivers, which combine to form the Jefferson River.
Throughout the river’s course, it is constantly gaining water. At the upper end of the study area, the Big Hole is a small mountain stream. By the time it reaches the confluence with the Beaverhead it has grown greatly in size. As it progresses downstream, gaining water, and entering various canyon reaches, the Big Hole expresses a diverse set of geomorphic processes. Working with this diversity was the primary challenge with this project.
Interestingly, this study, along with county desires to adopt regulatory flood plain mapping for the river corridor, is resulting in a State-level effort to identify cost-effective strategies for developing regulatory floodplain mapping for the Big Hole River. A review by the State of the GIS-based inundation modeling used in this project and its compliance with State laws for adopting regulatory flood plain boundaries is showed some inconsistencies with State regulations (e.g. lack of an engineer’s stamp and not using a modern hydraulic model). Since rural communities usually lack the funding necessary to perform a complete mapping of floodplain and floodway, this State-led effort is focused on identifying cost-effective approaches to updating the Approximate Zone-A mapping, while still meeting State and Federal requirements.