GAI was hired by TNC in 2007 to investigate restoration of natural hydrology to the Hellroaring Creek alluvial fan located in Centennial Valley, MT. Historic flow paths and patterns indicated the dominant stream channel used to regularly avulse across the entire fan. With the introduction of irrigation efforts were taken to consolidate perennial flows to the E. Fork channel. This created increased channel energy (stream power) in E. Fork which in turn lead to more avulsion on the upper fan and channel incision below Red Rock Pass Road during peak flow events.

Figure 1:

Figure 2: Human efforts to confine the perennial flow channel increased stream power and resulted in a channel downcutting cycle and disconnection from its historic floodplain.

Figure 3: The GAI project team recommended that TNC undertake a pilot demonstration project to stabilize banklines and creation of inset floodplain surfaces in incised channel.

Figure 4: Eight days after project installation Hellroaring Cr. experienced an out-of-bank peak flow event, testing the soft engineering methods installed. This event resulted in a modest rate of treatment failure (approximately 8% of banklines). Deemed a success, a larger restoration project was then initiated in 2010.