In 2011, an outbreak of Eurasian watermilfoil (EWM), an aquatic invasive species included on Montana’s list of noxious species, was identified in the lower extents of the Jefferson Slough in Jefferson County, Montana. In 2013, Confluence was contracted by Jefferson County to conduct a hydrologic and geomorphic study of Jefferson Slough, a 13-mile long former side channel of the Jefferson River. This assessment was conducted to provide supporting data for a watershed management plan aimed at addressing altered flow regimes, excessive sedimentation and aquatic habitat degradation, and the outbreak of EWM. This population of milfoil has been identified as the primary source of plant fragments contributing to other milfoil infestations found in the lower Boulder River, the lower Jefferson River, and Canyon Ferry Reservoir. Addressing this source of milfoil is critical to prevent further colonization by this aggressive species in the Missouri River drainage.
The hydrologic study consisted of installing six TruTrack continuous water level and temperature data loggers. Periodic stage and discharge measurements were collected using a Marsh-McBirney flow meter at the TruTrack monitoring locations and irrigation returns. The geomorphic study entailed breaking the 13-mile channel into geomorphic reaches and documenting physical channel characteristics affecting sediment transport within each reach. Confluence, in partnership with Applied Geomorphology, Inc. combined LiDAR survey data with GPS surveyed cross-sections and longitudinal profiles in each reach to determine relative sediment transport capacity along the length of the Slough. These sediment transport characteristics were used to develop a series of design criteria aimed at improving sediment transport and ultimately reducing EWM habitat suitability.
Jefferson Slough Eurasian Watermilfoil Management Plan
Confluence utilized results from the hydrologic and geomorphic assessment to generate a series of conceptual solutions to control and eventually eradicate Eurasian watermilfoil from Jefferson Slough. Conceptual alternatives included the following direct and indirect control actions:
- Applying herbicide treatments
- Dewatering the channel and subjecting milfoil plants to freezing conditions
- Reducing incoming sediment loads by constructing sediment traps
- Implementing tributary restoration projects to reduce fine sediment delivery to Jefferson Slough
- Improving sediment transport through milfoil-infested segments of the channel by modifying channel dimensions
- Trapping and removing milfoil fragments from the channel using conveyor belt screens
- Altering the hydrologic regime of Jefferson Slough by modifying irrigation infrastructure to generate periodic flushing flows.
Following public and agency input, a series of preferred alternatives was selected and approved by landowners along the Slough. Confluence prepared the Jefferson Slough EWM Management Plan, outlining all direct and indirect actions to achieve the goals of controlling, and eventually eradicating Eurasian milfoil from this waterbody. The Management Plan will be utilized by Jefferson County and the Jefferson River Watershed Council to further educate the public and obtain funding for implementation