Watershed Assessment of the Middle Powder Subbasin, Montana 2006

in the ecoregion contains the quality and integrity of its biological communities and habitats. In fact, the Powder River is the reference standard for the Missouri River aquatic classification (Stagliano 2005). Six globally rare to uncommon mayfly species (all are Montana Species of Concern) occur in the Powder River. We found one of these species at all aquatic sampling sites; two of the other species were recorded only at one site. The Powder River is also the primary spawning area for the lower Yellowstone River population of Sauger and represents substantial habitat for the sturgeon chub, both Montana Species of Concern. However, sturgeon chub, a former Endangered Species Act candidate, appear to be in decline in the Powder River. In the 1970s, they constituted 5% of all fish sampled in the Powder River. In 2005, neither we nor the USGS (2005) captured a single sturgeon chub within 40 miles of the Wyoming border. The goal of this study was to provide both landscape-level assessments of watershed health and integrity and site-specific evaluations of riparian and aquatic condition along the Powder River and its tributaries within the 4th-code Middle Powder Hydrologic Unit. This was accomplished using both field sampling and broad-scale GIS analysis. Our broad-scale GIS assessment examined underlying biological diversity, measured current conditions, and evaluated potential threats. Several key findings emerged from the GIS data analysis: 1. Relatively uniform natural diversity across 5th unit HUCs 2. Threats are highest in the Powder River floodplain, which is also the most important habitat 3. While there are no dams across the Powder River, there is a substantial number of diversions, which has contributed to chronic dewatering conditions 4. Roads could threaten aquatic health if not engineered and maintained properly - 66.7% of all tributary streams are within 20 meters of a road 5. Grazing is the dominant land use - between 94% and 96% of the land in natural cover is grazed, regardless of ownership (private or public) status 6. Native grasslands and shrub steppe dominate; evergreen forests are common, but wetlands rare. 7. Noxious weeds, especially leafy spurge, salt cedar and knapweed, have established themselves throughout the Powder River corridor, but are relatively uncommon elsewhere in the subbasin. The fine-scale rapid assessments plots focused on riparian areas but also evaluated uplands. These surveys generally confirmed the GIS analysis findings that all the study area watersheds were in reasonably good condition, with the lowest levels of disturbance found in the more remote southwestern watershed and the highest levels found in the northeastern watershed near Broadus with its associated agricultural activity. The Powder River cottonwood forest is ecologically significant in its extent. There are also a few tributaries with good condition riparian forests and considerable shrub structure. However, we found that cottonwood stands are disappearing or becoming decadent through most of the subbasin and regeneration is scarce. Shrub structure important for habitat is absent in most locations. We identified several management opportunities to support wetland and watershed health: 1. The Powder River is unique and important habitat for some Species of Concern. We recommend additional work on the distribution and water chemistry tolerances of the sturgeon chub and the Species of Concern mayflies. We also recommend that these species be included as a component of any biomonitoring approach during CBM development in the Powder River basin. 2. As CBM development begins, road-building and equipment movement between sites will greatly facilitate noxious weed transport. Vigilant monitoring and control will be necessary to prevent incursions of noxious weeds into weed-free parts of the watershed. 3. Russian olive and salt cedar are established and will degrade future riparian habitat if not controlled now while the infestation is limited. Additionally, the extensive cottonwood forest may virtually disappear if Powder River hydrology is not restored so cottonwoods can establish. The shrub component can return if grazing impacts are limited. 4. Because HUCs 060 and 070 have the highest scores on our Composite Watershed Condition index, and a high percentage of public ownership, we recommend they be prioritized for ongoing monitoring and assessment. 6. Many permittees already follow good grazing management practices to protect riparian resources. We recommend that these practices be encouraged, coupled with frequent utilization monitoring, and the use of physical barriers where necessary. 7. The Powder River reach upstream of Rough Creek (Site 5) was the most biologically intact aquatic site, followed by the Powder River reach at the Wyoming border (Site 1) and Site (2), the Dry Creek reach. We recommend choosing these as future aquatic monitoring sites Agreement Number :