2015 Missouri River Macroinvertebrate Study

Baseline Macroinvertebrate Monitoring Report 2015 for the Upper Missouri River, MT

EXECUTIVE SUMMARY - UMOWA began the Missouri River Baseline Monitoring program in 2015. This study was initiated to fill the need of establishing baseline monitoring stations, especially for benthic macroinvertebrates and water chemistry, in the Blue Ribbon section of the Missouri River between Wolf Creek and Cascade with additional sites above Canyon Ferry Reservoir. Very few macroinvertebrate samples have been collected below the Wolf Creek Bridge, but there were four sites below Holter Dam and two sites above Canyon Ferry that the MT Department of Environmental Quality (MDEQ) sampled qualitatively one time in the mid 2000’s that are available to use as comparisons to this study (MDEQ 2007). Project goals are: 1) to conduct standardized and quantitative macroinvertebrate surveys as baseline information to serve as the baseline standard against future monitoring efforts. 2) to assess aquatic biointegrity with key community indicators and comparing these against previously collected qualitative samples by MDEQ and PPL. These 2015 data represent the first year of seasonal reach-scale conditions documented to occur (i.e. pre-impact).

In spring, summer and fall of 2015, UMOWA quantitatively collected macroinvertebrate samples, habitat data and water chemistry measures at nine sites (seven below Wolf Creek and two above Canyon Ferry). Early spring run-off events from Little Prickly Pear Creek and the Dearborn River had significant effects on the densities of insects below those tributaries. These tributaries also contributed to the diversity of the insect fauna to the Missouri River, especially adding mayfly, stonefly and caddisfly taxa (EPT taxa). High densities and standing crops of Baetis (BWOs), Tricorythodes (Tricos), Chironomidae (midges), and Ephemerella (PMDs) in the May samples within the Wolf Creek to Craig section hatched out by the summer, leading to lower summer numbers of these taxa. These species gradually built up their populations into the fall sample period, but non-insect taxa still dominated the benthos. Percentages of sowbugs (Caecidotea), scuds (Gammarus, Hyalella and Crangonyx) and other non-insect taxa in the samples were highest during this fall period when EPT taxa were lowest. Of the 19 species of caddisflies that were collected across all sites, the net spinning caddis, Cheumatopsyche or Hydropsyche were usually the most prevalent. Seven stonefly taxa were collected throughout the reach, especially below the tributaries. Occurrence and diversity of stoneflies is sporadic and increases with increasing distance from Holter Dam. Highest total taxa and EPT richness were reported at sites least affected by the dams, particularly the Missouri River at York’s Island and Cascade, 18 and 37 miles respectively, downstream from dam effects. These sites contained a unique benthic fauna, reporting six mayfly species (three shared) that were collected nowhere else in the study.

Three additional semi-quantitative sites were added during the spring sampling period to fully understand insect communities of different macro-habitats; a silted-gravel area and boulder/riprap shoreline upstream of the Dearborn River and the cobble riffle at MidCannon FAS. The boulder/cobble shoreline areas were the only habitats where we consistently collected juvenile salmonids in the nets (predominately Rainbow Trout). Crayfish (Orconectes virilis) biomass and densities were highest in these boulder areas and only occupied benthic habitats where the substrate contained cobbles larger than ~6 inch diameter. Overall, macroinvertebrate communities collected in 2015 resemble those reported in the MDEQ samples from 2005 with similar taxa composition, but with significant reductions in caddisfly percentages in the samples and increases in the percentage of midges and sowbugs comprising the benthic samples. This change may reflect an increase in sediment build-up in many areas of the stream channel outside of the thalweg, compounded with large contributions of aquatic vegetation trapping sediments. HBI tolerance scores indicated significant organic pollution at five of the nine (>50%) monitoring sites, but this metric can also act as a surrogate for sediment impairment.