Baseline Macroinvertebrate Monitoring 2015-2017 for the Upper Missouri River: Building a Long-term Data Set – Flushing Flow Study


UMOWA began the Missouri River Baseline Monitoring program in 2015, and has continued this effort into 2018. In 2015 we established seven long-term, baseline monitoring stations for sampling benthic macroinvertebrates and water chemistry within the Blue Ribbon section of the Missouri River between Wolf Creek and Cascade and two sites upstream of Canyon Ferry Lake. Aside from a long-term PPL monitoring site below Holter Dam, until the 2015 study, very few macroinvertebrate samples have been collected below the Wolf Creek Bridge (see MDEQ 2007).



Establish a Baseline

to conduct standardized and quantitative macroinvertebrate surveys to serve as the baseline standards for future monitoring efforts within this Missouri River section.

Population Estimates

To establish seasonal macroinvertebrate population estimates and community characteristics to assess aquatic biointegrity.


To understand the insect dynamics in relation to increasing aquatic macrophyte beds, algae and minimal sediment-flushing lows.


Baseline Macroinvertebrate Monitoring 2015-2017 for the Upper Missouri River: Building a Long-term Data Set – Flushing Flow Study

In June, July and October of 2017, we collected 70 quantitative, replicated macroinvertebrate samples, habitat data and water chemistry measurements at seven Missouri River sites. UMOWA also collected water chemistry (WQ) samples seasonally in summer and fall in 2017.

Total nitrogen and phosphorus levels for the fall samples were above MDEQ’s numeric nutrient standard across the sites (see UMOWA WQ Report). Other than indirect effects of increased weed bed extents, we can find no direct correlation with the Missouri River’s (WQ) and macroinvertebrate population characteristics. Spring run-off events from Little Prickly Pear Creek and the Dearborn River continue to have significant effects on the densities and diversity of insect communities in the Missouri River below those tributaries, especially by adding mayfly (E), stonefly (P) and caddisfly (T) taxa (EPT taxa).

At the Missouri River upstream of Little Prickly Pear Creek, we observed the highest densities of macroinvertebrates (>20,000 per m2) and New Zealand mudsnails (>300 per m2) since the study began; these densities were significantly “knocked-back” by high flows in June 2017, compared to summer of 2016. High densities and standing crops of Baetis (BWOs), Tricorythodes (Tricos), Chironomidae (midges), and Ephemerella (PMDs) in the spring samples within the Wolf Creek to Craig section “hatch-out” by the summer period leading to lowered summer densities of these taxa, and a dominance of non-insect taxa (scuds, sowbugs, worms and snails).

EPT taxa (mostly mayflies) gradually build up their lower summer populations though the fall sampling period, but are likely hampered by aquatic vegetation growth; therefore, non-insect taxa still dominate the fall benthos at most sites upstream from the Dearborn River and in the weed beds. 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 contributions were lowest. Of the 20 species of caddisflies that were collected across all sites, the net spinning caddis, Cheumatopsyche and the long-horned caddis, Oecetis were usually the most prevalent. Caddisflies were more common during the spring and summer sampling periods at some sites, but as a percentage of total EPT were usually much lower than mayflies. While stonefly taxa are not common within the Wolf Creek to Dearborn reach, 8 taxa were reported below the tributaries, especially in the spring samples. Stonefly diversity and EPT taxa richness in general increases with increasing distance from Holter Dam. Highest total taxa and EPT richness were reported at sites least affected by the dam, particularly the Missouri River near Hardy Bridge and at Cascade which are 28 and 37 miles downstream from Holter dam, respectively. The Cascade site also contained a unique benthic fauna reporting four mayfly species that were collected nowhere else in the study.

We sampled aquatic weed-beds with Hess samples (n=3) at two sites (MO_LPPC_US and MO_HARDY) during the summer and fall sampling periods in 2016 to better understand insect communities using this abundant macro-habitat. Average macroinvertebrate densities across the sites’ weed bed samples (n=12) was ~8,300 per m2; this is more abundant than the surrounding bottom substrate at Hardy Bridge (~6,500 per m2), but not as high as the Little Prickly Pear upstream benthic densities of ~20,000 per m2. While aquatic weed beds are dominated by scuds, sowbugs, worms, snails and damselflies, there are 4 Baetidae mayfly species that are preferentially found in this habitat at low numbers: Callibaetis, Acerpenna pygmaea, Plauditus punctiventris and Pseudocloeon. These species add to the overall EPT diversity of the reach. Weed beds also provide the preferred habitat for damselflies (Enallagma and Ishnura) in this river section, and they averaged ~500 individuals per m2 of weedbed. Crayfish (Orconectes virilis) biomass and densities were highest at sites where benthic substrate contained cobbles larger than ~6 inch diameter (Mid- Cannon, Hardy Bridge and Cascade). The riffle beetle, Optioservus quadrimaculatus contributed significant numbers (avg. ~1,000 individuals per m2) to the macroinvertebrate communities of the Missouri River sites between Wolf Creek and Craig; these are not captured in any of the metrics analyzed, since they are non-EPT insects.

Overall, macroinvertebrate communities collected in 2017 resemble those reported in 2015 and 2016 with similar taxa composition; there are some steady increases in total and EPT taxa richness at the Missouri R. below Little Prickly Pear, D/S of the Dearborn, Hardy Creek and Cascade.

We recorded significant reductions in Chironomidae (midges) and the % of EPT taxa in the samples, especially from sites from Wolf Creek to upstream of the Dearborn River. Increases in the percentage of non-insect taxa, largely from Turbellarian flatworms, worms and sowbugs, comprising the benthic samples was substantial in the Missouri River from Craig and upstream of the Dearborn River sites. This community shift reflects an increase in sediment build-up in many gravel areas of the stream channel, compounded with large contributions of aquatic vegetation trapping sediments. Even the flushing flow plot study could not seem to effectively reduce the numbers of non-insects in the substrates, and in the Craig FFS plot, aquatic worms actually increased after the gravels were flushed. HBI tolerance scores indicated significant organic pollution at eight of the nine (>88%) monitoring sites, and this metric has also been used as a surrogate for sediment impairment.

New Zealand mudsnails which have persisted in low densities at multiple sites in the Missouri River from Wolf Creek to Cascade have begun to substantially increase their populations upstream of Craig, especially upstream and downstream of the Little Prickly Pear Creek. High densities of the NZMS in 2016 at the MO_LPPC_US site were reduced by 66% after the flushing flows of June 2017. We postulate that NZMS population increases, overall non-insect macroinvertebrate density increases, significant caddisfly decreases and the expansion of rooted weed-beds in the Wolf Creek to Dearborn section is potentially correlated with lower spring flushing flows from Holter Dam since 2011. Future monitoring events will be focused on determining macroinvertebrate community responses (i.e., caddisflies, NZMS, non-insect taxa) to the 2017 flows, and the potentially larger flushing flows of 2018.