Submersed aquatic vegetation surveys of the Upper Missouri River; from Holter Dam to Cascade, Montana and spanning portions of both Lewis & Clark and Cascade Counties, were conducted in May and in August 2019. These surveys utilized multi-band sonar, in conjunction with point- intercept sampling, and were conducted as a pilot assessment of submersed aquatic vegetation presence and spatial location; relative abundance and diversity and provided a characterization of river-bottom substrate composition.
Nearly the entirety of the surveyed area; some 8.25 square kilometers, would be considered suitable for aquatic plant growth as the surveyed area is predominantly comprised of extensive gravel bars and larger cobble beds, interspersed throughout with soft substrate zones conducive to aquatic plant growth. While deep areas (≥10 meters) exist within the study area, the calculated average depth of ±2.5 meters, is well suited to providing adequate sunlight to the river bottom in most locations. During the initial spring 2019 survey, the dominant submersed aquatic vegetation species was Northern milfoil (Myriophyllum sibiricum) followed by Leafy pondweed (Potamogeton foliosus) and Common watermoss (Fontinalis antipyretica). 99% of all recorded submersed aquatic plants occurred at depths ≤4 meters and the majority, 85%, were found at depths of 0-1 meter. Minor populations of the invasive (noxious; 2B)a species Curlyleaf pondweed (Potamogeton crispus) were noted along with extensive, incipient stands of filamentous algae (likely Lyngbya or Cladophora species).
During the follow-up survey conducted in August of 2019, the submersed aquatic vegetation population composition remained nearly the same, species was Northern milfoil (Myriophyllum sibiricum) followed by Leafy pondweed (Potamogeton foliosus) and Common watermoss (Fontinalis antipyretica). Mirroring the May survey, 99% of all surveyed SAV were detected at depths ≤ 5m and the majority (93%) occurring within the top meter of the water column. Unlike the May survey, SAV populations increased dramatically below the surface with 36% occurring at -1-2m and 13% respectively occurring deeper than 2 meters.
While the previously noted stands of P. crispus were still present, their life cycle was near completion at this time and so were much less extensive. In contrast, during this later survey extensive, nearly monotypic stands of a relatively cosmopolitan algae species, Ulva intestinalis (formerly Enteromorpha intestinalis), which was formerly thought to occur only rarely within Montana, were noted throughout the study area. Downstream of the confluence of the Dearborn River (47° 7’41.87″N 111°54’41.30″W), SAV was relatively absent, as compared to upstream river mile segments, and were dominated nearly in their entirety by filamentous and macrophytic algal species; primarily U. intestinalis.
Combined data from both survey events was used to confirm the presence of at least nine of eleven previously reported (2010, unpublished) submersed and floating aquatic species collected by Montana Fish, Wildlife and Parks. As this data set only encompasses a single season, limited inferences may be made about the potential effects of inter-seasonal peak discharge regime at Holter Dam; However, preliminary data suggests that the impacts on submersed aquatic vegetation are limited to within the first few kilometers of the river downstream from Holter Dam. Interestingly, the effect of gravel contribution to the Upper Missouri River from the Dearborn River appears to be quite extensive as rooted submersed plants only occurred in the river channel periphery. Submersed plant population occurrence and density was less than 50% of that found per river mile surveyed upstream of the Dearborn-Upper Missouri confluence; SAV and overall (includes moss and algae counts) species count was also significantly less than what was detected upstream, suggesting that seasonal discharge from the unmanaged Dearborn might provide an adequate model of the Missouri River study area were the Bureau of Reclamation Projects at Canyon Ferry and Holter not present.
A second season of hydroacoustic surveys, coupled with point-intercept and random sampling methods will be conducted during the 2020 water season with the intention of capturing river channel bathymetry, bottom composition, and SAV occurrence and distribution both PRE- and POST- peak seasonal discharge is planned. Combined data from the 2019 and 2020 data sets will be analyzed and reported during the winter 2020-21.