The Aquatic Plant Study

There is increasing concern regarding the excessive, heavy aquatic plant growth in the Missouri River, especially in the late summer and fall. This is so excessive in many locations that it is difficult to navigate a boat through these plant mats and nearly impossible to fish there. In 2019, UMOWA initiated an extensive, scientifically based analysis of this phenomenon by hiring Amaruq Environmental Services, a professional plant research company to design and implement the project.

Aquatic Plant Study Summary 2019-2020

Description. Four, side-scanning sonar floats and plant surveys were conducted in May and August over a two-year period (2019-2020) to document the presence and extent of submerged aquatic vegetation. The section of the Upper Missouri River that was surveyed covered 34.5 river miles from Holter Dam to Cascade. These surveys were conducted to develop baseline data on submersed aquatic vegetation populations, their relative densities pre- and post-runoff, and to document sediment transport following controlled releases at Holter Dam. 


  • Combined data from both years confirmed the presence of nine of eleven previously reported submerged and floating aquatic species collected by Montana Fish, Wildlife, and Parks in 2010; including minor populations of the invasive, curly-leafed pondweed. 
  • In general, aquatic plant diversity is very low during the early spring (May), prior to higher runoff releases (late spring/early summer) from Holter Dam. Once discharges begin to decrease, plant densities and diversity (species richness) increase correlating with water temperatures and photoperiod. 
  • Nutrient uptake by the vast beds of submersed aquatic vegetation in the summer has a noticeable effect on Nitrogen and Phosphorus concentrations observed during the UMOWA water quality sampling. 
  • Sediment transport has been shown to play a limited role in the local establishment of new plants, but it is the prolonged and extensive flows associated with undammed tributaries such as the Dearborn River and Little Prickly Pear creek that provide adequate scouring of established plant beds. Plant bed densities below these tributaries are significantly reduced compared to infestations to the extent seen near Craig and above Mid-Canon. 
  • This 2-year data suggests that the impact of “moderate flushing flow” operations at Holter Dam on aquatic weed populations downstream is limited to the first mile of the Upper Missouri River. 
  • This data also suggests that peak flows, in excess of 11,000 CFS, and likely greater than 15,000 CFS (as calculated by R2 Resources 1994), are needed to adequately scour established plant populations from extant sites within the river. Further, the implications of annual river scouring to river biological dynamics cannot be understated. 


UMOWA’s full 2020 report on the Aquatic Plant Study is coming soon!


Check out UMOWA’s 2019 Aquatic Plant Study bathymetric and vegetation maps:

2019 Mile 1 bathymetry

2019 Mile 2 bathymetry

2019 Combined Bathymetry

May 2019 Vegetation

2019 August Vegetation

2020 Hydroacoustic Survey Update

Obtaining a Comparable Basis

The study entailed sampling the entire length of the Missouri River from Holter Dam to Cascade, Montana. The study focused on documenting the presence of aquatic plants (native and invasive) to map the prevalence and biomass of this vegetation. From 2018-2020, the project aimed to obtain a comparable basis as a way to analyze the results and determine the effects of outside factors that may influence this vegetation. We have expanded our Water Quality Analysis program and are concerned that the recent findings of elevated nutrients (nitrogen and phosphorous) may be contributing to excessive plant growth. UMOWA is committed to working on this issue and making a positive impact on this critical situation.


Submersed Aquatic Vegetation Survey 2019, Upper Missouri River, Montana



Your membership, support and generous contributions make the projects we have undertaken and your continued financial assistance will ensure our capability to continue this vital work.