A shoreline-to-shoreline survey of aquatic weeds, river sediments, and depth was conducted 23-24 MAY 2020 along an estimated 34-mile segment of the Upper Missouri River. This was done ahead of planned spring (June) increases in discharge at Holter Dam. These spring discharges, which historically have occurred within the first three weeks of June, are thought to have direct impact on the extent and severity of downstream nuisance aquatic weed populations through numerous processes.
This survey followed similar work conducted at the direction of the Upper Missouri Watershed Alliance, or UMOWA, in 2019, which noted Northern milfoil (Myriophyllum sibiricum), Leafy pondweed (Potamogeton foliosus), and Common watermoss (Fontinalis antipyretica) to be the species most commonly found in nuisance populations. Compared to survey data from 2019, aquatic weed growth throughout the study area had declined significantly in the period leading up to increased spring discharges. Compared to May 2019, an approximate 75% reduction in overall species occurrence and population densities was seen, while very few survey points had aquatic weeds growing in excess of 45% of the available water column. One population of the invasive species Curlyleaf pondweed (Potamogeton crispus). was recorded. Populations of filamentous algae (likely Lyngbya or Cladophora species) were noted throughout the study area. A second survey will be conducted late August 2020, to document the extent of aquatic weed growth this season and analyze for changes from 2019.
