Ohio River Valley Water Sanitation Commission ORSANCO

ORSANCO HAB Investigation – August 2025

Explore the Story Map

For maps, photos, and ongoing updates from this event, visit our Interactive HAB Story Map. This resource is updated regularly with field observations, sampling data, and trends.

ORSANCO HAB Investigation – August 2025

Recent Investigation

On August 20, 2025, ORSANCO staff conducted a harmful algal bloom (HAB) investigation along the Ohio River, covering the stretch from Markland Locks & Dam to Paducah, KY. The purpose was to assess conditions and collect samples for toxin analysis and algal enumeration.

Field Observations

  • River Conditions: The river appeared green, with low flow and very warm water.

  • Algal Presence: Small amounts of Microcystis were visible, but no obvious surface scums were found. Other algae accumulations observed were not consistent with Microcystis (different color/texture). Lab enumeration will confirm species composition.

  • Toxin Sampling: Samples indicated low levels of microcystin present from Ohio River Mile 846-902.5. The levels were all below the recreation threshold of 8.0 ug/L. The sample collected near the Paducah Water intake did not detect any toxins.

  • Dissolved Oxygen: Readings showed variability — ORSANCO’s datasonde at Markland L&D recorded 3.82 mg/L at 7:30 a.m., while the USGS datasonde across the river measured 2.2 mg/L at 2:45 p.m.

  • Fish Health: Dead carp were observed at all locations investigated. This is not unusual for this time of year and may not be directly related to the algal conditions observed.

Next Steps

ORSANCO will continue processing toxin and algae samples. Results will be shared with partners to assess potential impacts on recreation, drinking water, and aquatic life.

ORSANCO HAB Program

Harmful algal blooms (HABs) are a serious water quality concern across the Ohio River Basin. ORSANCO and our partners are working year-round to monitor, predict, and respond to these events to protect both people and ecosystems. ORSANCO sampling has identified over 300 different species of algae in the Ohio River, grouped into 8 divisions. The most common are:

  • Diatoms (Bacillariophyta)

  • Green algae (Chlorophyta)

  • Blue-green algae (Cyanobacteria)

Cyanobacteria are of particular concern because some species produce toxins (cyanotoxins). These toxins can be harmful if swallowed and pose risks to people and pets through drinking water or recreation (like swimming or boating). When a bloom is dominated by cyanobacteria, it is considered a harmful algal bloom. This event has not produced toxic levels.

For more information regarding ORSANCO’s Harmful Algal Bloom program visit: orsanco.org/programs/harmful-algal-blooms

Photo taken at boat ramp located in Rosiclare, Illinois