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Water Quality Issues

Algae Blooms

Algae blooms can occur naturally in our environment, but their frequency, duration and intensity are increased by nutrient pollution. They occur when excess nitrogen and phosphorus cause an overgrowth of algae in a short period of time. The overgrowth of algae consumes oxygen and blocks sunlight from underwater plants. When the algae eventually die, that process also consumes oxygen in the water. This leads to low levels of dissolved oxygen that other aquatic life needs to survive. The following creates an optimal environment for algae blooms: excess nutrients (nitrogen and phosphorus), high temperatures, and standing or slow-moving water.

Some algae blooms can contain cyanobacteria, often referred to as blue-green algae. While most algae often seen in ponds and lakes during summer months are not toxic, some cyanobacteria species can produce cyanotoxins during algae blooms which can be harmful at elevated levels if ingested by dogs, wildlife, and humans, or during wading and other recreational contact with water.

Learn about how to recreate safely when blue-green algae is present.


We measure E. coli, a type of bacteria that can be found naturally in the environment, as an indicator of possible fecal contamination. While E. coli is often harmless, persistently high levels can indicate fecal pollution and an increased risk of illness when recreating in a waterbody.

Potential Sources:

  • Pets
  • Trash and urban wildlife
  • Livestock
  • Faulty sewers and septic systems

Learn more about E. coli in the Boulder St. Vrain watershed.

Printable E. coli fact sheets: E. coli Fact Sheet (English) / Hoja informativa sobre E. coli (Spanish).

Climate Change

Climate change is impacting our waterways through increased pollution and sediment runoff and increased intensity of droughts and precipitation events. Climate change is causing more intense droughts and precipitation events, which then impacts the flow of water through our watershed. Reduced stream and river flows can increase the concentration of pollutants in water. Flooding can occur during heavy rain events when water enters the watershed too quickly for the land to absorb it. These are just a couple of examples that show how having too little or too much water can impact water quality.


Certain metals can impact drinking water quality or be toxic to aquatic life when there are high concentrations in our waterbodies. Metals are naturally occurring in our environment, but human activities can contribute excess metals that become a water quality issue.

Potential Sources:

  • Active and legacy mines
  • Soil erosion and rock weathering
  • Road salts and urban runoff
  • Various industrial sources


Nutrients, such as nitrogen and phosphorus, occur naturally in the environment and are important for healthy aquatic systems. Human activities contribute excess nutrients to waterbodies that can lead to harmful algae growth, reducing oxygen for aquatic life and sometimes producing harmful toxins.

Potential Sources:

  • Fertilizers
  • Detergents
  • Pets, urban wildlife, and livestock
  • Faulty sewers and septic systems
  • Treated wastewater