Westchester County Department of County Planning

Weather can affect the physical, biological and chemical conditions in streams. Some examples:

• Flooding - Flooding increases water levels. Fast flowing surface water entering waterbodies often adds pollution and sediment, but it can also dilute any pollution present in the water.
• Drought - A long period without any or very little rain will lower the water levels, possibly making some stream stretches dry up. This affects numerous water chemistry components and stresses animals living in the streams.
• Cloudy weather - Plants produce food for themselves from sunlight and give off oxygen as a byproduct through the process known as photosynthesis. Prolonged cloudy weather may affect the amount of dissolved oxygen due to plants, because of the slower rate of photosynthesis caused by decreased levels of sunlight.
• Rain - Rainstorms and showers tend to dilute pollution, increase turbidity, and add nitrates and other non-point source pollution to waterbodies.
• Wind - Strong wind creates surface waves that will increase dissolved oxygen levels.
• Temperature - As water temperature decreases, its ability to hold oxygen increases.

Some types of bacteria are naturally found in streams and waterbodies. At low levels bacteria are essential to the health of a waterbody. As bacteria levels increase, the likelihood of an outside contamination source increases. For example, fecal coliform bacteria, a bacteria present in human and animal intestines, is rare in unpolluted water. High levels of fecal coliform bacteria indicate sewage (a source of fecal coliform) entering the waterbody. 

In addition, disease causing pathogens flourish in growing conditions similar to fecal coliform bacteria. Therefore a high level of fecal coliform in stream water indicates that more serious pathogens, possibly from the same source, could be contaminating the water.

Possible bacteria pollution sources:

• Animal and pet waste
• Septic system failure
• Poor pasture management
• Urban runoff
• Sewage from stormwater overflows

Alkalinity is a measure of the amount of dissolved weak bases, such as carbonates, bicarbonates and hydroxides.

Freshwater usually has alkalinity levels between 20-200 mg/L (milligrams per liter of calcium carbonate) depending on the composition of surrounding soils and bedrock. Alkalinity levels in the 100-200 mg/L range will restabilize pH changes in a stream.

The abundance or lack of alkalinity in a stream determines how well it can neutralize acidic pollution and resist changes in pH, referred to as the buffering capacity of a stream. Increases in acidic substances will decrease the amount of available buffers, or weak bases.

Over time surrounding alkaline soils and rocks, such as limestone, can restore the alkalinity/buffering capacity of a stream by naturally adding weak bases, such as carbonates, to the water. However, even momentary decreases in pH are harmful to macroinvertebrates, especially the pollution intolerant macroinvertebrates.

Modifications to waterbodies occur as land is developed and waterbodies are used for recreation, travel and drinking water. Stream and river modifications, such as dams and reservoirs, channelization, straightening, piping and rerouting can adversely affect water quality in various ways.  Learn more about land modifications that affect water quality.