A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 5
Getting a Handle on Hydrology

As described in Chapter Three, stream flow greatly influences the character of a stream. By its own merits, it is an important parameter for understanding stream water quality. Stream flow also is necessary for calculating pollutant loadings – an important tool for interpreting stream water quality data. The following chapter describes how to measure flow and how to set up a staff gage for measuring stream height. It also explains how to use staff gage measurements to predict stream flows and how to calculate pollutant loads using stream flow data.

Measuring Stream Flow

Stream flow measurements can provide important information for both streams and lakes. In addition to allowing a comparison between pollutant loading and concentration, the changing relationship between precipitation and stream flow can be an important indicator of impacts from developing watersheds.

As watersheds develop, an ever-increasing portion of the land is covered by buildings, concrete, and asphalt. These impermeable surfaces prevent rainwater from seeping into the ground, and instead tend to channel and speed water on its way to the nearest stream. In a developed watershed, a small amount of rain may cause a rapid rise in flow in nearby streams, whereas the same amount of rain may have caused an imperceptible change in stream flow previous to development.

Taking streamflow measurements can be a fairly involved process if done right, but there also are simple methods that can be used to provide rough estimates for comparison. Two methods are described here. The first is a slightly modified version of the official U.S. Geological Survey (USGS) method. It is the method that most professionals use. The second is a simple method for obtaining a rough estimate that doesn’t require any expensive equipment. For both methods, the first step is to choose a good spot for making the measurements.

Selecting a Station for Streamflow Measurement

Selecting the proper location for measuring stream flow can be as important to collecting accurate information as the method used to take the measurements. Ideally, all of the following criteria should be met. In reality, they rarely are. However, it is important to understand the limitations of the site you select and the potential effects on streamflow measurements.

• The site should be readily and safely accessible. Never enter a stream if the water is too high or moving too fast for you to feel comfortable. Remember that a segment that was safely crossed in summer may be inaccessible in winter. Expect a difference and make a conscious decision each time you enter a stream.
• The site should lie in a section of stream that is free flowing:
  • The stream should be straight for enough distance to have relatively uniform flow. (The USGS) recommends 300 feet. In most smaller streams, you will be lucky to find a 100-foot straight section.)
  • The station should be located a sufficient distance upstream of tributaries and tidal action to ensure flow is not affected by either.
  • The stream should be confined to one channel. (Check to be sure there are no side channels or evidence that these may form during high flow conditions.)
  • Streambanks should be high and stable enough to contain maximum flows.
  • The streambank and channel should be relatively free of thick brush or vegetation that may slow the water and make measurements difficult.
  • Flow should be uniform and free of eddies, slack water, and excessive turbulence.
  • The streambed should be uniform. (Check for large boulders or logs, and consistency in depth and velocity.)

The next section discusses measuring stream flow with a meter.

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