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.
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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Contents | Lakes | Streams | From the Field to the Lab | Hydrology
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