Stream Flow Monitoring / Measurement

Documenting and monitoring stream flow is an integral part of developing water budgets, conducting loading calculations,  evaluating the relationship between groundwater and surface water, and critical in evaluating impacts from urban runoff.

There are a number of methods to document stream flow, but the most typical method for field evaluations is to develop a cross-section of a stream segment or channel.  The volume of water that moves through the channel is then calculated by dividing the channel into smaller units of known or approximated areas (width * depth) and measuring the flow within each area (velocity - distance over time). When these values, area * velocity, are multiplied together the result is flow (length^3/ time), such as ft^3/second.
 

Step 1: Selecting the Channel Location

The area to be monitored should be a stable stream channel or at least not significant alteration.  The portion of the stream to monitor should not be a braided section of the stream or an meander area.  If possible, the section to be monitored should have flow that is parallel to the stream channel orientation and not within a pool area or other area altered by structures that may create backwater areas or reverse the flow of the water.  The site should be accessible   
 

Step 2. Developing a Cross-Section of the Site and Establishing a Reference or Staff Gauge

These methods utilize a velocity-area approach to measuring stream flow (or volume of water passing a set point in a given period of time). That is, to determine flow, the monitors combine information about stream area with information about the velocity of the water at their site.

If possible, the section of the stream used for this measurement should be relatively stable, i.e., not actively downcutting or meandering.  In addition, it would be advisable to select a stream where the flow is nearly parallel to the stream channel and not immediately after or before a meander or rapid.  When developing the cross-section, it may be necessary to remove or relocate some of the stream channel bottom material to create a more uniform bottom.  It is also advisable to extend the cross-section to a point that is above the flood level for the stream.

Step 3. Measuring Interval and Depth

Monitors measure the area of the stream at their site by measuring the width across the stream and the depth at several locations across the measured width.  For a small stream, the width interval between measurements is typically 6 inches. For larger streams, intervals of 1 - foot can be used. 

The stream is divided into rectangular grids and the flow at 60% off the stream depth is measured.  Flow can be measured with a number of different flow measuring devices.
 

Data Sources:

Pennsylvania Precipitation Data/Maps
US Geological Survey, Water Resources of Pennsylvania
SRBC Streamflow Information
 

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